Longitudinal evidence that fatherhood decreases testosterone in human males

Reproductive endocrinology and aging in human males: An evolutionary perspective

Due to its important role in fertility, reproductive endocrine function has been subject to natural selection in all organisms including human males. Moreover, reproductive endocrine function is subject to change as males age. Indeed, the biology of aging is also subject to natural selection. As males age, hormone function such as variation in testosterone can change as the result of general somatic degradation. However these changes are not universal and can differ between human male populations depending on lifestyle and ecological context. The degree to which this variation is adaptive remains an open question but recent evolutionary anthropology research has provided some clarity. While knowledge of evolutionary approaches has limitations, the benefits of understanding the origins and comparative context of reproductive endocrine function in older human males are significant. This paper discusses our present comprehension of reproductive endocrinology and aging in human males, with a focus on human diversity across varied lifestyles, ecologies, and environments. In addition, comparative great ape research is examined. Current research challenges and future directions related to the importance of evolutionary biology and human diversity for understanding human male aging are discussed.

Do Parental Hormone Levels Synchronize During the Prenatal and Postpartum Periods? A Systematic Review

Physiological synchrony is the phenomenon of linked physiological processes among two or more individuals. Evidence of linkage between dyads has been found among a broad range of physiological indices, including the endocrine systems. During the transition to parenthood, both men and women undergo hormonal changes that facilitate parenting behavior. The present review sought to address the question as to whether hormonal synchronization occurs among expecting or new parents. A systematic literature search yielded 13 eligible records. The evidence of cortisol synchrony during the prenatal period, with additional testosterone, prolactin, and progesterone covariations in the time leading up to childbirth, was found to be most significant. During the postpartum period, parental synchrony was reported for oxytocin, testosterone, and cortisol levels. The implications of these covariations were found to translate into adaptive parenting behaviors and the facilitation of romantic bond. Associations with infant development were also reported, suggesting far-reaching effects of hormonal synchrony outside the parental dyad. The results highlight the importance of physiological interrelatedness during this sensitive period, underscoring the need for further research in this field. In view of the limited data available in this research domain, we have put forward a framework for future studies, recommending the adoption of standardized research protocols and repeated collections of specimens.

Effects of family life cycle events on individual Body Mass Index trajectories: Evidence from China

PURPOSE

Body mass index (BMI) is an important predictor of one's physiological health. China is a family-centric nation compared to Western societies and has already entered an aged society. Exploring the characteristics and patterns of BMI changes during household events in China provides critical insights into the biological and social determinants of health, which can help enhance the scientific validity of health promotion measures and contribute to the realization of healthy aging goals in China.

METHODS

Using data from the China Health and Nutrition Survey (CHNS) from 1993 to 2015, this article utilizes two-level growth curve models with piecewise spline specifications for age to examine the effects of family life cycle events on BMI trajectories for age groups and gender differences.

RESULTS

Compared to continuing status, experiencing transition in an individual's family life cycle could lead to more fluctuating variations in their BMI trajectories, generally, there is a faster increase in BMI during youth and a faster decline during old age. As for gender heterogeneity, males are more affected by divorce, widowhood, and empty nest, whereas females' BMI changes are influenced by entering/maintaining marriage and parenthood.

CONCLUSIONS

A long-term perspective has revealed the significance of family events on BMI throughout the life course. Future research should focus on the nutrition and health of specific populations, especially elderly individuals in vulnerable groups.

The intersection between alexithymia, testosterone reactivity, and coparenting in fathers predicts child's prosocial behavior

The development of prosocial skills in children is a key predictor of long-term social, cognitive, and emotional functioning. However, the role of fathers' psychological characteristics in fostering prosocial development, including during the prenatal period, and the mechanisms underlying their influence, remain relatively unexplored. This study aimed to examine whether a higher tendency of alexithymia, a difficulty to identify and verbalize emotions, in expectant fathers predicts prosocial behavior of two-year-old toddlers through the quality of coparenting and whether greater testosterone increase during a stressful parenting task moderates this indirect effect. A sample of 105 couples and their children was tracked longitudinally starting from the third trimester of pregnancy (T1), at three months (T2), and at two years postnatally (T3). Using self-report questionnaires, fathers reported on alexithymia (T1) and mothers and fathers reported on coparenting quality (T2). Additionally, fathers provided saliva samples before and after engaging in a stressful parenting task (the Inconsolable Doll Task) to measure testosterone reactivity (T1). Children's prosocial behavior was observed during an out-of-reach task (T3). A moderated mediation analysis using structural equation modeling showed that higher levels of alexithymia pre-birth predicted lower coparenting quality three months after birth, which in turn predicted lower prosocial behavior of two-year-old children, but only among fathers with mean or high testosterone increases. This study illuminates a potential mechanism by which fathers' alexithymia and testosterone reactivity forecast their toddlers' prosocial behavior.

Fatherhood and Cardiovascular Health, Disease, and Mortality: Associations From the Multi-Ethnic Study of Atherosclerosis

Introduction

Emerging literature links fatherhood to men's health but lacks comprehensive assessment of health outcomes, especially among multiethnic populations. This study's objective was to evaluate the associations of fatherhood (age at onset and status) with cardiovascular health scores, incident cardiovascular disease, cardiovascular disease death, and all-cause mortality, examining differences by race/ethnicity.

Methods

The study sample included men from Multi-Ethnic Study of Atherosclerosis, prospective cohort study that enrolled adults aged 45-84 years without known cardiovascular disease at baseline. Cardiovascular health was defined using the American Heart Association's Life's Essential 8 scores (0-100), excluding sleep (cardiovascular health score).

Results

In this sample of 2,814 men, mean age at cardiovascular health assessment was 62.2 years, 82% were fathers, 24% self-identified as Black, 13% self-identified Chinese, 22% self-identified Hispanic, and 41% self-identified White. Fathers who were aged <20 years and 20-24 years at their oldest child's birth had worse overall cardiovascular health than fathers who were aged >35 years (adjusted mean score of 61.1 vs 64.7 [p=0.01] and 61.0 vs 64.7 [p<0.001], respectively). Fathers had worse overall cardiovascular health (adjusted mean score of 63.2 vs 64.7, p=0.03) and more nicotine exposure (63.1 vs 66.6, p=0.04) than nonfathers. In age-adjusted models, fathers overall (hazard ratio=0.82; 95% CI=0.69, 0.98) and Black fathers (hazard ratio=0.73; 95% CI=0.53, 0.999) had a lower rate of all-cause mortality rate than nonfathers, but these associations were no longer significant in fully adjusted models.

Conclusions

Fatherhood is a social determinant of health, and understanding its influence may provide opportunities to improve men's health, particularly among men of color.

Modulation of the attentional response to baby schema by testosterone in pedohebephilic men and its relation to the nurturing system

Previous fMRI research found increased brain responses in men with pedophilic interest to non-sexual pictures of child and animal faces. This raised the question of whether an aberrant nurturing system could be linked to pedophilia. To further explore this hypothesis, 20 pedohebephilic and 23 teleiophilic men performed a target detection task with adult versus infant human and animal faces, which measured selective attention towards the baby schema by comparing reaction times to infant versus adult targets that were presented amongst distractors of the other category. Since the response to baby schema can be influenced by steroid hormones, saliva samples were additionally collected to determine endogenous testosterone, progesterone, estradiol and cortisol. Contrary to expectations, all men did not react faster to infant than adult faces. Yet, pedohebephilic men were more distracted by infant's faces than teleiophilic men. Pedohebephilic men with higher testosterone were faster in orienting attention to infant targets in the context of adult distractors. This association was not observed in teleiophilic men. Our results support the idea of an overactive nurturing system in pedophilia, which may be influenced by the endogenous testosterone level.

The design and implementation of a multi-center, pragmatic, individual-level randomized controlled trial to evaluate Baby2Home, an mHealth intervention to support new parents

BACKGROUND

Becoming a parent is a transformative experience requiring multiple transitions, including the need to navigate several components of health care, manage any mental health issues, and develop and sustain an approach to infant feeding. Baby2Home (B2H) is a digital intervention built on the collaborative care model (CCM) designed to support families during these transitions to parenthood.

OBJECTIVES

We aim to investigate the effects of B2H on preventive healthcare utilization for the family unit and patient-reported outcomes (PROs) trajectories with a focus on mental health. We also aim to evaluate heterogeneity in treatment effects across social determinants of health including self-reported race and ethnicity and household income. We hypothesize that B2H will lead to optimized healthcare utilization, improved PROs trajectories, and reduced racial, ethnic, and income-based disparities in these outcomes as compared to usual care.

METHODS

B2H is a multi-center, pragmatic, individual-level randomized controlled trial. We will enroll 640 families who will be randomized to: [1] B2H + usual care, or [2] usual care alone. Preventive healthcare utilization is self-reported and confirmed from medical records and includes attendance at the postpartum visit, contraception use, depression screening, vaccine uptake, well-baby visit attendance, and breastfeeding at 6 months. PROs trajectories will be analyzed after collection at 1 month, 2 months, 4 months, 6 months and 12 months. PROs include assessments of stress, depression, anxiety, self-efficacy and relationship health.

IMPLICATIONS

If B2H proves effective, it would provide a scalable digital intervention to improve care for families throughout the transition to new parenthood.

Parental status and markers of brain and cellular age: A 3D convolutional network and classification study

Recent research shows prominent effects of pregnancy and the parenthood transition on structural brain characteristics in humans. Here, we present a comprehensive study of how parental status and number of children born/fathered links to markers of brain and cellular ageing in 36,323 UK Biobank participants (age range 44.57-82.06 years; 52% female). To assess global effects of parenting on the brain, we trained a 3D convolutional neural network on T1-weighted magnetic resonance images, and estimated brain age in a held-out test set. To investigate regional specificity, we extracted cortical and subcortical volumes using FreeSurfer, and ran hierarchical clustering to group regional volumes based on covariance. Leukocyte telomere length (LTL) derived from DNA was used as a marker of cellular ageing. We employed linear regression models to assess relationships between number of children, brain age, regional brain volumes, and LTL, and included interaction terms to probe sex differences in associations. Lastly, we used the brain measures and LTL as features in binary classification models, to determine if markers of brain and cellular ageing could predict parental status. The results showed associations between a greater number of children born/fathered and younger brain age in both females and males, with stronger effects observed in females. Volume-based analyses showed maternal effects in striatal and limbic regions, which were not evident in fathers. We found no evidence for associations between number of children and LTL. Classification of parental status showed an Area under the ROC Curve (AUC) of 0.57 for the brain age model, while the models using regional brain volumes and LTL as predictors showed AUCs of 0.52. Our findings align with previous population-based studies of middle- and older-aged parents, revealing subtle but significant associations between parental experience and neuroimaging-based surrogate markers of brain health. The findings further corroborate results from longitudinal cohort studies following parents across pregnancy and postpartum, potentially indicating that the parenthood transition is associated with long-term influences on brain health.

The role of exogenous testosterone and social environment on the expression of sociosexuality and status-seeking behaviors in young Chilean men

Testosterone plays an important role as a social hormone. Current evidence suggests that testosterone is positively related to sociosexuality increasing the psychological attitudes toward investing in short-term versus long-term mating and promotes status-seeking behaviors both by dominance and prestige. In addition, the social environment may play an important role in the expression of mating effort through changes in sociosexuality and status-seeking behaviors. However, the causal relationships among the mentioned variables are still debated. We employed a double-blind, placebo-controlled within-individual design, in order to test and integrate the proposed causal relationships between testosterone and social environment over short-term and long-term mating orientation and dominant and prestigious status-seeking behaviors in a sample of 95 young Chilean men. We did not find evidence that the administration of exogenous testosterone increased short-term or decreased long-term mating orientation as expected. Moreover, exogenous testosterone did not affect either aggressive or cooperative behavior failing to support the social status hypothesis. We also did not find any relationship between short or long-term mating orientation with status-seeking behaviors. Finally, we found support for the effect of social environment on sociosexual attitudes but not over status-seeking behaviors. Thus, men reported higher levels of short-term mating orientation in the presence of a woman compared to a man and no differences were found for long-term mating orientation. We argue that sociosexuality may be expressed flexibly, but contextual factors such as the presence of women seem more important than changes in testosterone levels.

Stability and inter-family associations of hair endocannabinoid and N-acylethanolamines across the perinatal period in mothers, fathers, and children

Analysis of endocannabinoids (ECs) and N-acylethanolamines (NAEs) in hair is assumed to retrospectively assess long-term EC/NAE concentrations. To inform their use, this study investigated stability of EC/NAE hair concentrations in mothers, fathers, and their children across the perinatal period as well as associations between family members. In a prospective cohort study, EC (AEA, 1-AG/2-AG) and NAE (SEA, PEA, OEA) levels were quantified in hair samples taken four times in mothers (n = 336) and their partners (n = 225) from pregnancy to two years postpartum and in offspring (n = 319) from shortly after birth to two years postpartum. Across the perinatal period, maternal and paternal hair ECs/NAEs showed poor multiple-test consistency (16-36%) and variable relative stability, as well as inconsistent absolute stability for mothers. Regarding children, hair ECs/NAEs evidenced poor multiple-test consistency (4-19%), no absolute stability, and either no or variable relative stability. Hair ECs/NAEs showed small to medium significant associations across the perinatal period within couples and parent-child dyads. Findings suggest hair ECs/NAEs during the perinatal period possess variable stability in adults, albeit more stability in fathers than mothers in this time. This highlights the need to further investigate factors associated with changes in hair ECs/NAEs across time. The first two years of life may be a dynamic phase for the endocannabinoid system in children, potentially characterized by complex within-family correspondence that requires further systematic investigation.

Becoming a first-time father during the COVID-19 pandemic in France

The role of fathers in Western societies has undergone significant change over time. However, their psychopathology remains largely misunderstood and difficult to identify. This study aims to explore the lived experiences of first-time fathers during the COVID-19 pandemic. Twenty-seven first-time French fathers were recruited for the study, which involved a narrative interview, the Rorschach projective test, the Edinburgh Postnatal Depression Scale (EPDS), and a semi-structured interview. Narrative interviews revealed several challenges faced by these fathers, including the pressure of paternal responsibility, the need to be actively involved in the prenatal activities and caregiving (haptonomy, skin-to-skin contact), and concerns about the future of their couple and family as a triad. The Rorschach tests showed numerous perinatal responses and difficulties in identifying phallic representations among the fathers. Additionally, the EPDS scores indicated that 15% of fathers showed signs of depression, while 52% exhibited signs of anxiety. This study also examines the impact of the COVID-19 pandemic and its associated health context on creating the initial triad. Lastly, the case of one father is presented to illustrate the need for diagnostic tools to address the psychopathology of fathers, as narrative or semi-structured interviews have often fallen short of addressing this issue.

Evolutionary perspectives on substance and behavioural addictions: Distinct and shared pathways to understanding, prediction and prevention

Addiction poses significant social, health, and criminal issues. Its moderate heritability and early-life impact, affecting reproductive success, poses an evolutionary paradox: why are humans predisposed to addictive behaviours? This paper reviews biological and psychological mechanisms of substance and behavioural addictions, exploring evolutionary explanations for the origin and function of relevant systems. Ancestrally, addiction-related systems promoted fitness through reward-seeking, and possibly self-medication. Today, psychoactive substances disrupt these systems, leading individuals to neglect essential life goals for immediate satisfaction. Behavioural addictions (e.g. video games, social media) often emulate ancestrally beneficial behaviours, making them appealing yet often irrelevant to contemporary success. Evolutionary insights have implications for how addiction is criminalised and stigmatised, propose novel avenues for interventions, anticipate new sources of addiction from emerging technologies such as AI. The emerging potential of glucagon-like peptide 1 (GLP-1) agonists targeting obesity suggest the satiation system may be a natural counter to overactivation of the reward system.

Men's physical health and life history transitions in the Philippines: Evidence for 'marital selection' but not protective effects of partnering and fatherhood

In Euro-American societies, married people typically have lower overall risks for total mortality and for certain chronic conditions compared to non-married people. However, people becoming partnered and parents also tend to gain weight in Euro-American settings. Few studies have tested whether links between physical health and life history status translate to other cultural contexts where the socio-ecological dynamics of family life may differ. This limits the application of these insights to men's well-being in global public health. To help address this gap, we drew on a large, long-running birth cohort study of Filipino men, using data collected at three waves between 2005 and 2014 when men were 21.5-30.5 years old (N = 607, obs. = 1760). We tested for the effects of the transition to partnering (marriage/cohabitation) and fatherhood on men's physical health (waist circumference, fat-free mass index, and grip strength). Men becoming partnered or partnered fathers (P/PF) had comparable longitudinal physical health trajectories to men remaining single non-fathers. However, men who became P/PF by their mid 20s had higher fat-free mass index values than single non-fathers at each wave, with the largest effect observed when all men were single non-fathers at baseline. Men who became P/PF by their early 30s were also stronger than the reference group at baseline. Thus, men who were more muscular and stronger at baseline were more likely to transition to P/PF status, consistent with a 'marital selection' model. In complementary analyses, men did not exhibit adverse health effects when they became partnered fathers as young adults or parents to infants, respectively. These findings suggest that links between health and life history transitions in this setting differ from those commonly observed in Euro-American societies. While transitions to marriage and fatherhood are promising windows for interventions to improve men's health, our results highlight the importance of tailoring such approaches to local dynamics.

No evidence for relationship between paternal post-partum depressive symptoms and testosterone or cortisol in first-time fathers

Male life history strategies are regulated by the neuroendocrine system. Testosterone (T) and cortisol regulate male behaviors including parenting and facilitate managing tradeoffs at key transitions in development such as first-time fatherhood. Both hormones demonstrate marked fluctuations in the postnatal period, and this presents an opportunity to investigate the role of T and cortisol in postpartum depressive symptoms-comparably less studied in fathers than in mothers in the evolutionary literature. Prior work on depressive symptoms has yet to integrate insights from the "dual hormone hypothesis (DHH)" which has focused on how T and cortisol interact to jointly regulate traits associated with dominance and status-seeking (i.e., mating effort) but has yet to be included in models of parenting effort. In this research, we use secondary data to investigate the relationship between DHH and traits ostensibly opposed to status seeking (i.e., depressive symptoms). First-time fathers (n = 193) provided morning saliva samples 10 months following parturition and reported on the presence of depressive symptoms (BDI-II). Responses were decomposed into three factors: cognitive, affective, and somatic. Using hybrid latent variable structural equation modeling, we did not find evidence that T predicted variability in cognitive, affective, or somatic depressive symptom factors. We found a null effect for cortisol as well. Finally, we could not find evidence that the DHH variable (T × cortisol interaction) predicted any variability in cognitive, affective, or somatic depressive symptoms. While we did not find evidence to support our hypotheses using a secondary data set, this study contributes to research on the neuroendocrinology of depression in fathers. Discussion focuses on the limitations of sample demographics, timing of saliva and self-report collection, and the lack of extant theory specific to paternal postpartum depression.

The next generation of developmental psychopathology research: Including broader perspectives and becoming more precise

The current Special Issue marks a major milestone in the history of developmental psychopathology; as the final issue edited by Cicchetti, we have an opportunity to reflect on the remarkable progress of the discipline across the last four decades, as well as challenges and future directions for the field. With contemporary issues in mind, including rising rates of psychopathology, health disparities, and international conflict, as well as rapid growth and accessibility of digital and mobile technologies, the discipline of developmental psychopathology is poised to advance multidisciplinary, developmentally- and contextually- informed research, and to make substantial progress in supporting the healthy development of individuals around the world. We highlight key future directions and challenges for the next generation of developmental psychopathology research including further investigation of culture at multiple levels of analysis, incorporation of macro-level influences into developmental psychopathology research, methods advances to address heterogeneity in translational research, precision mental health, and the extension of developmental psychopathology research across the lifespan.

Testosterone's role in modulating human behaviors relevant to mating and parenting

Testosterone (T) is linked to human mating and parenting. Here, we comprehensively reviewed evidence on whether, in men and women, (1) basal T levels are related to mating and parenting behaviors, (2) T responds to reproduction-relevant cues, (3) acute changes in T map onto subsequent mating and parenting behaviors, and (4) single-dose exogenous T administration causally affects mating and parenting behaviors. We examined whether the available evidence supports trade-off interpretations of T's adaptive function whereby high T levels correspond to greater mating/reproductive effort and competition and low T levels to greater parenting effort and nurturance. We found mixed support for trade-off hypotheses, suggesting that T's function in modulating human mating and parenting might be more nuanced and highly dependent on context and individual trait differences. Results were largely similar for men and women, although studies with women were scarcer than those with men for most behaviors we reviewed.

Father contribution to human resilience

Fathers have been an important source of child endurance and prosperity since the dawn of civilization, promoting adaptation to social rules, defining cultural meaning systems, teaching daily living skills, and providing the material background against which children developed; still, the recent reformulation in the role of the father requires theory-building. Paternal caregiving is rare in mammals, occurring in 3-5% of species, expresses in multiple formats, and involves flexible neurobiological accommodations to ecological conditions and active caregiving. Here, we discuss father contribution to resilience across development. Our model proposes three tenets of resilience - plasticity, sociality, and meaning - and discussion focuses on father-specific contributions to each tenet at different developmental stages; newborn, infant, preschooler, child, and adolescent. Father's style of high arousal, energetic physicality, guided participation in daily skills, joint adventure, and conflict resolution promotes children's flexible approach and social competence within intimate bonds and social groups. By expanding children's interests, sharpening cognitions, tuning affect regulation, encouraging exploration, and accompanying the search for identity, fathers support the sense of meaning, enhancing the human-specific dimension of resilience. We end by highlighting pitfalls to paternal contribution, including absence, abuse, rigidity, expectations, and gender typing, and the need to formulate novel theories to accommodate the "involved dad."

Deconstructing sex: Strategies for undoing binary thinking in neuroendocrinology and behavior

The scientific community widely recognizes that "sex" is a complex category composed of multiple physiologies. Yet in practice, basic scientific research often treats "sex" as a single, internally consistent, and often binary variable. This practice occludes important physiological factors and processes, and thus limits the scientific value of our findings. In human-oriented biomedical research, the use of simplistic (and often binary) models of sex ignores the existence of intersex, trans, non-binary, and gender non-conforming people and contributes to a medical paradigm that neglects their needs and interests. More broadly, our collective reliance on these models legitimizes a false paradigm of human biology that undergirds harmful medical practices and anti-trans political movements. Herein, we continue the conversations begun at the SBN 2022 Symposium on Hormones and Trans Health, providing guiding questions to help scientists deconstruct and rethink the use of "sex" across the stages of the scientific method. We offer these as a step toward a scientific paradigm that more accurately recognizes and represents sexed physiologies as multiple, interacting, variable, and unbounded by gendered preconceptions. We hope this paper will serve as a useful resource for scientists who seek a new paradigm for researching and understanding sexed physiologies that improves our science, widens the applicability of our findings, and deters the misuse of our research against marginalized groups.

Prenatal testosterone synchrony in first-time parents predicts fathers' postpartum relationship quality

There is evidence that men's testosterone levels decline across the transition to fatherhood and that this decline may reflect fathers' investment in the new family. There is also emerging evidence that cohabiting couples show synchrony or within-couple associations in testosterone levels during the perinatal period. Hormonal synchrony may act as a mechanism that supports fathers' biological preparation for parenthood, perhaps by facilitating perinatal declines in paternal testosterone. However, few studies have examined testosterone synchrony and change within couples. A sample of 97 U.S. couples expecting their first child provided testosterone samples during pregnancy, and of those couples, 78 couples also provided testosterone at seven months postpartum. Couples reported on relationship satisfaction both at prenatal and postpartum visits. Bayesian multilevel modeling revealed within-couple testosterone synchrony both during pregnancy and postpartum. Testosterone synchrony during pregnancy predicted a greater drop in fathers' testosterone levels from prenatal to postpartum and higher paternal postpartum relationship quality. Fathers' lower prenatal testosterone levels also subsequently predicted higher self-reported postpartum relationship quality for both parents. In sum, this study finds that couples' testosterone levels show synchrony across the transition to parenthood in ways that are associated with couple relationship quality and men's neuroendocrine preparation for fatherhood.

Pregnancy programs the brain for mothering

Progesterone and estrogen exert separate effects on the brains of pregnant mice.

Testing the Challenge Hypothesis in Stumptail Macaque Males: The Role of Testosterone and Glucocorticoid Metabolites in Aggressive and Mating Behavior

The "challenge hypothesis" predicts higher male-male aggressive behavior along with increases in testosterone levels during times of reproductive challenges and social instability. In addition, in some primate species, higher glucocorticoid levels can be observed as well, but this is usually modulated by dominance rank. We studied rank-related aggressive behavior, mating activity, and fecal testosterone and glucocorticoid metabolites (fTm and fGCm) in male stumptail macaques (Macaca arctoides) in order to test some predictions of the "challenge hypothesis". Over a 20-month period, we collected data on aggressive behavior and copulation, as well as fecal samples (n = 700) to quantify fTm and fGCm in seven adult stumptail males living in captivity. During periods of mating activity, male-to-male aggression increased in higher- and middle-ranking males. Neither fTm nor fGCm levels predicted male-to-male aggression. fGCm levels (but not fTm) were positively associated with male-to-female aggression; however, this association was pronounced during periods of mating activity. fGCm levels differed according to social rank, with middle-ranking males having the highest levels. Both hormones were higher during periods of mating activity, but only in higher- and middle-ranking males. Taken together, our findings partially support the challenge hypothesis in a non-seasonal primate and shed some light on the unique social and mating system of the stumptail macaque.

Facial Attractiveness, but not Facial Masculinity, is Used as a Cue to Paternal Involvement in Fathers

Purpose

Facial femininity in men is purportedly used as a cue by women as a signal of paternal involvement. However, evidence for this claim is questionable. Previous findings have shown that paternal involvement is linked to testosterone, but have not investigated facial masculinity directly, while other studies have found that facial masculinity is negatively associated with perceptions of paternal involvement but do not assess the accuracy of this judgement. Here, we assess whether facial masculinity in men is used as a cue to paternal involvement, and whether this cue is accurate.

Methods

We collected facial photographs of 259 men (156 of which were fathers) who also completed self-report measures of paternal involvement. Facial images were then rated by a separate group of raters on facial masculinity, attractiveness, and perceived paternal involvement. Shape sexual dimorphism was also calculated from the images using geometric morphometrics.

Results

We found that facial masculinity was not associated with perceptions of paternal involvement, nor was it related with self-reported paternal involvement. Interestingly, facial attractiveness was negatively associated with perceptions of paternal involvement, and we found partial evidence that facial attractiveness was also negatively associated with self-reported paternal involvement.

Conclusion

These findings challenge the hypothesis that sexual dimorphism is used as a cue to paternal involvement, and perhaps indicate that facial attractiveness is more important for this judgement instead.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40750-023-00217-y.

The Holy Father (and Mother)? Multiple Tests of the Hypothesis That Parenthood and Parental Care Motivation Lead to Greater Religiosity

Parenting is a universal element of human life. However, the motivational and attitudinal implications of parenthood remain poorly understood. Given that many major religions prescribe parent-benefiting norms restricting sexual promiscuity and socially disruptive behavior, we hypothesized that both parenthood and parental care motivation would predict higher levels of religiosity. Studies 1 to 3 (N >2,100 U.S. MTurkers; two preregistered) revealed that parental status and motivation were robustly associated with religiosity in Americans, and that age-related increases in religiosity were mediated by parenthood. Study 4a (376 students) found a moderated experimental effect, such that emotionally engaged participants showed increases in religiosity in response to a childcare manipulation. Study 4b then replicated this effect in recoded data from Studies 1 and 2. Study 5 used data from the World Values Survey (N = 89,565) and found further evidence for a relationship between parenthood and religiosity. These findings support functional accounts of the relationship between parenthood and mainstream religiosity.

Functional relationships between estradiol and paternal care in male red-bellied lemurs, Eulemur rubriventer

Fathers contribute substantially to infant care, yet the mechanisms facilitating paternal bonding and interactions with infants are not as well understood as they are in mothers. Several hormonal changes occur as males transition into parenthood, first in response to a partner's pregnancy, and next in response to interacting with the newborn. These changes may prepare fathers for parenting and help facilitate and maintain paternal care. Experimental studies with monkeys and rodents suggest that paternal care requires elevated estradiol levels, which increase when a male's partner is pregnant and are higher in fathers than non-fathers, but its role in the expression of paternal behaviors throughout infant development is unknown. To assess estradiol's role in paternal care, we analyzed the relationship between paternal estradiol metabolites and 1) offspring age, and 2) paternal care behavior (holding, carrying, huddling, playing, grooming), in wild, red-bellied lemurs (Eulemur rubriventer). We collected 146 fecal samples and 1597 h of behavioral data on 10 adult males who had newborn infants during the study. Estradiol metabolites increased four-fold in expectant males, and in new fathers they fluctuated and gradually decreased with time. Infant age, not paternal behavior, best predicted hormone levels in new fathers. These results suggest that hormonal changes occur in expectant males with facultative paternal care, but they do not support the hypothesis that estradiol is directly associated with the day-to-day expression of paternal care. Future research should explore estradiol's role in facilitating behaviors, including infant-directed attention and responsiveness, or preparing fathers for infant care generally.

Development of the Paternal Brain in Humans throughout Pregnancy

Previous studies have demonstrated that paternal caregiving behaviors are reliant on neural pathways similar to those supporting maternal care. Interestingly, a greater variability exists in parental phenotypes in men than in women among individuals and mammalian species. However, less is known about when or how such variability emerges in men. We investigated the longitudinal changes in the neural, hormonal, and psychological bases of expression of paternal caregiving in humans throughout pregnancy and the first 4 months of the postnatal period. We measured oxytocin and testosterone, paternity-related psychological traits, and neural response to infant-interaction videos using fMRI in first-time fathers and childless men at three time points (early to mid-pregnancy, late pregnancy, and postnatal). We found that paternal-specific brain activity in prefrontal areas distinctly develops during middle-to-late pregnancy and is enhanced in the postnatal period. In addition, among fathers, the timing of the development of prefrontal brain activity was associated with specific parenting phenotypes.

Dual Mating Strategies Observed in Male Clients of Female Sex Workers

Humans have a complex and dynamic mating system, and there is evidence that our modern sexual preferences stem from evolutionary pressures. In the current paper we explore male use of a dual mating strategy: simultaneously pursuing both a long-term relationship (pair-bonding) as well as short-term, extra-pair copulations (variety-seeking). The primary constraint on such sexual pursuits is partner preferences, which can limit male behavior and hence cloud inferences about male preferences. The aim of this study was to investigate heterosexual male mating preferences when largely unconstrained by female partner preferences. In service of this goal, female full-service sex workers (N = 6) were surveyed on the traits and behaviors of their male clients (N = 516) and iterative cluster analysis was used to identify male mating typologies. Two clusters emerged: clients seeking a pair-bonding experience and clients seeking a variety experience. Results also suggested that romantically committed men were more likely to seek a variety experience than a relationship experience. We conclude that men desire both pair-bonding and sexual variety, and that their preference for one might be predicted by fulfilment of the other. These findings have implications for relationships, providing insight into motivations for male infidelity.

The Myth of Women's Advantage in Using Child-Directed Speech: Evidence of Women Versus Men in Single-Sex-Parent Families

PURPOSE

Differences between child-directed speech (CDS) by women and men are generally explained by either biological-evolutionary or gender-social theories. It is difficult to tease these two explanations apart for different-sex-parent families because women are usually also the main caregivers. Thus, this study aims to examine the influence of parental sex on CDS by investigating men and women who are in same-sex-parent families.

METHOD

Twenty same-sex-parent families participated in the study-10 families in which the parents were two men and 10 families in which the parents were two women. The families were matched for toddler age (range: 9-24 months) and sex. CDS was recorded using the Language Environment Analysis (LENA) device for 16 hr during a day. Each parent was also audio-recorded during a 30-min play session with his or her child.

RESULTS

No difference was found between men and women across all the LENA measures, namely, adult word count, conversational turns count, and child vocalization count. The analysis of speech samples during parent-child play showed no difference between men and women in mean length of utterance and number of nouns, verbs, and adjectives. Pragmatic speech acts of initiations, responses to infants' actions, or vocalizations were similar in both sexes. Women used more "teaching" utterances than men, and men who were main caregivers used more "teaching" utterances than men who were secondary caregivers. Across both sexes, secondary caregivers used more "requests for actions" compared to main caregivers.

CONCLUSIONS

The present findings support a functional-social approach and not a biological approach for explaining the use of CDS by men and women. These findings have clinical implications on the involvement of men in early intervention programs.

Bride price, dowry, and young men with time to kill: A commentary on men's marriage postponement in India

Rising numbers of young unmarried men in India reflect a marriage squeeze that goes beyond the shortage of brides created by sex-selective abortion. We describe a decline in men's marriageability caused by their falling economic prospects at the same time as families of brides are increasingly seeking grooms with stable employment. We group young men into those without jobs or much education, those with education but no work, and the privileged few with education as well as employment. This classification resolves some of the seeming contradictions in the qualitative literature on marriage in India. Some of this literature talks about the rising prevalence of bride price and some about the persistence of dowry, while some papers reflect in general on the costs of being young, male, and aimless. Our commentary includes a review of the growing literature on the physiological and (perhaps) consequently behavioural and health outcomes of men's anomie.

The utility of dried blood spot measurement of bone turnover markers in biological anthropology

OBJECTIVES

Bone is a dynamic organ under continual turnover influenced by life history stage, energy dynamics, diet, climate, and disease. Bone turnover data have enormous potential in biological anthropology for testing evolutionary and biocultural hypotheses, yet few studies have integrated these biomarkers. In the present article we systematically review the current availability, future viability, and applicability of measuring bone turnover markers (BTMs) in dried blood spot (DBS) samples obtained from finger prick whole blood.

METHODS

Our review considers clinical and public health relevance, biomarker stability in DBS, assay availability, and cost. We consider biomarkers of bone formation such as osteocalcin (bone matrix protein), PINP (N-terminal propeptide of type I collagen), and alkaline phosphatase (osteoblast enzyme), as well as biomarkers of bone resorption such as CTX (marker of collagen breakdown) and TRACP5b (tartrate-resistant acid phosphatase 5b; osteoclast enzyme).

RESULTS

Two BTMs have been validated for DBS: osteocalcin (formation) and TRACP5b (resorption). Prime candidates for future development are CTX and PINP, the formation and resorption markers used for clinical monitoring of response to osteoporosis treatment.

CONCLUSION

BTMs are a field-friendly technique for longitudinal monitoring of skeletal biology during growth, reproduction and aging, combining minimized risk to study participants with maximized ease of sample storage and transport. This combination allows new insights into the effects of energy availability, disease, and physical activity level on bone, and questions about bone gain and loss across life history and in response to environmental factors; these issues are important in human biology, paleoanthropology, bioarchaeology, and forensic anthropology.

Minimally invasive biomarkers in human population biology research, part 2: An introduction to the special issue

Over the past several decades, biomarkers have become indispensable tools in human biology, allowing researchers to investigate biocultural and evolutionary questions and to generate basic data on health and well-being. Human biologists are central players in biomarker methods development, pioneering the creation of "field friendly," minimally invasively collected biomarker approaches, as well as leading the innovative use of biomarkers, most notably to map the complex pathways from social and environmental conditions to altered physiology to health effects. This special issue, Part 2 of a collection focused on minimally invasively collected biomarkers, is comprised of nine papers that jointly contribute to driving the science of biomarker methods development and application. This compilation of papers engages with topics such as biological normalcy, ecoimmunology, and the ethics of working with vulnerable and underserved populations. It also focuses attention on research areas at present not emphasized in human biology such as bone turnover markers as a window onto osteoporosis and osteoarthritis and the use of cancer-related biomarkers in population screening and epidemiology. Taken together, these papers help draw the roadmap for future biomarker work, emphasizing: (1) the need for systematic and transparent approaches to assay validation, with open access publication; (2) simultaneous measurement of multiple biomarkers and expanded use of instrument platforms that increase the range of physiological, genomics, and omics markers available to researchers; and, (3) increased attention to ethics and researcher responsibilities, encouraging a mindset that recognizes our obligation to provide benefits to individuals and communities and to help redress past abuses.

Minimally invasive biomarkers in human and non-human primate evolutionary biology: Tools for understanding variation and adaptation

BACKGROUND

The use of minimally invasive biomarkers (MIBs - physiological biomarkers obtained from minimally invasive sample types) has expanded rapidly in science and medicine over the past several decades. The MIB approach is a methodological strength in the field of human and non-human primate evolutionary biology (HEB). Among humans and our closest relatives, MIBs provide unique opportunities to document phenotypic variation and to operationalize evolutionary hypotheses.

AIMS

This paper overviews the use of MIBs in HEB. Our objectives are to (1) highlight key research topics which successfully implement MIBs, (2) identify promising yet under-investigated areas of MIB application, and (3) discuss current challenges in MIB research, with suggestions for advancing the field.

DISCUSSION AND CONCLUSIONS

A range of MIBs are used to investigate focal topics in HEB, including energetics and life history variation/evolution, developmental plasticity, and social status and dominance relationships. Nonetheless, we identify gaps in existing MIB research on traits such as physical growth and gut function that are central to the field. Several challenges remain for HEB research using MIBs, including the need for additional biomarkers and methods of assessment, robust validations, and approaches that are standardized across labs and research groups. Importantly, researchers must provide better support for adaptation and fitness effects in hypothesis testing (e.g., by obtaining complementary measures of energy expenditure, demonstrating redundancy of function, and performing lifetime/longitudinal analyses). We point to continued progress in the use of MIBs in HEB to better understand the past, present, and future of humans and our closest primate relatives.

Is paternal oxytocin an oxymoron? Oxytocin, vasopressin, testosterone, oestradiol and cortisol in emerging fatherhood

How do hormonal levels in men change from pregnancy to after the birth of their firstborn child, and what is the role of oxytocin, alone or in interplay with other hormones, in explaining variance in their parenting quality? We explored in 73 first-time fathers the development of five hormones that have been suggested to play a role in parenting: oxytocin (OT), vasopressin (AVP), testosterone (T), oestradiol (E2) and cortisol (Cort). In an extended group of fathers (N = 152) we examined associations with fathers' behaviour with their 2-month-old infants. OT and E2 showed stability from the prenatal to the postnatal assessments, whereas AVP and T decreased significantly, and Cort decreased marginally. OT on its own or in interplay with other hormones was not related to paternal sensitivity. Using an exploratory approach, the interaction between T and E2 emerged as relevant for fathers' sensitive parenting. Among fathers with high E2, high T was associated with lower sensitivity. Although we did not find evidence for the importance of OT as stand-alone hormone or in interplay with other hormones in this important phase in men's lives, the interaction between T and E2 in explaining variation in paternal behaviour is a promising hypothesis for further research. This article is part of the theme issue 'Interplays between oxytocin and other neuromodulators in shaping complex social behaviours'.

The Road Not Taken: What Developmental Psychology Might Learn From Darwin's Insights Concerning Sexual Selection

Developmental Psychology is the branch of psychology that studies, not only human behavior, but how and why human behavior changes over time. This essay seeks to review to what extent Developmental Psychology has failed to perceive human behavior through the lens of evolutionary theory in general, and in particular sexual selection as first described by Darwin and later elaborated on by many, including Robert Trivers and Geoffrey Miller; the essay asserts that this failure has resulted in many wrong turns and missed opportunities. In some cases, major developmental theorists (e.g., Freud, Erikson) were bedeviled by sex-based differences which they saw but could not explain and which compromised the parsimony of their stage theories. In the case of stage theories of moral development, some major theorists (e.g., Piaget, Kohlberg) were able to offer simpler explanations of moral development only by limiting their studies to male subjects. And, while Developmental Psychology textbooks thoroughly describe sex differences in the timing of morphological changes in puberty, writers seldom discuss why the timing is different in the two sexes, universally, and functionally. On the other hand, several domains of developmental focus, including play, mate choice, parenting, and spatial cognition, have seen successful research efforts that utilized sexually selected predispositions as foundational assumptions. The essay concludes with a discussion of how a more evolutionary and functional view of human behavior might move the field of Developmental Psychology to an even more robust and accurate understanding of how humans change over the course of a lifetime.

Evidence for an adolescent sensitive period to family experiences influencing adult male testosterone production

Across vertebrates, testosterone is an important mediator of reproductive trade-offs, shaping how energy and time are devoted to parenting versus mating/competition. Based on early environments, organisms often calibrate adult hormone production to adjust reproductive strategies. For example, favorable early nutrition predicts higher adult male testosterone in humans, and animal models show that developmental social environments can affect adult testosterone. In humans, fathers’ testosterone often declines with caregiving, yet these patterns vary within and across populations. This may partially trace to early social environments, including caregiving styles and family relationships, which could have formative effects on testosterone production and parenting behaviors. Using data from a multidecade study in the Philippines (n = 966), we tested whether sons’ developmental experiences with their fathers predicted their adult testosterone profiles, including after they became fathers themselves. Sons had lower testosterone as parents if their own fathers lived with them and were involved in childcare during adolescence. We also found a contributing role for adolescent father–son relationships: sons had lower waking testosterone, before and after becoming fathers, if they credited their own fathers with their upbringing and resided with them as adolescents. These findings were not accounted for by the sons’ own parenting and partnering behaviors, which could influence their testosterone. These effects were limited to adolescence: sons’ infancy or childhood experiences did not predict their testosterone as fathers. Our findings link adolescent family experiences to adult testosterone, pointing to a potential pathway related to the intergenerational transmission of biological and behavioral components of reproductive strategies.

Relatively rapid effects of testosterone on men's ratings of female attractiveness depend on relationship status and the attractiveness of stimulus faces

Attractiveness judgements influence desires to initiate and maintain romantic relationships. Testosterone also predicts relationship initiation and maintenance; such effects may be driven by the hormone's modulation of attractiveness judgements, but no studies have investigated causal (and situation-dependent) effects of the hormone on these judgements. Using a placebo-controlled cross-over design, our preregistered analyses revealed order- and relationship- dependent effects: single heterosexual men judged the women as more appealing when testosterone was administered first (and placebo second), but marginally less appealing when placebo was administered first (and testosterone second). In a more complex model incorporating the women's attractiveness (as rated by an independent set of observers), however, we show that testosterone increases the appeal of women -but this effect depends upon the men's relationship status and the women's attractiveness. In partnered men (n = 53) who tend to derogate attractive alternatives (by rating them as less appealing), testosterone countered this effect, boosting the appeal of these attractive alternatives. In single men (n = 53), conversely, testosterone increased the appeal of low-attractive women. These differential effects highlight the possibility of a newly discovered mechanism whereby testosterone promotes male sexual reproduction through different routes depending on relationship status, promoting partner up- rather than down-grading when partnered and reducing choosiness when single. Further, such effects were relatively rapid [within 85 (±5) minutes], suggesting a potential non-genomic mechanism of action.

Cortisol and testosterone concentrations during the prenatal and postpartum period forecast later caregiving quality in mothers and fathers

Given that parental caregiving quality affects child development from birth onwards, it is important to detect parents who are at risk for low-quality caregiving as early as possible, preferably before or soon after birth. This study investigated whether cortisol (CORT) and testosterone (T) measured during the last trimester of pregnancy and six weeks postpartum were associated with observed caregiving quality at child age 3 in mothers (N = 63) and fathers (N = 45). CORT and T were measured during an interaction with a simulator infant (pregnancy) and their own infant (postpartum). In mothers, no associations were found with CORT and T during pregnancy, but higher postpartum CORT during a mother-infant interaction was related to higher caregiving quality during toddlerhood. In fathers, the association between T during pregnancy and caregiving quality in toddlerhood was more negative for fathers with low CORT. In contrast to mothers, higher postpartum CORT in fathers was associated with lower caregiving quality in toddlerhood. These findings proved robust after applying the Benjamini-Hochberg procedure to control for false discovery rate. Our findings indicate that CORT and T during the perinatal period can forecast caregiving quality in both mothers and fathers. Moreover, our results provided evidence for the dual-hormone hypothesis, but only in fathers. These findings contribute to our growing understanding on how endocrine measures explain individual differences in caregiving quality in mothers and fathers.

Testosterone administration does not alter the brain activity supporting cognitive and affective empathy

Although there is evidence that testosterone has deteriorating effects on cognitive and affective empathy, whether testosterone administration influences both routes to understanding others has not yet been simultaneously investigated. We conducted a functional magnetic resonance imaging (fMRI) pharmacological study using a within-subjects, randomized, placebo-controlled, double-blind crossover design to examine the effects of 100 mg transdermal testosterone administration on brain activation during a task that examines affective and cognitive empathy simultaneously in a sample of 23 healthy right-handed adult men. Relative to placebo, testosterone did not alter affective or cognitive empathy functional brain networks. Instead, the task yielded activation in the canonical networks associated with both types of empathy. Affective empathy yielded activation in the inferior and middle frontal gyri, inferior temporal gyri, and the cingulate cortex. Cognitive empathy was associated with activation of the temporoparietal junction, medial prefrontal cortex, middle and inferior temporal gyri, and temporal pole. Behaviourally, testosterone administration decreased error rates and increased participants' confidence in their responses regardless of response accuracy. Independent of testosterone administration, participants reported higher affective responses during emotionally negative scenarios. Even though our results provide further evidence that testosterone administration in healthy men does not alter brain activity underlying cognitive and affective empathy, testosterone administration does influence the empathic concern and hence socio-cognitive processes. The reproducibility and variability of the current and previous findings should nevertheless be addressed in upcoming studies.

Integrating the female masculinization and challenge hypotheses: Female dominance, male deference, and seasonal hormone fluctuations in adult blue-eyed black lemurs (Eulemur flavifrons)

In the decades since female social dominance was first described in strepsirrhine primates, researchers have sought to uncover the proximate and ultimate explanations for its development. In the females of various female-dominant species, androgens have been implicated as regulators of behavior and/or predictors of seasonal fluctuations in aggression (the 'Female Masculinization Hypothesis'). Males, more generally, respond to changing social demands via seasonal fluctuations in androgen-mediated behavior (the 'Challenge Hypothesis'), that may also entail changes in activation of the hypothalamic-pituitary-adrenal axis. Here, we explore if androgens, glucocorticoids, and intersexual behavior fluctuate seasonally in the female-dominant, blue-eyed black lemur (Eulemur flavifrons), with potential consequences for understanding female aggression and male deference. Across two studies conducted during the breeding and nonbreeding seasons, we assessed rates of mixed-sex, dyadic social behavior (aggression and affiliation) and concentrations of fecal glucocorticoid metabolites (Study 1) and serum sex hormones (androstenedione, testosterone, and estradiol; Study 2). Our results align with several predictions inspired by the Female Masculinization and Challenge Hypotheses for intersexual relations: During the breeding season, specifically, both aggression and androstenedione peaked in females, while female-initiated affiliation decreased, potentially to facilitate female resource access and reproductive control. By comparison, all target hormones (androgens, estrogen, and glucocorticoids) peaked in males, with glucocorticoid concentrations potentially increasing in response to the surge in female aggression, and unusually high estrogen concentrations year-round potentially facilitating male deference via male-initiated affiliation. These results suggest complex, seasonally and hormonally mediated behavior in Eulemur flavifrons.

Neurobiological Correlates of Fatherhood During the Postpartum Period: A Scoping Review

During the postpartum period, the paternal brain suffers extensive and complex neurobiological alterations, through the experience of father-infant interactions. Although the impact of such experience in the mother has been increasingly studied over the past years, less is known about the neurobiological correlates of fatherhood-that is, the alterations in the brain and other physiological systems associated with the experience of fatherhood. With the present study, we aimed to perform a scoping review of the available literature on the genetic, neuroendocrine, and brain correlates of fatherhood and identify the main gaps in the current knowledge. PubMed, Scopus, and Web of Science electronic databases were searched for eligible studies on paternal neuroplasticity during the postpartum period, over the past 15 years. Reference lists of relevant key studies and reviews were also hand-searched. The research team independently screened the identified studies based on the established inclusion criteria. Extracted data were analyzed using tables and descriptive synthesis. Among the 29 studies that met our inclusion criteria, the vast majority pertained to neuroendocrine correlates of fatherhood (n = 19), followed by brain activity or connectivity (n = 7), association studies of candidate genes (n = 2), and brain structure correlates (n = 1). Collectively, studies published during the past 15 years suggest the existence of significant endocrine (testosterone, oxytocin, prolactin, and cortisol levels) and neurofunctional alterations (changed activity in several brain networks related to empathy and approach motivation, emotional processing and mentalizing, emotion regulation, dorsal attention, and default mode networks) as a result of fatherhood, as well as preliminary evidence of genetic variability accounting for individual differences during the postpartum period in fathers. No studies were so far published evaluating epigenetic mechanisms associated with the paternal brain, something that was also the focus of the current review. We highlight the need for further research that examines neuroplasticity during the experience of fatherhood and that considers both the interplay between hormones and simultaneous assessment of the different biomarkers (e.g., associations between hormones and neural activity); data collection protocols and assessment times should also be refined.

Creation and Validation of the Japanese Cute Infant Face (JCIF) Dataset

Research interest in cuteness perception and its effects on subsequent behavior and physiological responses has recently been increasing. The purpose of the present study was to produce a dataset of Japanese infant faces that are free of portrait rights and can be used for cuteness research. A total of 80 original facial images of 6-month-old infants were collected from their parents. The cuteness level of each picture was rated on a 7-point scale by 200 Japanese people (100 men and 100 women in their 20s–60s). Prototypical high- and low-cuteness faces were created by averaging the top 10 and bottom 10 faces according to the mean cuteness ratings. Then, 50 composite faces were made by mixing two faces randomly chosen from the 60 unused middle-cuteness faces. The normative cuteness ratings of these composite faces were obtained from 229 Japanese men and women in their 20s–60s. The shape of each composite face was transformed to be cuter (+50%) or less cute (–50%) along a continuum between the high- and low-cuteness prototypical faces. A two-alternative forced-choice task (N = 587) confirmed that cuteness discrimination was better than the chance level for all 50 face pairs. Moreover, the results showed that young men had poorer sensitivity to cuteness differences in infant faces than older men and women of any age. This Japanese Cute Infant Face (JCIF, “jay-sif”) dataset, including composite face images and normative rating scores, is publicly available online.

A meta-analysis of the association between male dimorphism and fitness outcomes in humans

Humans are sexually dimorphic: men and women differ in body build and composition, craniofacial structure, and voice pitch, likely mediated in part by developmental testosterone. Sexual selection hypotheses posit that, ancestrally, more 'masculine' men may have acquired more mates and/or sired more viable offspring. Thus far, however, evidence for either association is unclear. Here, we meta-analyze the relationships between six masculine traits and mating/reproductive outcomes (96 studies, 474 effects, N = 177,044). Voice pitch, height, and testosterone all predicted mating; however, strength/muscularity was the strongest and only consistent predictor of both mating and reproduction. Facial masculinity and digit ratios did not significantly predict either. There was no clear evidence for any effects of masculinity on offspring viability. Our findings support arguments that strength/muscularity may be sexually selected in humans, but cast doubt regarding selection for other forms of masculinity and highlight the need to increase tests of evolutionary hypotheses outside of industrialized populations.

The Neural Basis of Human Fatherhood: A Unique Biocultural Perspective on Plasticity of Brain and Behavior

With the growing involvement of fathers in childrearing and the application of neuroscientific tools to research on parenting, there is a need to understand how a father's brain and neurohormonal systems accommodate the transition to parenthood and how such neurobiological changes impact children's mental health, sociality, and family functioning. In this paper, we present a theoretical model on the human father's brain and the neural adaptations that take place when fathers assume an involved role. The neurobiology of fatherhood shows great variability across individuals, societies, and cultures and is shaped to a great extent by bottom-up caregiving experiences and the amount of childrearing responsibilities. Mechanisms of mother-father coparental brain coordination and hormonal correlates of paternal behavior are detailed. Adaptations in the father's brain during pregnancy and across the postpartum year carry long-term implications for children's emotion regulation, stress management, and symptom formation. We propose a new conceptual model of HEALthy Father Brain that describes how a father's brain serves as a source of resilience in the context of family adversity and its capacity to "heal", protect, and foster social brain maturation and functionality in family members via paternal sensitivity, attunement, and support, which, in turn, promote child development and healthy family functioning. Father's brain provides a unique model on neural plasticity as sustained by committed acts of caregiving, thereby affording a novel perspective on the brain basis of human affiliation.

Human studies of mitochondrial biology demonstrate an overall lack of binary sex differences: A multivariate meta-analysis

Mitochondria are maternally inherited organelles that play critical tissue-specific roles, including hormone synthesis and energy production, that influence human development, health, and aging. However, whether mitochondria from women and men exhibit consistent biological differences remains unclear, representing a major gap in knowledge. This meta-analysis systematically examined four domains and six subdomains of mitochondrial biology (total 39 measures), including mitochondrial content, respiratory capacity, reactive oxygen species (ROS) production, morphometry, and mitochondrial DNA copy number. Standardized effect sizes (Hedge's g) of sex differences were computed for each measure using data in 2258 participants (51.5% women) from 50 studies. Only two measures demonstrated aggregate binary sex differences: higher mitochondrial content in women's WAT and isolated leukocyte subpopulations (g = 0.20, χ² p = .01), and higher ROS production in men's skeletal muscle (g = 0.49, χ² p < .0001). Sex differences showed weak to no correlation with age or BMI. Studies with small sample sizes tended to overestimate effect sizes (r = -.17, p < .001), and sex differences varied by tissue examined. Our findings point to a wide variability of findings in the literature concerning possible binary sex differences in mitochondrial biology. Studies specifically designed to capture sex- and gender-related differences in mitochondrial biology are needed, including detailed considerations of physical activity and sex hormones.

Early Life Adversity and Males: Biology, Behavior, and Implications for Fathers' Parenting

Fathers have an important and unique influence on child development, but influences on fathers' parenting have been vastly understudied in the scientific literature. In particular, very little empirical research exists on the effects of early life adversity (ELA; e.g. childhood maltreatment, parental separation) on later parenting among fathers. In this review, we draw from both the human and non-human animal literature to examine the effects of ELA, specifically among males, in the following areas: 1) neurobiology and neurocognitive functioning, 2) hormones and hormone receptors, 3) gene-environment interactions and epigenetics, and 4) behavior and development. Based on these findings, we present a conceptual model to describe the biological and behavioral pathways through which exposure to ELA may influence parenting among males, with a goal of guiding future research and intervention development in this area. Empirical studies are needed to improve understanding of the relationship between ELA and father's parenting, inform the development of paternal and biparental interventions, and prevent intergenerational transmission of ELA.

Putative Mental, Physical, and Social Mechanisms of Hormonal Influences on Postpartum Sexuality

Purpose of Review

Much research has documented changes in postpartum sexuality, including changes in sexual functioning and satisfaction for both the birthing parent and their partner(s). These changes are often linked to postpartum changes in hormonal and immune responses, which can have both direct and indirect effects on sexuality.

Recent Findings

Here, we review how postpartum sexuality may be changed via mental, physical, and social/relationship effects of a variety of hormones, including estrogens, progestogens, androgens, cortisol, and oxytocin. We also review the ways in which inflammation may act alongside hormones to influence postpartum sexuality.

Summary

We argue that, as each of these factors strongly influence the action of others, the next phase of research in postpartum sexuality must examine the bidirectional interactions of hormones and their effects on behavior, cognition, and social relationships.

Self-Protection as an Adaptive Female Strategy

Many male traits are well explained by sexual selection theory as adaptations to mating competition and mate choice, whereas no unifying theory explains traits expressed more in females. Anne Campbell's "staying alive" theory proposed that human females produce stronger self-protective reactions than males to aggressive threats because self-protection tends to have higher fitness value for females than males. We examined whether Campbell's theory has more general applicability by considering whether human females respond with greater self-protectiveness than males to other threats beyond aggression. We searched the literature for physiological, behavioral, and emotional responses to major physical and social threats, and found consistent support for females' responding with greater self-protectiveness than males. Females mount stronger immune responses to many pathogens; experience a lower threshold to detect, and lesser tolerance of, pain; awaken more frequently at night; express greater concern about physically dangerous stimuli; exert more effort to avoid social conflicts; exhibit a personality style more focused on life's dangers; react to threats with greater fear, disgust and sadness; and develop more threat-based clinical conditions than males. Our findings suggest that in relation to threat human females have relatively heightened protective reactions compared to males. The pervasiveness of this result across multiple domains suggests that general mechanisms might exist underlying females' unique adaptations. An understanding of such processes would enhance knowledge of female health and well-being.