Wealth, fertility and adaptive behaviour in industrial populations

Childbirth timing and completed family size by the mode of conception-the role of medically assisted reproduction: a population-based cohort study in Australia

Background

With declining total fertility rates to below replacement levels amongst all high-, middle- and low-income countries, coupled with increasing use of medically assisted reproduction (MAR) treatments globally, we describe the impact of these treatments on completed family size and childbearing timing in a country with unlimited publicly funded access to MAR.

Methods

We utilised a unique longitudinal propensity score-weighted population-based birth cohort that included nulliparous mothers who gave birth after all major forms of MAR treatments (assisted reproductive technologies [ART], ovulation induction [OI], and intrauterine insemination [IUI]) and after natural conception (reference category) in Australia, 2003-2017. We followed first-time mothers over their reproductive lifespan (15-50 years). The primary outcome was completed family size (i.e., the mean cumulative number of children per mother of our cohort) and the fertility gap (i.e., adjusted difference in completed family size between MAR conceptions and the reference).

Findings

Our cohort includes 481,866 first-time mothers, mean follow-up of 13.8 years. ART mothers (n = 25,296) were six years older (mean age: 34.6 years) than mothers who conceived naturally (28.7 years (reference)) while OI/IUI mothers were only 2.2 years older (31.0 years) than the reference. ART mothers had up to 27% smaller completed family size (2.54 children) compared to OI/IUI mothers (2.98 children) and natural conception mothers (3.23 children). ART mothers who resided in the lower socioeconomic areas were less likely to reach a similar family size to the natural conception mothers (fertility gap of 0.83 fewer children per ART mother compared to natural conception mothers) than ART mothers who resided in the higher socioeconomic areas (0.43 fewer children).

Interpretation

Greater awareness of the limitations of MAR treatment to resolve childlessness and achieve desired family size is needed. Furthermore, with policymakers increasingly turning to MAR treatment as a tool to reverse declining fertility rates, their potential impact should not be overestimated.

Funding

Australian National Health and Medical Research Council.

Impact of energy availability and physical activity on variation in fertility across human populations

Human reproduction is energetically costly, even more so than other primates. In this review, we consider how the energy cost of physical activity impacts reproductive tasks. Daily energy expenditure appears to be constrained, leading to trade-offs between activity and reproduction expenditures in physically active populations. High workloads can lead to suppression of basal metabolic rate and low gestational weight gain during pregnancy and longer interbirth intervals. These responses lead to variation in fertility, including age at first reproduction and interbirth interval. The influence of energetics is evident even in industrialized populations, where cultural and economic factors predominate. With the decoupling of skills acquisition from food procurement, extrasomatic resources and investment in individual offspring becomes very costly. The result is greater investment in fewer offspring. We present a summary of age at first reproduction and interbirth interval trends across a diverse, global sample representing 44 countries and two natural fertility populations. While economic factors impact fertility, women in energy-rich, industrialized populations are capable of greater reproductive output than women in energy-stressed populations. Thus, energetic factors can be disentangled from cultural and economic impacts on fertility. Future research should focus on objective measurements of energy intake, energy expenditure, and physical activity in a broader sample of populations to elucidate the role of energetics in shaping reproductive outcomes and health.

Linkages between men's wealth status and the ideal number of children: A trend and multilevel analysis of survey data in Nigeria

Most African societies practice a patriarchal family system that endows a man with authority and dominance in the family and society with a defined role of being the breadwinner of the home. A man is expected to have a great influence in determining the ideal number of children in the family and take a domineering role in decision-making, especially those related to household resource allocation. Therefore, this study examines the relationship between men's wealth status and an ideal number of children. The study used secondary data from the National Demographic Health Survey (NDHS) from 2003 to 2018. The objectives were achieved using descriptive and inferential statistics, including frequency, mean, ANOVA, and multilevel analysis techniques. Wealth status significantly influenced the ideal number of children considering the crude and adjusted regression analysis. After adjusting for individual-level and contextual factors, the odd ratio of ideal number of children was significantly lower among men in the richest categories of the wealth index. Moreover, men with two wives and above, uneducated men, Northern residents, men living in high community family norms, low community family planning, high community poverty, and low community level of education desired a high number of children. The analyses suggest the need for a consideration of community structures to provide lucrative employment for men and would experience an appreciable fertility decline in line with the objectives and targets stated in Nigeria's population policies and programmes.

Association of undernutrition and female infertility in East Africa: Finding from multi-country demographic and health surveys

Introduction

Infertility is one of the public health problems affecting a significant number of women in the reproductive age group. Although female fertility is predominantly affected by gynecological and systemic diseases, lifestyle and nutritional factors also play an important role in secondary female infertility. Therefore, this study aimed to determine the pooled prevalence of secondary female infertility and its association with undernutrition using nationwide data from the Demographic and Health Surveys (DHS) of eastern African countries.

Methods

The data of ten East African countries that comprise a weighted sample of 38,020 women data were accessed from measure DHS. Data processing and analysis were performed using STATA 15 software. A multilevel mixed-effect logistic regression model was fitted to examine the association between undernutrition and secondary infertility. Variables with a p-value < 0.05 were declared as significant factors associated with secondary infertility. Model comparison was done based on Akaike and Bayesian Information Criteria (AIC and BIC). To measure variation (random effects), Community-level variance with standard deviation and intra-cluster correlation coefficient (ICC) was used.

Result

The proportion of women who have secondary infertility was 16.32% with 95%CI (15.96, 16.69), of which 26.94% were undernourished. This study found that being undernutrition (AOR = 1.74; 95%CI: 1.54-1.98) and overweight (AOR = 1.72; 95%CI: 1.62-1.86) were significantly associated with secondary infertility. Women aged >35 years (AOR = 3.47; 95%CI: 2.66-4.55), and rural residents (AOR = 1.16; 95%CI: 1.02-1.37) are other factors that are positively associated with secondary infertility. However, primary education (AOR = 0.87; 95%CI: 0.77-0.97) and richer wealth index (AOR = 0.84; 95%CI: 0.73-0.97) are protective factors for secondary infertility.

Conclusion

This study indicated that there is a strong association between secondary infertility and undernutrition growing in Eastern Africa. Therefore, Health information dissemination and awareness creation on the impact of malnutrition on infertility should be given to the community and health care providers. Given this, it may lead to integrating nutrition counseling into both clinical settings for infertility management as well as national dietary guidelines for individuals of reproductive age.

Fertility Dynamics and Life History Tactics Vary by Socioeconomic Position in a Transitioning Cohort of Postreproductive Chilean Women

Globally, mortality and fertility rates generally fall as resource abundance increases. This pattern represents an evolutionary paradox insofar as resource-rich ecological contexts can support higher numbers of offspring, a component of biological fitness. This paradox has not been resolved, in part because the relationships between fertility, life history strategies, reproductive behavior, and socioeconomic conditions are complex and cultural-historically contingent. We aim to understand how we might make sense of this paradox in the specific context of late-twentieth-century, mid–demographic transition Chile. We use distribution-specific generalized linear models to analyze associations between fertility-related life-history traits—number of offspring, ages at first and last reproduction, average interbirth interval, and average number of live births per reproductive span year—and socioeconomic position (SEP) using data from a cohort of 6,802 Chilean women born between 1961 and 1970. We show that Chilean women of higher SEP have shorter average interbirth intervals, more births per reproductive span year, later age at first reproduction, earlier ages at last reproduction, and, ultimately, fewer children than women of lower SEP. Chilean women of higher SEP consolidate childbearing over a relatively short time span in the middle of their reproductive careers, whereas women of lower SEP tend to reproduce over the entirety of their reproductive lifespans. These patterns may indicate that different SEP groups follow different pathways toward declining fertility during the demographic transition, reflecting different life-history trade-offs in the process.

Can impulsivity evolve in response to childhood environmental harshness?

Previous studies have suggested that human impulsivity is an adaptive response to childhood environmental harshness: individuals from families of low socioeconomic status (SES) tend to be more impulsive. However, no studies have tested the evolvability of this reaction norm. This study examined whether (a) impulsivity is associated with higher fitness among individuals from low SES families, while (b) it is associated with lower fitness among individuals from high SES families. We assessed three indices of impulsivity (temporal discounting, risk taking and fast/slow life history strategy), childhood SES and five proxy indices of fitness (number of children, lifelong singlehood, annual household income, subjective SES and life satisfaction) of 692 middle-aged participants (40-45 years old). None of the results supported the evolvability of the impulsivity reaction norm, although low childhood SES was associated with lower fitness on every proxy measure. Impulsivity (operationalised as the fast life history strategy) was associated with lower fitness regardless of childhood SES.

Weighted Bayesian Poisson Regression for The Number of Children Ever Born per Woman in Bangladesh

Number of children ever born to women of reproductive age forms a core component of fertility and is vital to the population dynamics in any country. Using Bangladesh Multiple Indicator Cluster Survey 2019 data, we fitted a novel weighted Bayesian Poisson regression model to identify multi-level individual, household, regional and societal factors of the number of children ever born among married women of reproductive age in Bangladesh. We explored the robustness of our results using multiple prior distributions, and presented the Metropolis algorithm for posterior realizations. The method is compared with regular Bayesian Poisson regression model using a Weighted Bayesian Information Criterion. Factors identified emphasize the need to revisit and strengthen the existing fertility-reduction programs and policies in Bangladesh.

Supplementary Information

The online version contains supplementary material available at 10.1007/s44199-022-00044-2.

Historical Context Changes Pathways of Parental Influence on Reproduction: An Empirical Test from 20th-Century Sweden

Several studies have found that parental absences in childhood are associated with individuals’ reproductive strategies later in life. However, these associations vary across populations and the reasons for this heterogeneity remain debated. In this paper, we examine the diversity of parental associations in three ways. First, we test whether different kinds of parental availability in childhood and adolescence are associated with women’s and men’s ages at first birth using the intergenerational and longitudinal Uppsala Birth Cohort Study (UBCoS) dataset from Sweden. This cultural context provides a strong test of the hypothesis that parents influence life history strategies given that robust social safety nets may buffer parental absences. Second, we examine whether investments in education help explain why early parental presence is associated with delayed ages at first birth in many post-industrial societies, given that parents often support educational achievement. Third, we compare parental associations with reproductive timing across two adjacent generations in Sweden. This historical contrast allows us to control for many sources of heterogeneity while examining whether changing educational access and norms across the 20th-century change the magnitude and pathways of parental influence. We find that parental absences tend to be associated with earlier first births, and more reliably so for women. Many of these associations are partially mediated by university attendance. However, we also find important differences across cohorts. For example, the associations with paternal death become similar for sons and daughters in the more recent cohort. One possible explanation for this finding is that fathers start influencing sons and daughters more similarly. Our results illustrate that historical changes within a population can quickly shift how family affects life history.

Balancing fertility and livelihood diversity in mixed economies

Mixed economies provide a unique context for testing theories of fertility change. Because they have a stake in two traditions, mixed-economy households balance the demands of both a labor-based subsistence economy, which benefits from a large family, and a wage-labor economy, which benefits from reduced fertility. Additionally, household size changes over the course of its life-cycle and shapes available economic opportunities. Here we argue that in mixed economies, fertility may reflect opportunities for livelihood diversity rather than simply responding to the restricted socioeconomic benefits of small families. While low fertility may in some cases have an economic benefit, low fertility can also limit the livelihood diversity of a household which is a key strategy for long-term economic success. We test this prediction with longitudinal data from a Maya community undergoing both a sustained decline in fertility and rapid integration into the market economy. Using household-level fertility, number of adults, and livelihood diversity at two time points, we find that household size is positively related to livelihood diversity, which in turn is positively related to household income per-capita. However, household size also has a negative association with income per capita. The results reflect a balancing act whereby households attempt to maximize the economic diversity with as few members as possible. Broadly, these results suggest that theories of fertility decline must account for how households pool resources and diversify economic activities in the face of increasing market integration, treating fertility as both an outcome and an input into economic and reproductive decision-making.

Why the Son-bias in Caregiving? Testing Sex-differences in the Associations Between Paternal Caregiving and Child Outcomes in England

Studies show that fathers across Western populations tend to provide more care to sons than daughters. Following a human behavioral ecological framework, we hypothesize that son-biases in fathering may (at least in part) be due to differences in fitness returns to paternal direct investments by child's sex. In this study, we investigate sex-differences in the associations between paternal caregiving and children's outcomes in stable, two-parent families. Using data from the Avon Longitudinal Study of Parents and Children, we test whether paternal caregiving in early childhood is associated with different effects on children's school test scores and behavioral difficulties by children's sex. Overall, we find that paternal caregiving is associated with higher school test scores and lower behavioral difficulty scores, but the association between paternal caregiving and school test scores was stronger for boys. Our findings highlight possible sex-differences in returns to paternal caregiving for certain domains of child outcomes in England.

Socioeconomic determinants of cumulative fertility in Ghana

The pace of decline in fertility rates in sub-Saharan Africa appears to have slowed or stalled in the last few decades. This study examines the socioeconomic associated with cumulative fertility in Ghana. Negative binomial regression models were used to estimate determinants of cumulative fertility using data from the Ghana Demographic and Health Surveys of 2003, 2008, and 2014. The composition of children ever born by women did not change considerably over the study periods. Socioeconomic disparities in educational attainment, household wealth, employment, and employer status are significantly associated with cumulative fertility risks in Ghana. The current age of women, age at sexual debut, and marital status, among others, are also linked to cumulative fertility levels. Place and region of residence are also linked to fertility in Ghana. Cumulative fertility levels in Ghana are underscored by considerable socioeconomic disparities among women of reproductive age. Fertility regulation policies should hinge on improving the socio-economic wellbeing of women in Ghana.

The Impact of Grandparental Death on the Fertility of Adult Children

The increasingly central role of vertical family kinship in Western societies underscores the potential value of intergenerational linkages that tie grandparents to the fertility of their adult children. Recent research has examined the changing demography of grandparenthood and the roles fulfilled by living grandparents, but the complex implications of grandparental death-a key feature of intergenerational linkages over the life course-have drawn less attention. In this article, we explore whether and how childbearing of adult women is affected by the death of grandparents-their own parent(s) or their spouse's parent(s). We develop a novel conceptual framework that presents the pathways of influence and considers the overall impact of grandparental death on childbearing of adult children. We then estimate fixed-effects models to identify causal relationships between grandparental death and childbearing, using linked micro-level census and population register data from Israel for the period 1986-2014. We find that grandparental death leads to a reduction of approximately 5 percentage points in the five-year probability of childbirth. The effects of grandparental death are negative across all parities examined and are broadly similar across grandparent's gender and kinship relation. Additional effects are identified, including how the impact of grandparental death varies with time since the previous birth as well as residential proximity prior to death. We explain how our findings regarding the effect of grandparental death offer insight into the role of living grandparents. Our results suggest that policy-makers concerned with low fertility should explore mechanisms that reinforce potential sources of support from grandparents.

Disentangling wealth effects on fertility in 64 low- and middle-income countries

Studies have shown mixed associations between wealth and fertility, a finding that has posed ongoing puzzles for evolutionary theories of human reproduction. However, measures of wealth do not simply capture economic capacity, which is expected to increase fertility. They can also serve as a proxy for market opportunities available to a household, which may reduce fertility. The multifaceted meaning of many wealth measures obscures our ability to draw inferences about the relationship between wealth and fertility. Here, we disentangle economic capacity and market opportunities using wealth measures that do not carry the same market-oriented biases as commonly used asset-based measures. Using measures of agricultural and market-based wealth for 562,324 women across 111,724 sampling clusters from 151 DHS surveys in 64 countries, we employ a latent variable structural equation model to estimate (a) latent variables capturing economic capacity and market opportunity and (b) their effects on completed fertility. Market opportunities had a consistent negative effect on fertility, while economic capacity had a weaker but generally positive effect on fertility. The results show that the confusion between operational measures of wealth and the concepts of economic capacity can impede our understanding of how material resources and market contexts shape reproduction.

When does family size matter? Sibship size, socioeconomic status and education in England

There is still no clear understanding of the relationship between sibship size and child outcomes. Research from across disciplines, and across settings, reports conflicting results suggesting that the relationship is complex and ecologically dependent. Evolutionary models predict that parents will make reproductive decisions based on their ability to invest in each child, but that this is not necessarily equal across children. Here we use data from the Next Steps study linked to National Pupil Database to examine the relationship between sibship size and Key Stage 4 (GCSE) maths and English grades in England for children born in 1989/1990. We were interested to further examine if and how associations might differ at the ends of the socioeconomic spectrum and we also tested if direct measures of parental investment could mitigate any negative impact of larger families. Multilevel ordinary least squares regression models with a random effect for school show that sibship sizes are associated with school grades, as is socioeconomic status. Moreover, the association between sibship size and grades holds true across the socioeconomic spectrum. Birth order was only weakly associated with school results, and only significant in some models. Parental investment is important, however, and might offset the some of the negative impact of larger families, for both maths and English attainment.

Do Criminals Live Faster Than Soldiers and Firefighters? : A Comparison of Biodemographic and Psychosocial Dimensions of Life History Theory

A high risk of morbidity-mortality caused by a harsh and unpredictable environment is considered to be associated with a fast life history (LH) strategy, commonly linked with criminal behavior. However, offenders are not the only group with a high exposure to extrinsic morbidity-mortality. In the present study, we investigated the LH strategies employed by two groups of Polish men: incarcerated offenders (N = 84) as well as soldiers and firefighters (N = 117), whose professions involve an elevated risk of injury and premature death. The subjects were asked to complete the Mini-K (used as a psychosocial LH indicator) and a questionnaire which included a number of biodemographic LH variables. Although biodemographic and psychosocial LH indicators should be closely linked with each other, the actual connection between them is unclear. Thus, this study was driven by two aims: comparing LH strategies in two groups of men with a high risk of premature morbidity-mortality and investigating the relationship between the biodemographic and psychosocial LH dimensions. The study showed that incarcerated men employed faster LH strategies than soldiers and firefighters, but only in relation to biodemographic variables (e.g., number of siblings, age of sexual initiation, life expectancy). No intergroup differences emerged regarding psychosocial LH indicators. Moreover, the correlation analysis showed a weak association between biodemographic and psychosocial LH indicators. The results strengthen the legitimacy of incorporating biodemographic LH traits into research models and indicate the need for further research on the accuracy of the Mini-K. The possible explanations for the intergroup differences in LH strategies are discussed.

Sex Differences in the Association of Family and Personal Income and Wealth with Fertility in the United States

Evolutionary theory predicts that social status and fertility will be positively related. It also predicts that the relationship between status and fertility will differ for men and women. This is particularly likely in modern societies given evidence that females face greater trade-offs between status and resource acquisition and fertility than males. This paper tests these hypotheses using newly released data from the 2014 wave of the Survey of Income and Program Participation by the US Census, which has the first complete measures of fertility and number of childbearing partners for a large, representative, national probability sample of men and women and also contains comprehensive measures of economic status as measured by personal and family resources, including income from all sources and all assets. Multivariate analyses show that personal income is positively associated with total fertility and number of childbearing unions for men only. For men, personal net worth is positively associated with number of childbearing unions; it is also positively associated with fertility for married men with a spouse present. These findings support evolutionary predictions of a positive relationship between status, access to mates, and reproductive success for males. Whereas personal income and personal net worth are negatively associated with total fertility and number of childbearing unions for women, family income (net of personal income) is positively associated with total fertility for women. For married men living with a spouse, family income (net of personal income) is negatively associated with total fertility. These findings are consistent with evolutionary theory given the existence of greater trade-offs between production and reproduction for women in an advanced industrial society. For women and men, family net worth (net of personal net worth) is negatively associated with number of childbearing unions and fertility. Implications are discussed.

The Severity of Post-abortion Stress in Spontaneous, Induced and Forensic Medical Center Permitted Abortion in Shiraz, Iran, in 2018

Background:

Abortion and loss of pregnancy in the first trimester may affect maternal mortality and morbidity. This study aimed to determine the severity of post-abortion stress in spontaneous abortion, induced abortion, and Forensic Medical Center (FMC) referral abortions immediately after abortion and after 1 month of follow-up in Shiraz, Iran, in 2018.

Materials and Methods:

This cross-sectional study was conducted on 104 mothers selected through convenience sampling method in 2018. The data collection tools included a demographic characteristics questionnaire and the Mississippi Post-Traumatic Stress Disorder (M-PTSD) Scale that were filled out by mothers immediately and 1 month after the abortion. Data were analyzed using one-way ANOVA and post-hoc LSD test in SPSS software. Moreover, p < 0.05 was considered as statistically significant.

Results:

The mean (SD) of post-traumatic stress scores was 83.87 (18.35) and 77.40 (9.88) in spontaneous abortion, 82.28 (13.27) and 75.71 (14.73) in FMC permitted abortions, and 86.66 (10.10) and 74.98 (12.99) in induced abortions immediately and 1 month after abortion, respectively. Stress was reduced in the three groups of mothers, after one month of severe value. The score for frequency of stress was 3.10% in FMC-permitted abortions and 5.10% in induced abortions; moreover, no stress was observed in the spontaneous abortion cases.

Conclusions:

Stress was gradually reduced over time. The level of PTSD was lower after 1 month in women who had experienced spontaneous abortion. Given that 1 month after abortion, women are still often moderately stressed, follow-up care, and appropriate counseling for these women are necessary.

Morphometric traits predict educational attainment independently of socioeconomic background

Background

Aim of this study is to describe the relationship between anthropometric traits and educational attainment among Estonian schoolchildren born between 1937 and 1962. We asked whether height, cranial volume and face width (a testosterone-dependent trait), measured in childhood predict later educational attainment independently of each other, family socioeconomic position (SEP) and sex. Associations between morphometric traits and education and their interactions with biosocial variables are of scholarly importance because higher education is nearly universally associated with low fertility in women, and often with high fertility in men. Hence, morphometric traits associated with educational attainment are targeted by natural selection and describing the exact nature of these associations is relevant for understanding the current patterns of evolution of human body size.

Methods

Data on morphometric measurements and family background of 11,032 Estonian schoolchildren measured between seven and 19 years of age were obtained from the study performed by Juhan Aul between 1956 and 1969. Ordinal logistic regression was used for testing the effects of morphometric traits, biosocial variables and their interaction on the cumulative probability of obtaining education beyond primary level.

Results

Of biosocial variables, family SEP was the most important determinant of educational attainment, followed by the sex, rural vs urban origin and the number of siblings. No significant interactions with morphometric traits were detected, i.e., within each category of SEP, rural vs urban origin and sex, taller children and those with larger heads and relatively narrower faces were more likely to proceed to secondary and/or tertiary education. The effect of height on education was independent of cranial volume, indicating that taller children did not obtain more educations because their brains were larger than those of shorter children; height per se was important.

Conclusions

Our main finding – that adjusting for other morphometric traits and biosocial variables, morphometric traits still robustly predicted educational attainment, is relevant for understanding the current patterns of evolution of human body size. Our findings suggest that fecundity selection acting on educational attainment could be partly responsible for the concurrent selection for smaller stature and cranial volume in women and opposite trends in men.

Cognitive ability and fertility among Swedish men born 1951-1967: evidence from military conscription registers

We examine the relationship between cognitive ability and childbearing patterns in contemporary Sweden using administrative register data. The topic has a long history in the social sciences and has been the topic of a large number of studies, many reporting a negative gradient between intelligence and fertility. We link fertility histories to military conscription tests with intelligence scores for all Swedish men born 1951-1967. We find a positive relationship between intelligence scores and fertility, and this pattern is consistent across the cohorts we study. The relationship is most pronounced for the transition to a first child, and men with the lowest categories of IQ scores have the fewest children. Using fixed effects models, we additionally control for all factors that are shared by siblings, and after such adjustments, we find a stronger positive relationship between IQ and fertility. Furthermore, we find a positive gradient within groups at different levels of education. Compositional differences of this kind are therefore not responsible for the positive gradient we observe-instead, the relationship is even stronger after controlling for both educational careers and parental background factors. In our models where we compare brothers to one another, we find that, relative to men with IQ 100, the group with the lowest category of cognitive ability have 0.56 fewer children, and men with the highest category have 0.09 more children.

Differences in Tsimane children's growth outcomes and associated determinants as estimated by WHO standards vs. within-population references

Anthropometric measures are commonly converted to age stratified z-scores to examine variation in growth outcomes in mixed-age and sex samples. For many study populations, z-scores will differ if calculated from World Health Organization (WHO) growth standards or within-population references. The specific growth reference used may influence statistical estimates of growth outcomes and their determinants, with implications for biological inference. We examined factors associated with growth outcomes in a sample of 152 Tsimane children aged 0-36 months. The Tsimane are a subsistence-scale population in the Bolivian Amazon with high rates of infectious disease and growth faltering. To examine the influence of growth reference on statistical inferences, we constructed multiple plausible models from available infant, maternal, and household attributes. We then ran identical models for height-for-age (HAZ), weight-for-age (WAZ), and weight-for-height (WHZ), with z-scores alternately calculated from WHO and robust Tsimane Lambda-Mu-Sigma growth curves. The distribution of WHO relative to Tsimane HAZ scores was negatively skewed, reflecting age-related increases in lower HAZ. Standardized coefficients and significance levels generally agreed across WHO and Tsimane models, although the strength and significance of specific terms varied in some models. Age was strongly, negatively associated with HAZ and WAZ in nearly all WHO, but not Tsimane models, resulting in consistently higher R2 estimates. Age and weaning effects were confounded in WHO models. Biased estimates of determinants associated with WHO HAZ may be more extreme in small samples and for variables that are strongly age-patterned. Additional methodological considerations may be warranted when applying WHO standards to within-population studies, particularly for populations with growth patterns known to systematically deviate from those of the WHO reference sample.

Worldwide fertility declines do not rely on stopping at ideal parities

A key demographic hypothesis has been that fertility declines rely on stopping at target parities, but emerging evidence suggests that women frequently reduce fertility without specific numeric targets. To assess the relative importance of these two paths to fertility decline, we develop a novel mixture model to estimate: (1) the proportion of women who stop at a target parity; and (2) mean completed fertility among those who do not. Applied to Demographic and Health Survey data from women aged 45-49 in 84 low- and middle-income countries, and to United States Census cohorts, the model shows considerable variation in the proportion stopping at specific parities (1-84 per cent). The estimates also show that declines in completed fertility are largely attributable to women who do not stop at target parities, suggesting that stopping at ideal parities may be less important than parity-independent decisions for a wide range of fertility transitions.

Nicotinamide's Ups and Downs: Consequences for Fertility, Development, Longevity and Diseases of Poverty and Affluence

To further explore the role of dietary nicotinamide in both brain development and diseases, particularly those of ageing. Articles cover neurodegenerative disease and cancer. Also discussed are the effects of nicotinamide, contained in meat and supplements and derived from symbionts, on the major transitions of disease and fertility from ancient times up to the present day. A key role for the tryptophan - NAD 'de novo' and immune tolerance pathway are discussed at length in the context of fertility and longevity and the transitions from immune paresis to Treg-mediated immune tolerance and then finally to intolerance and their associated diseases. Abstract: Nicotinamide in human evolution increased cognitive power in a positive feedback loop originally involving hunting. As the precursor to metabolic master molecule NAD it is, as vitamin B3, vital for health. Paradoxically, a lower dose on a diverse plant then cereal-based diet fuelled population booms from the Mesolithic onwards, by upping immune tolerance of the foetus. Increased tolerance of risky symbionts, whether in the gut or TB, that excrete nicotinamide co-evolved as buffers for when diet was inadequate. High biological fertility, despite disease trade-offs, avoided the extinction of Homo sapiens and heralded the dawn of a conscious, creative, and pro-fertility culture. Nicotinamide equity now would stabilise populations and prevent NAD-based diseases of poverty and affluence.

IS FERTILITY AFTER THE DEMOGRAPHIC TRANSITION MALADAPTIVE?

Fitness is always relative to the fitness of others in the group or breeding population. Even in very low-fertility societies, individual fitness as measured by the share of genes in subsequent generations may still be maximized. Further, sexual selection theory from evolutionary biology suggests that the relationship between status and fertility will differ for males and females. For this reason it is important to examine the relationship between status and fertility separately for males and females-something few demographic studies of fertility do. When male fertility is measured separately, high-status men (as measured by their wealth and personal income) have higher fertility than low-status men, even in very low-fertility societies, so individual males appear to be maximizing their fitness within the constraints posed by a modern society. Thus male fertility cannot be considered maladaptive. When female fertility is measured separately, in both very high- and very low-fertility societies, there is not much variance across women of different statuses in completed fertility. Only in societies currently changing rapidly (with falling fertility rates) is somewhat high variance across women of different statuses in completed fertility found. What is seen across all phases of the demographic transition appears to be a continuation of two somewhat different evolved human reproductive strategies-one male, one female-in changing social and material contexts. Whether contemporary female fertility is maladaptive remains an open question.

The Reproductive Ecology of Industrial Societies, Part II : The Association between Wealth and Fertility

Studies of the association between wealth and fertility in industrial populations have a rich history in the evolutionary literature, and they have been used to argue both for and against a behavioral ecological approach to explaining human variability. We consider that there are strong arguments in favor of measuring fertility (and proxies thereof) in industrial populations, not least because of the wide availability of large-scale secondary databases. Such data sources bring challenges as well as advantages, however. The purpose of this article is to illustrate these by examining the association between wealth and reproductive success in the United States, using the National Longitudinal Study of Youth 1979. We conduct a broad-based exploratory analysis of the relationship between wealth and fertility, employing both cross-sectional and longitudinal approaches, and multiple measures of both wealth (income and net worth) and fertility (lifetime reproductive success and transitions to first, second and third births). We highlight the kinds of decisions that have to be made regarding sample selection, along with the selection and construction of explanatory variables and control measures. Based on our analyses, we find a positive effect of both income and net worth on fertility for men, which is more pronounced for white men and for transitions to first and second births. Income tends to have a negative effect on fertility for women, while net worth is more likely to positively predict fertility. Different reproductive strategies among different groups within the same population highlight the complexity of the reproductive ecology of industrial societies. These results differ in a number of respects from other analyses using the same database. We suggest this reflects the impossibility of producing a definitive analysis, rather than a failure to identify the “correct” analytical strategy. Finally, we discuss how these findings inform us about (mal)adaptive decision-making.

The Reproductive Ecology of Industrial Societies, Part I : Why Measuring Fertility Matters

Is fertility relevant to evolutionary analyses conducted in modern industrial societies? This question has been the subject of a highly contentious debate, beginning in the late 1980s and continuing to this day. Researchers in both evolutionary and social sciences have argued that the measurement of fitness-related traits (e.g., fertility) offers little insight into evolutionary processes, on the grounds that modern industrial environments differ so greatly from those of our ancestral past that our behavior can no longer be expected to be adaptive. In contrast, we argue that fertility measurements in industrial society are essential for a complete evolutionary analysis: in particular, such data can provide evidence for any putative adaptive mismatch between ancestral environments and those of the present day, and they can provide insight into the selection pressures currently operating on contemporary populations. Having made this positive case, we then go on to discuss some challenges of fertility-related analyses among industrialized populations, particularly those that involve large-scale databases. These include “researcher degrees of freedom” (i.e., the choices made about which variables to analyze and how) and the different biases that may exist in such data. Despite these concerns, large datasets from multiple populations represent an excellent opportunity to test evolutionary hypotheses in great detail, enriching the evolutionary understanding of human behavior.

The offspring quantity-quality trade-off and human fertility variation

The idea that trade-offs between offspring quantity and quality shape reproductive behaviour has long been central to economic perspectives on fertility. It also has a parallel and richer theoretical foundation in evolutionary ecology. We review the application of the quantity–quality trade-off concept to human reproduction, emphasizing distinctions between clutch size and lifetime fertility, and the wider set of forces contributing to fertility variation in iteroparous and sexually reproducing species like our own. We then argue that in settings approximating human evolutionary history, several factors limit costly sibling competition. Consequently, while the optimization of quantity–quality trade-offs undoubtedly shaped the evolution of human physiology setting the upper limits of reproduction, we argue it plays a modest role in accounting for socio-ecological and individual variation in fertility. Only upon entering the demographic transition can fertility limitation be clearly interpreted as strategically orientated to advancing offspring quality via increased parental investment per child, with low fertility increasing descendant socio-economic success, although not reproductive success. We conclude that existing economic and evolutionary literature has often overemphasized the centrality of quantity–quality trade-offs to human fertility variation and advocate for the development of more holistic frameworks encompassing alternative life-history trade-offs and the evolved mechanisms guiding their resolution.

Status competition, inequality, and fertility: implications for the demographic transition

The role that social status plays in small-scale societies suggests that status may be important for understanding the evolution of human fertility decisions, and for understanding how such decisions play out in modern contexts. This paper explores whether modelling competition for status--in the sense of relative rank within a society--can help shed light on fertility decline and the demographic transition. We develop a model of how levels of inequality and status competition affect optimal investment by parents in the embodied capital (health, strength, and skills) and social status of offspring, focusing on feedbacks between individual decisions and socio-ecological conditions. We find that conditions similar to those in demographic transition societies yield increased investment in both embodied capital and social status, generating substantial decreases in fertility, particularly under conditions of high inequality and intense status competition. We suggest that a complete explanation for both fertility variation in small-scale societies and modern fertility decline will take into account the effects of status competition and inequality.

The cultural evolution of fertility decline

Cultural evolutionists have long been interested in the problem of why fertility declines as populations develop. By outlining plausible mechanistic links between individual decision-making, information flow in populations and competition between groups, models of cultural evolution offer a novel and powerful approach for integrating multiple levels of explanation of fertility transitions. However, only a modest number of models have been published. Their assumptions often differ from those in other evolutionary approaches to social behaviour, but their empirical predictions are often similar. Here I offer the first overview of cultural evolutionary research on demographic transition, critically compare it with approaches taken by other evolutionary researchers, identify gaps and overlaps, and highlight parallel debates in demography. I suggest that researchers divide their labour between three distinct phases of fertility decline--the origin, spread and maintenance of low fertility--each of which may be driven by different causal processes, at different scales, requiring different theoretical and empirical tools. A comparative, multi-level and mechanistic framework is essential for elucidating both the evolved aspects of our psychology that govern reproductive decision-making, and the social, ecological and cultural contingencies that precipitate and sustain fertility decline.

Understanding variation in human fertility: what can we learn from evolutionary demography?

Decades of research on human fertility has presented a clear picture of how fertility varies, including its dramatic decline over the last two centuries in most parts of the world. Why fertility varies, both between and within populations, is not nearly so well understood. Fertility is a complex phenomenon, partly physiologically and partly behaviourally determined, thus an interdisciplinary approach is required to understand it. Evolutionary demographers have focused on human fertility since the 1980s. The first wave of evolutionary demographic research made major theoretical and empirical advances, investigating variation in fertility primarily in terms of fitness maximization. Research focused particularly on variation within high-fertility populations and small-scale subsistence societies and also yielded a number of hypotheses for why fitness maximization seems to break down as fertility declines during the demographic transition. A second wave of evolutionary demography research on fertility is now underway, paying much more attention to the cultural and psychological mechanisms underpinning fertility. It is also engaging with the complex, multi-causal nature of fertility variation, and with understanding fertility in complex modern and transitioning societies. Here, we summarize the history of evolutionary demographic work on human fertility, describe the current state of the field, and suggest future directions.