No association between adult sex steroids and hand preference in humans

BACKGROUND

There is ongoing debate about the effects of hormones on the lateralization of the developing brain. In humans, there are conflicting theories of how testosterone during development should affect lateralization. Empirical studies linking prenatal and postnatal testosterone levels to hand preference (a proxy for lateralization) are similarly mixed. Links between hand preference and health may also suggest a mediating role of steroid hormones such as testosterone and estradiol. Studies to date of adult steroid hormones and handedness have been hindered by samples that contain small numbers of non-right-handers.

RESULTS

In the largest study of the phenomenon to date, I find that the testosterone (n = 7290) and estradiol (n = 3700) levels of left- and mixed-handed adults are no different to those of right-handers. All Bayesian 95% highest density intervals contained 0.

DISCUSSION AND CONCLUSIONS

The results have implications for studies that show elevated risk of hormonal-related mental and physical disorders in left-handed individuals.

Brain Lateralization: A Comparative Perspective

Comparative studies on brain asymmetry date back to the 19th century but then largely disappeared due to the assumption that lateralization is uniquely human. Since the reemergence of this field in the 1970s, we learned that left-right differences of brain and behavior exist throughout the animal kingdom and pay off in terms of sensory, cognitive, and motor efficiency. Ontogenetically, lateralization starts in many species with asymmetrical expression patterns of genes within the Nodal cascade that set up the scene for later complex interactions of genetic, environmental, and epigenetic factors. These take effect during different time points of ontogeny and create asymmetries of neural networks in diverse species. As a result, depending on task demands, left- or right-hemispheric loops of feedforward or feedback projections are then activated and can temporarily dominate a neural process. In addition, asymmetries of commissural transfer can shape lateralized processes in each hemisphere. It is still unclear if interhemispheric interactions depend on an inhibition/excitation dichotomy or instead adjust the contralateral temporal neural structure to delay the other hemisphere or synchronize with it during joint action. As outlined in our review, novel animal models and approaches could be established in the last decades, and they already produced a substantial increase of knowledge. Since there is practically no realm of human perception, cognition, emotion, or action that is not affected by our lateralized neural organization, insights from these comparative studies are crucial to understand the functions and pathologies of our asymmetric brain.

Prenatal and pubertal testosterone affect brain lateralization

After decades of research, the influence of prenatal testosterone on brain lateralization is still elusive, whereas the influence of pubertal testosterone on functional brain lateralization has not been investigated, although there is increasing evidence that testosterone affects the brain in puberty. We performed a longitudinal study, investigating the relationship between prenatal testosterone concentrations in amniotic fluid, pubertal testosterone concentrations in saliva, and brain lateralization (measured with functional Transcranial Doppler ultrasonography (fTCD)) of the Mental Rotation, Chimeric Faces and Word Generation tasks. Thirty boys and 30 girls participated in this study at the age of 15 years. For boys, we found a significant interaction effect between prenatal and pubertal testosterone on lateralization of Mental Rotation and Chimeric Faces. In the boys with low prenatal testosterone levels, pubertal testosterone was positively related to the strength of lateralization in the right hemisphere, while in the boys with high prenatal testosterone levels, pubertal testosterone was negatively related to the strength of lateralization. For Word Generation, pubertal testosterone was negatively related to the strength of lateralization in the left hemisphere in boys. For girls, we did not find any significant effects, possibly because their pubertal testosterone levels were in many cases below quantification limit. To conclude, prenatal and pubertal testosterone affect lateralization in a task-specific way. Our findings cannot be explained by simple models of prenatal testosterone affecting brain lateralization in a similar way for all tasks. We discuss alternative models involving age dependent effects of testosterone, with a role for androgen receptor distribution and efficiency.

Cerebral laterality for language is related to adult salivary testosterone levels but not digit ratio (2D:4D) in men: A functional transcranial Doppler ultrasound study

The adequacy of three competing theories of hormonal effects on cerebral laterality are compared using functional transcranial Doppler sonography (fTCD). Thirty-three adult males participated in the study (21 left-handers). Cerebral lateralization was measured by fTCD using an extensively validated word generation task. Adult salivary testosterone (T) and cortisol (C) concentrations were measured by luminescence immunoassay and prenatal T exposure was indirectly estimated by the somatic marker of 2nd to 4th digit length ratio (2D:4D). A significant quadratic relationship between degree of cerebral laterality for language and adult T concentrations was observed, with enhanced T levels for strong left hemisphere dominance and strong right hemisphere dominance. No systematic effects on laterality were found for cortisol or 2D:4D. Findings suggest that higher levels of T are associated with a relatively attenuated degree of interhemispheric sharing of linguistic information, providing support for the callosal and the sexual differentiation hypotheses rather than the Geschwind, Behan and Galaburda (GBG) hypothesis.

Quantifying cerebral asymmetries for language in dextrals and adextrals with random-effects meta analysis

Speech and language-related functions tend to depend on the left hemisphere more than the right in most right-handed (dextral) participants. This relationship is less clear in non-right handed (adextral) people, resulting in surprisingly polarized opinion on whether or not they are as lateralized as right handers. The present analysis investigates this issue by largely ignoring methodological differences between the different neuroscientific approaches to language lateralization, as well as discrepancies in how dextral and adextral participants were recruited or defined. Here we evaluate the tendency for dextrals to be more left hemisphere dominant than adextrals, using random effects meta analyses. In spite of several limitations, including sample size (in the adextrals in particular), missing details on proportions of groups who show directional effects in many experiments, and so on, the different paradigms all point to proportionally increased left hemispheric dominance in the dextrals. These results are analyzed in light of the theoretical importance of these subtle differences for understanding the cognitive neuroscience of language, as well as the unusual asymmetry in most adextrals.

Measurement of testosterone in human sexuality research: methodological considerations

Testosterone (T) and other androgens are incorporated into an increasingly wide array of human sexuality research, but there are a number of issues that can affect or confound research outcomes. This review addresses various methodological issues relevant to research design in human studies with T; unaddressed, these issues may introduce unwanted noise, error, or conceptual barriers to interpreting results. Topics covered are (1) social and demographic factors (gender and sex; sexual orientations and sexual diversity; social/familial connections and processes; social location variables), (2) biological rhythms (diurnal variation; seasonality; menstrual cycles; aging and menopause), (3) sample collection, handling, and storage (saliva vs. blood; sialogogues, saliva, and tubes; sampling frequency, timing, and context; shipping samples), (4) health, medical issues, and the body (hormonal contraceptives; medications and nicotine; health conditions and stress; body composition, weight, and exercise), and (5) incorporating multiple hormones. Detailing a comprehensive set of important issues and relevant empirical evidence, this review provides a starting point for best practices in human sexuality research with T and other androgens that may be especially useful for those new to hormone research.

Hand preference in humans is associated with testosterone levels and androgen receptor gene polymorphism

Exposure of the central nervous system to androgens during the early developmental period has been proposed to play a role in the establishment of hand preference in males. Existing data, however, are inconclusive. In the present investigation, handedness was assessed in a large sample of left-, mixed-, and right-handed men (N=180) using a standardized handedness inventory. Saliva sampling was used to assay levels of bioavailable testosterone and DNA genotyping was carried out to quantify AR-CAG repeat length, a genetic marker of the capacity of the androgen receptor to respond to testosterone. Strongly left-handed males were found to have lower levels of bioavailable testosterone than right-handed males, while males with mixed handedness exhibited a weaker androgen receptor, but no significant difference from right-handers in circulating testosterone levels. These findings support the view that testosterone could play a role in the development of hand preference in males. Furthermore, because the AR gene lies on the X chromosome, it provides a potential theoretical bridge to genetic theories of handedness that postulate the existence of an X-linked locus important in the establishment of hand preference.

Sex hormone activity in alcohol addiction: integrating organizational and activational effects

There are well-known sex differences in the epidemiology and etiopathology of alcohol dependence. Male gender is a crucial risk factor for the onset of alcohol addiction. A directly modifying role of testosterone in alcohol addiction-related behavior is well established. Sex hormones exert both permanent (organizational) and transient (activational) effects on the human brain. The sensitive period for these effects lasts throughout life. In this article, we present a novel early sex hormone activity model of alcohol addiction. We propose that early exposure to sex hormones triggers structural (organizational) neuroadaptations. These neuroadaptations affect cellular and behavioral responses to adult sex hormones, sensitize the brain's reward system to the reinforcing properties of alcohol and modulate alcohol addictive behavior later in life. This review outlines clinical findings related to the early sex hormone activity model of alcohol addiction (handedness, the second-to-fourth-finger length ratio, and the androgen receptor and aromatase) and includes clinical and preclinical literature regarding the activational effects of sex hormones in alcohol drinking behavior. Furthermore, we discuss the role of the hypothalamic-pituitary-adrenal and -gonadal axes and the opioid system in mediating the relationship between sex hormone activity and alcohol dependence. We conclude that a combination of exposure to sex hormones in utero and during early development contributes to the risk of alcohol addiction later in life. The early sex hormone activity model of alcohol addiction may prove to be a valuable tool in the development of preventive and therapeutic strategies.

Seasonal variation of salivary testosterone in men, normally cycling women, and women using hormonal contraceptives

Humans' endogenous testosterone concentrations vary over a number of temporal scales, with little known about variation longer than monthly cycles. Past studies of seasonal or circannual variation have principally used male participants and have produced inconsistent results. Thus, little is known about how testosterone concentrations fluctuate throughout the year, whether such variation differs between men and women, and whether there are influences of hormonal contraceptive use. The present study collected saliva samples from a large sample (N=718) of men and women, each collected at one time point within a relatively uniform distribution over a full calendar year. Both men and normally-cycling women displayed seasonal variation in salivary testosterone concentrations, such that testosterone concentrations are maximal in the fall and minimal in the summer. Notably, normally-cycling women had testosterone concentrations that were over 100% greater at their maximum in fall compared to their minimum in summer. Women using hormonal contraceptives not only had consistently lower endogenous testosterone concentrations, but also showed a flatter seasonal testosterone profile. The implications for studies of psychology and human behavioral endocrinology are discussed.

Left-Handedness and Male-Male Competition: Insights from Fighting and Hormonal Data

Male-male competition can shape some behavioral or morphological traits of males. Here we investigate if this competition could play a role in the persistence of the polymorphism of handedness in human populations. A negative frequency-dependent selection mechanism has been hypothesized, based on the fact that left-handed men may benefit from a “surprise” advantage during fighting interactions because they are rare in human populations. This advantage may thereby enhance the probability of survival of left-handed men and/or their reproductive success through an increase in social status. In this study, we first explored the association between hand preference and lifetime fighting behavior in a population of 1,161 French men. No effect of hand preference on the probability of fighting was detected, suggesting that the innate propensity to fight does not differ between left- and right-handers. However, among men who had been involved in at least one fight during their lifetime, left-handers reported significantly more fights than right-handers. To explore the biological basis of this behavior, we also investigated the testosterone concentration in saliva samples from 64 French university students. Consistent with frequencies of fights, we found a significantly higher average testosterone concentration in left-handers than in right-handers. We suggest that these behavioral and hormonal differences may be acquired throughout life due to previous experiences in a social context and may favor the persistence of left-handers in humans.

Decreased prevalence of left-handedness among females with male co-twins: evidence suggesting prenatal testosterone transfer in humans?

Studies of singletons suggest that right-handed individuals may have higher levels of testosterone than do left-handed individuals. Prenatal testosterone levels are hypothesised to be especially related to handedness formation. In humans, female members from opposite-sex twin pairs may experience elevated level of prenatal exposure to testosterone in their intrauterine environment shared with a male. We tested for differences in rates of left-handedness/right-handedness in female twins from same-sex and opposite-sex twin pairs. Our sample consisted of 4736 subjects, about 70% of all Finnish twins born in 1983-1987, with information on measured pregnancy and birth related factors. Circulating testosterone and estradiol levels at age 14 were available on 771 and 744 of these twins, respectively. We found significantly (p=.006) lower prevalence of left-handedness in females from opposite-sex pairs (5.3%) compared to females from same-sex pairs (8.6%). The circulating levels of neither testosterone nor estradiol related to handedness in either females or males. Nor were there differences in circulating testosterone or estradiol levels between females from opposite-sex and same-sex twin pairs. Birth and pregnancy related factors for which we had information were unrelated to handedness. Our results are difficult to fully explain by postnatal factors, but they offer support to theory that relates testosterone to formation of handedness, and in a population-based sample, are suggestive of effects of prenatal testosterone transfer.

Evidence for hemispheric specialization in the marmoset (Callithrix penicillata) based on lateralization of behavioral/neurochemical correlations

A correlative study between behavioral, neurochemical and hormonal measures was conducted on male black tufted-ear marmoset monkeys (Callithrix penicillata). Behavioral analysis was performed in order to examine the effects of confrontation with a natural predator (taxidermized oncilla cat, Felis tigrina). The subjects were subjected to four trials without predator, six confrontation trials with predator present, and four trials with the predator removed. Handedness was analyzed by the frequency with which they performed scratching, grooming and hanging behaviors with the left or right hands. The animals' brains were subjected to ex vivo neurochemical analysis of several structures from both hemispheres. The content of monoamines, acetycholine and metabolites were analyzed by HPLC-ED. Plasma levels of cortisol and adrenocorticotrophic hormone (ACTH) were analyzed by chemoluminescence immunoassay. Testosterone plasma concentration was determined by radioimmunoassay. Higher levels of dopamine and acetylcholine were detected in the right caudate/putamen, in comparison to the left. For the remaining areas, similar levels were observed in both hemispheres. A hand preference between and within the behaviors scored was not detected. However, correlative analyses revealed complex interactions between the behavioral and neurochemical measures, particularly in the left hemisphere. Lateralized correlations were found in relation to brain site, type of behavior, neurochemical parameter and treatment condition, thus providing evidence for functional brain asymmetries in this species. Interhemispheric comparisons of neurochemical/behavioral correlations appear to be a promising approach towards delineating hemispheric specialization of functions in this, and perhaps, other species.

Actigraphic investigations on the activity-rest behavior of right- and left-handed students

The aim of this study was to explore differences between left-and right-handed subjects in sleep duration. Sleep and activity patterns were continuously registered for 12 days using actometers on 20 left-handed and 20 right-handed medical students in Berlin. Handedness was determined by a modified version of the Edinburgh handedness inventory. Each participant wore one actometer on each wrist. Actiwatch Sleep Analysis Software (CNT, UK) was used to evaluate the data, and statistical calculations were performed with a non-parametric variance analysis. A significant difference in mean sleep duration between left-handers (7.9 h) and right-handers (7.3 h) was determined (p=0.025 for measurement made on the dominant hand and p=0.013 for ones made on the non-dominant hand). In contrast, the maximal phase of daily activity (acrophase) did not show any difference between the two groups. The difference in sleep duration might be caused by either the greater effort required for left-handers to cope in a right-handed world or by structural brain differences.

Seasonality, waist-to-hip ratio, and salivary testosterone

Patterns of seasonal variation in testosterone (T) and T-dependent measures are poorly understood in humans and particularly in women, despite their importance in other animals. We examined seasonal fluctuations in salivary T in women and men, and waist-to-hip ratio (WHR) in women. Participants were 220 women and 127 men from central and West Coast North America. Results showed that T was significantly highest in autumn for both women and men, and that WHR in women closely matched the seasonal variation in T, with high values in the fall and summer. This suggests that T does show a reliable fluctuation over the seasons, which may result in meaningful fluctuations in behavioral, cognitive, and somatic variables associated with T.

Effects of implicit power motivation on men's and women's implicit learning and testosterone changes after social victory or defeat

Two studies examined interactions of implicit power motivation and experimentally varied victory or defeat in a contest on implicit learning of a visuomotor sequence associated with the contest outcome and changes in testosterone and self-reported affect. In men and women, power motivation predicted enhanced learning (sequence-execution accuracy) after a victory and impaired learning after a defeat. In men, power motivation predicted testosterone increases among winners and decreases among losers, and testosterone decreases mediated the negative effect of power motivation on learning in losers. In women, power motivation predicted postcontest testosterone increases, particularly among losers. In both men and women, self-reported affective states were influenced only by contest outcome and were unrelated to participants' testosterone changes or implicit learning.

Prenatal sex hormone effects on child and adult sex-typed behavior: methods and findings

There is now good evidence that human sex-typed behavior is influenced by sex hormones that are present during prenatal development, confirming studies in other mammalian species. Most of the evidence comes from clinical populations, in which prenatal hormone exposure is atypical for a person's sex, but there is increasing evidence from the normal population for the importance of prenatal hormones. In this paper, we briefly review the evidence, focusing attention on the methods used to study behavioral effects of prenatal hormones. We discuss the promises and pitfalls of various types of studies, including those using clinical populations (concentrating on those most commonly studied, congenital adrenal hyperplasia, androgen insensitivity syndrome, ablatio penis, and cloacal exstrophy), direct measures of hormones in the general population (assayed through umbilical cord blood, amniotic fluid, and maternal serum during pregnancy), and indirect measures of hormones in the general population (inferred from intrauterine position and biomarkers such as otoacoustic emissions, finger length ratios, and dermatoglyphic asymmetries). We conclude with suggestions for interpreting and conducting studies of the behavioral effects of prenatal hormones.

Waist-to-hip ratio is positively associated with bioavailable testosterone but negatively associated with sexual desire in healthy premenopausal women

OBJECTIVE

We examined whether high waist-to-hip ratio (WHR) in women is a biomarker for increased testosterone (T) and higher levels of sexual desire.

METHODS

Participants were 99 healthy nonobese premenopausal women. Trait levels of mean bioavailable T were estimated from three saliva samples collected at 8:00 am, 9:30 am, and 6:00 pm to 9:00 pm, controlling for phase of the menstrual cycle. Sexual desire was measured with the Sexual Desire Inventory.

RESULTS

WHR was positively, although moderately, correlated with bioavailable T. WHR, but not T, was negatively correlated with level of sexual desire.

CONCLUSIONS

These results confirm that the positive association between WHR and androgens seen in clinical populations also exists in the general population of healthy adult women. However, the relationship may not be strong enough in healthy women for WHR to serve as a useful biomarker of androgen levels in sexuality studies, especially given its association with other, perhaps negatively valued, morphologic attributes.

Prenatal exposure to testosterone and functional cerebral lateralization: a study in same-sex and opposite-sex twin girls

In animals it has been shown that exposure to sex hormones is influenced by intrauterine position. Thus fetuses located between two male fetuses are exposed to higher levels of testosterone (T) than fetuses situated between two female fetuses or one female and one male fetus. In a group of opposite-sex (OS) twin girls and same-sex (SS) twin girls a potential effect of prenatal exposure to testosterone (T) on functional cerebral lateralization was investigated. We hypothesized that prenatal exposure to T would result in a more masculine, i.e. a more lateralized pattern of cerebral lateralization in OS twin girls than in SS twin girls. An auditory-verbal dichotic listening task (DLT) was used as an indirect method to study hemispheric specialization. Firstly, we established a sex difference on the DLT. Compared with SS girls, OS twin boys showed a more lateralized pattern of processing verbal stimuli. Secondly, as predicted OS girls had a more masculine pattern of cerebral lateralization, than SS girls. These findings support the notion of an influence of prenatal T on early brain organization in girls.

Salivary testosterone is related to both handedness and degree of linguistic lateralization in normal women

The aim of this study was to test the hypothesis that individual differences in testosterone (T) are associated with different patterns of linguistic lateralization and hand preference. Twenty left-handed (LH) and 19 right-handed (RH) women filled in a handedness questionnaire and performed a consonant-vowel dichotic listening test (DL-CV). Salivary T was measured by radioimmunoassay (RIA). LH women showed significantly lower mean salivary T than RH women. T levels were negatively correlated with the absolute value of the DL laterality index. Subjects with right ear advantage (REA) were classified into strongly and weakly lateralized following Wexler et al. method (Brain Lang. 13 (1981) 13). When taking into account hand preference, a pattern emerged in that RH-strongly lateralized and LH-weakly lateralized women showed similar T levels. The lowest level appeared for LH-strongly lateralized women and the highest for RH-weakly lateralized women, being significantly different from each other. The results are discussed in terms of several theories that have proposed a link between testosterone and cerebral lateralization.

Replication of a premenstrual decrease in right-ear advantage on language-related dichotic listening tests of cerebral laterality

Five fused dichotic word tasks measured perceptual asymmetry in 30 women at 4 weekly intervals. The five tasks varied according to whether the stimuli presented were word-word pairs or nonsense word-pairs, or whether they consisted of neutral words paired with positive emotion-evoking words (e.g. hug-tug), neutral words paired with negative emotion-evoking words (e.g. till-kill), or neutral words paired with neutral words (e.g. bean-dean). Overall right-ear advantage (REA) decreased in the premenstrual week relative to the postmenstrual week, replicating previous results using identical measures. In addition, REA scores were similar at menstrual, postmenstrual and midcycle weeks. Additional data from 12 men suggests sex differences in task performance were small or non-existent. In both women and men, there were no effects of repeated testing on REA, but emotional proclivity indices, defined as the tendency to recall words of positive or negative affective tone, increased across the four test sessions. As found previously, there was a trend for women to hear fewer positive words during the premenstrual week. These data are consistent with other research suggesting that a progesterone-mediated decrease in functional asymmetry occurs in the luteal phase. Future research manipulating task demands (e.g. memory load) or the affective valence of the stimuli may be useful in understanding the observed changes in hemispheric advantage.

Sex steroids and human behavior: implications for developmental psychopathology

In a variety of mammalian species, prenatal androgens organize brain structures and functions that are later activated by steroid hormones in postnatal life. In humans, studies of individuals with typical and atypical development suggest that sex differences in reproductive and nonreproductive behavior derive in part from similar prenatal and postnatal steroid effects on brain development. This paper provides a summary of research investigating hormonal influences on human behavior and describes how sex differences in the prevalences and natural histories of developmental psychopathologies may be consistent with these steroid effects. An association between patterns of sexual differentiation and specific forms of psychopathology suggests novel avenues for assessing the effects of sex steroids on brain structure and function, which may in turn improve our understanding of typical and atypical development in women and men.