During early development, testosterone plays an important role in sexual differentiation of the mammalian brain and has enduring influences on behavior. Testosterone exerts these influences at times when the testes are active, as evidenced by higher concentrations of testosterone in developing male than in developing female animals. This article critically reviews the available evidence regarding influences of testosterone on human gender-related development. In humans, testosterone is elevated in males from about weeks 8 to 24 of gestation and then again during early postnatal development. Individuals exposed to atypical concentrations of testosterone or other androgenic hormones prenatally, for example, because of genetic conditions or because their mothers were prescribed hormones during pregnancy, have been consistently found to show increased male-typical juvenile play behavior, alterations in sexual orientation and gender identity (the sense of self as male or female), and increased tendencies to engage in physically aggressive behavior. Studies of other behavioral outcomes following dramatic androgen abnormality prenatally are either too small in their numbers or too inconsistent in their results, to provide similarly conclusive evidence. Studies relating normal variability in testosterone prenatally to subsequent gender-related behavior have produced largely inconsistent results or have yet to be independently replicated. For studies of prenatal exposures in typically developing individuals, testosterone has been measured in single samples of maternal blood or amniotic fluid. These techniques may not be sufficiently powerful to consistently detect influences of testosterone on behavior, particularly in the relatively small samples that have generally been studied. The postnatal surge in testosterone in male infants, sometimes called mini-puberty, may provide a more accessible opportunity for measuring early androgen exposure during typical development. This approach has recently begun to be used, with some promising results relating testosterone during the first few months of postnatal life to later gender-typical play behavior. In replicating and extending these findings, it may be important to assess testosterone when it is maximal (months 1 to 2 postnatal) and to take advantage of the increased reliability afforded by repeated sampling.

The ratio of length between the second and fourth fingers (2D:4D) is commonly used as an indicator of prenatal sex hormone exposure. Several approaches have been used to try to validate the measure, including examining 2D:4D in people with congenital adrenal hyperplasia (CAH), a suite of conditions characterised by elevated adrenal androgen production secondary to defective steroidogenesis. We present here a systematic review that examines the relationship between these two variables. Twelve articles relating to nine CAH cohorts were identified, and 2D:4D comparisons have been made between cases and controls in eight of these cohorts. Altogether, at least one 2D:4D variable has been compared between n=251 females with CAH and n=358 unaffected females, and between n=108 males with CAH and n=204 unaffected males. A previous meta-analysis (Hönekopp & Watson, 2010) reported lower right hand (R2D:4D) and left hand (L2D:4D) digit ratios in patients with CAH relative to sexmatched controls. Our meta-analysis showed the same direction of results; however, the effects were only statistically significant for R2D:4D in males and L2D:4D in females (R2D:4D: females, p = 0.072, g = 0.591; males, p = 0.019, g = 0.513; L2D:4D: females, p = 0.020, g = 0.245; males, p = 0.334, g = 0.218), and the average effect size had reduced by 46.70%. We also found no evidence to suggest the right-left difference in 2D:4D (D[R-L]) is associated with prenatal sex hormone exposure.

Testosterone levels during early development influence subsequent sex‐typical behavior. These influences were initially identified in experimental research on nonhuman species. Additional research—primarily investigating individuals exposed to atypical hormone environments due to genetic disorders or maternal treatment with hormones during pregnancy—suggested that testosterone also influences the development of sex‐typical behavior in humans. There is also interest in identifying relations between normal variability in the early hormone environment and normal variability in subsequent behavior. This article reviews studies that have assessed prenatal testosterone exposure in typically developing children using amniotic fluid sampling or maternal blood sampling. It concludes that both these approaches are promising, but both require larger samples than those used in most studies to date. Recommendations for future research also include using outcome measures that show sex differences, analyzing data within each sex, considering the time of day (as well as the time of gestation) when samples were taken, and reporting all the measures evaluated, not just those showing significant effects.

Some cognitive abilities exhibit reliable gender differences, with females outperforming males in specific aspects of verbal ability, and males showing an advantage on certain spatial tasks. Among these cognitive gender differences, differences in mental rotation are the most robust, and appear to be present even in infants. A large body of animal research suggests that gonadal hormones, particularly testosterone, during early development could contribute to this gender difference in mental rotation. Also, substantial evidence supports an influence of socialization on mental rotation performance. The present study investigated the relationship of two types of factors, early postnatal testosterone exposure and parental attitudes about gender, to mental rotation performance in 61 healthy infants (29 males, 32 females). We measured salivary testosterone at two time points: 1-2.5 months of age and 5-6 months of age. Infants' mental rotation performance and parents' attitudes about gender were assessed at 5-6 months of age. As predicted, testosterone concentrations were significantly higher in boys than girls in early infancy (d = 0.54), and boys performed significantly better than girls on mental rotation (d = 0.64). A significant positive correlation between testosterone at age 1-2.5 months and mental rotation was found only in boys (r = 0.50, p = .01). A significant negative correlation between parents' gender-stereotypical attitudes and mental rotation performance was found only in girls (r = -.57, p = .002). These findings suggest that the early postnatal testosterone surge (also known as "mini-puberty") may have organizational influences on mental rotation performance in boys, and that parents may influence their daughters' mental rotation abilities beginning very early in life.