Postnatal testosterone levels and temperament in early infancy.

Recent research showing associations between behavior and postnatal testosterone levels in male infants has suggested that the transient activation of the hypothalamic-pituitary-gonadal axis in early infancy may influence the expression of gender phenotypes in later development (i.e., the postnatal hormone hypothesis). As a further test of the relationship between postnatal hormones and behavior in infancy, we measured digit ratios and salivary testosterone in 76 male and female infants (3-4 months of age) and parents completed the Infant Behavior Questionnaire-Revised, a well-established measure of temperament in the first year of life. Consistent with our earlier findings, there were no significant sex differences in salivary testosterone levels and testosterone levels were unrelated to measures of behavior in female infants. However, in male infants, higher androgen levels predicted greater Negative Affectivity. Further examination of the four scales contributing to the measure of Negative Affectivity showed testosterone levels were a significant predictor of scores on the Distress to Limitations scale, but not of scores on Fear, Sadness, or Reactivity scales. This sex-specific association between salivary testosterone and behavior in infants is consistent with animal research showing higher prenatal androgens associated with typical male development lower the threshold of sensitivity to endogenous testosterone in postnatal life. In sum, these data provide additional support for the postnatal hormone hypothesis and suggest postnatal testosterone levels may influence the development of emotional regulation in male infants.