Sleep in infancy predicts gender specific social-emotional problems in toddlers.

Despite strong evidence linking sleep to developmental outcomes, the longitudinal relationship between sleep and emotional well-being remains largely unknown. To address this gap in our knowledge, the current study examined sleep in infancy, measured via actigraphy, as a predictor of social-emotional problems in toddlers. A total of 47 children (29 males) were included in this longitudinal study. At time one, actigraphy measures of sleep were obtained from 3- to 4-month-old infants. At time two, parents rated their 18- to 24-month-old toddler's social-emotional well-being using the Brief Infant Toddler Social Emotional Assessment. Results indicated that boys tended to have higher levels of externalizing behaviors than did girls. Additionally, boys with longer sleep durations also showed lower sleep efficiency. In girls, sleep duration in infancy was a significant predictor of autism spectrum disorder behaviors and approached significance as a predictor of externalizing problems in toddlerhood. Our findings are the first to show a relationship between sleep measured in infancy and autism spectrum disorder symptomatology measured in early childhood. They suggest that the etiology of social-emotional problems may differ between genders and raise the possibility that sleep/wake cycles may be differentially related to autism spectrum disorder symptoms in girls and boys.

Postnatal testosterone levels and disorder relevant behavior in the second year of life.

The objective of the current study was to investigate the relationship between testosterone collected at 3-4 months of age and sex-linked disorder-relevant behaviors in the second year of life. Eighty-four children participated at 3-4 (when salivary testosterone levels were obtained and second to fourth digit ratios were measured) and 18-24 months of age (when behavioral ratings of aggression and verbal ability were coded from two 8-min play sessions). Parents also completed the Brief Infant-Toddler Social and Emotional Assessment, and the four subscales (Internalizing, Externalizing, Dysregulation, and Autism Spectrum Disorder) were used to indicate child specific problems. Greater postnatal testosterone levels in early infancy were predictive of more male-typical behaviors in the second year of life (i.e., more autism spectrum behaviors, less time vocalizing, and more Internalizing Problems). These results support the hypothesis that early infancy may be another critical period for the development of gender-linked behavior.

Digit ratios (2D:4D), postnatal testosterone and eye contact in toddlers.

Previous research has shown an association between eye contact and prenatal testosterone measured in amniocenteses samples. The purpose of this study was to test the association between eye contact and prenatal androgen action measured via second to fourth digit ratios (2D:4D ratios), and to explore the relationship between eye contact and postnatal testosterone levels. Participants included 72 children, between the ages of 18 and 24 months, and their parents. Salivary testosterone levels were obtained when children were 3-months old. At 18-months, 2D:4D ratios were measured and parent-child dyads participated in an 8-min play session that was recorded and later coded for duration and frequency of eye contact. Results indicated that larger 2D:4D ratios (indicative of lower androgen levels) significantly predicted longer duration and more frequency of eye contact, while postnatal testosterone levels were unrelated to eye contact. These novel findings suggest prenatal androgens may influence the emergence of social development.

Early androgens, activity levels and toy choices of children in the second year of life.

The hypothesis that stronger preferences for active play styles contribute to stronger preferences for male-typical toys was examined in 47 boys and 37 girls at 19-months of age using ambulatory monitoring technology (i.e., actigraphy) to measure activity levels during contact with male-typical, female-typical, and gender-neutral toys. Digit ratios and salivary testosterone levels were measured earlier in children at 3-4 months of age. There were no significant sex differences in digit ratios, salivary testosterone levels, or overall activity levels during toy play. In contrast, contact times showed large sex differences in infants' toy preferences. The within-sex comparisons showed that infant girls had significant preferences for female-typical toys over male-typical toys, whereas infant boys showed only a small preference for male-typical toys over female-typical toys. More male-typical digit ratios in early infancy predicted higher activity counts during toy play and less female-typical toy preferences in girls. However, in both sexes, activity levels were unrelated to toy preferences suggesting that factors other than activity level preferences contribute to the early emergence of gender-linked toy preferences.

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.