Babies’ levels of testosterone predict sex-related differences in childhood expressive vocabulary

Testosterone measured a few months after birth predicts the sex-related differences that have been observed in early expressive vocabulary development, according to a study published this June in Psychoneuroendocrinology.

On average girls speak more words than boys during the first few years of life, which naturally results in a larger expressive vocabulary. Furthermore, a small vocabulary may predict subsequent language difficulties, which are also more prevalent in boys than girls.

Recent research has suggested that early male sex hormone (androgen) exposure may contribute to the sex difference in early expressive vocabulary development, in particular the testicular hormone, testosterone.

This is especially relevant in the early postnatal period called “Mini-puberty”, where a surge of testosterone in male infants occurs. This peaks at about 1–3 months of age and then declines to baseline by about 6 months of age. During these periods, the adrenal glands produce some androgens in both sexes, but in males the gonads produce larger amounts of testosterone.

The study, led by Karson Kung of the University of Cambridge, tested whether early postnatal testosterone concentrations influence sex differences in expressive vocabulary in childhood. Parents of 118 healthy infants were recruited, and saliva samples were taken from their infants for testosterone assays when they were 1–3 months old. Then again, when the children were 18–30 months old, the parents completed an online questionnaire assessing the children’s expressive vocabulary size.

The results showed that, as expected, there were differences between boys and girls in salivary testosterone at 1–3 months of age and in expressive vocabulary size at 18–30 months of age. Importantly, a negative relationship between testosterone during mini-puberty and expressive vocabulary was found for all children. Furthermore, mediation analysis suggested that the sex difference in expressive vocabulary can be partly attributed to the sex difference in testosterone during mini-puberty.

The researchers concluded, “These results suggest that testosterone during the early postnatal period contributes to early language development and neurobehavioral sexual differentiation in humans.” They also added, “Saliva sampling at 1–3 months postnatal may provide a useful approach to studying the influences of early androgen exposure. Further research could relate salivary testosterone during mini-puberty to other cognitive and behavioral outcomes that show sex differences.”