Digit ratio (2D:4D) and neonatal head circumference: Evidence for oestrogenization in human brain evolution?

The second-to-fourth digit ratio (2D:4D) is widely thought to be a correlate of first trimester sex steroids, such that it is negatively correlated to testosterone and positively correlated to oestrogen. The 2D:4D has been linked to many physical and behavioural traits, including intelligence. However, little attention has been paid to its association with neonatal characteristics. Here we focus on associations between neonatal 2D:4D and head circumference as the latter variable is correlated with intelligence.

Our sample was 225 full-term (37-41 weeks) newborns comprising 125 girls and 100 boys. Recumbent length, weight, head circumference and right and left second and fourth digit lengths were measured. In addition to biological measures, maternal education was recorded as a proxy for sociodemographic status. Hierarchical multiple linear regression analyses were conducted separately for each sex to assess associations.

For males, recumbent length and right 2D:4D were significantly positively associated with head circumference. Mother's education level was not related to head circumference. For females, recumbent length and birth weight were significant positive predictors of head circumference, digit ratios and mother's education showed no association. The independent variables accounted for more variance in head circumference for male newborns than for females.

We have found that right 2D:4D of newborns is positively correlated to head circumference, the effect was strongest for boys and independent of recumbent length and weight. High 2D:4D is associated with high prenatal oestrogen and head circumference is positively linked to intelligence. We discuss our findings in the context of oestrogenization in human brain evolution.