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Digit ratio (2D:4D) and neonatal head circumference: Evidence for oestrogenization in human brain evolution?
Barış Özener1, John T Manning2, Berna Ertuğrul1
1Istanbul University, Department of Anthropology, Istanbul, Türkiye.
Early Human Development
|January 13, 2026
Summary
The second-to-fourth digit ratio (2D:4D) in newborns, particularly boys, is positively linked to head circumference. This finding suggests a connection between prenatal hormone exposure, head size, and potential intelligence correlates.
Area of Science:
- Developmental biology
- Human evolution
- Prenatal development
Background:
- The second-to-fourth digit ratio (2D:4D) is a recognized marker of prenatal sex steroid exposure.
- 2D:4D ratio has been associated with various physical and behavioral traits, including intelligence.
- Limited research exists on the association between neonatal 2D:4D and head circumference.
Purpose of the Study:
- To investigate the relationship between neonatal 2D:4D and head circumference.
- To explore sex-specific associations between 2D:4D and head circumference.
- To examine the influence of prenatal hormone exposure on neonatal head development.
Main Methods:
- A sample of 225 full-term newborns (125 girls, 100 boys) was analyzed.
- Measurements included recumbent length, weight, head circumference, and digit lengths (2D and 4D).
- Hierarchical multiple linear regression was used for sex-specific analyses, with maternal education as a covariate.
Main Results:
- In males, right 2D:4D and recumbent length were positively associated with head circumference.
- In females, recumbent length and birth weight predicted head circumference; digit ratios and maternal education showed no association.
- Variance in head circumference was greater for male newborns compared to females.
Conclusions:
- Neonatal right 2D:4D is positively correlated with head circumference, most strongly in boys.
- This association is independent of recumbent length and weight, suggesting a direct link to prenatal factors.
- Findings support the role of prenatal oestrogenization in head development and its potential implications for intelligence.
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