Ashhar Alam/New Delhi

The growth of the human brain has been a defining feature of evolution, and new research indicates that hormonal influences before birth may have contributed to this development. Scientists have identified a link between prenatal hormone exposure and head size at birth with finger length offering an unexpected clue.

The study centres on the “digit ratio,” known scientifically as the 2D:4D ratio, which compares the length of the index finger to that of the ring finger. This ratio is widely regarded as an indirect marker of the balance between oestrogen and testosterone exposure during the first trimester of pregnancy.

Professor John Manning of Swansea University’s Applied Sports, Technology, Exercise and Medicine (A-STEM) research team, who has extensively studied digit ratios, collaborated with researchers from Istanbul University’s Department of Anthropology for the latest investigation. Their findings have been published in the journal Early Human Development.

To explore a possible evolutionary connection, the team analysed finger ratios and head circumference measurements in 225 newborns, including 100 boys and 125 girls. Head circumference at birth is considered a reliable indicator of brain size and has been associated with later cognitive development.

The analysis revealed a notable trend among male infants. Boys with higher 2D:4D ratios, suggesting greater exposure to prenatal oestrogen tended to have larger head circumferences. The same association was not observed in female newborns.

Professor Manning said the findings may support what researchers call the “estrogenized ape hypothesis,” which proposes that increases in brain size during human evolution occurred alongside skeletal traits linked to higher oestrogen influence.

At the same time, previous research has associated higher digit ratios in males with certain health risks, including cardiovascular issues, lower sperm counts and a greater likelihood of schizophrenia. According to Manning, the evolutionary push for larger brains may have involved biological trade-offs, potentially affecting aspects of male health and survival.

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The researchers suggest that prenatal oestrogen could have played a constructive role in shaping the expanding human brain, even if it came with physiological costs.

Professor Manning’s earlier studies have connected digit ratio to diverse outcomes, ranging from alcohol use patterns to Covid-19 recovery and athletic oxygen efficiency. Collectively, this body of work underscores how a subtle physical trait may reflect powerful developmental processes that begin before birth.