The progressive increase in brain volume represents one of the most defining characteristics of human evolutionary history. Recent empirical evidence suggests that this evolutionary development may be partly associated with elevated levels of prenatal estrogen exposure. A potential morphological marker of this hormonal influence appears to be the digit ratio of the hand.
John T. Manning, a researcher at the A-STEM research centre at Swansea University, has devoted a substantial portion of his work to investigating the 2D:4D ratio—the proportional relationship between the lengths of the second digit (index finger) and the fourth digit (ring finger). This ratio is considered a biomarker reflecting the balance between prenatal estrogen and testosterone exposure during the first trimester of fetal development.
In collaboration with researchers from the Department of Anthropology at Istanbul University, Manning examined whether the 2D:4D ratio correlates with neonatal head circumference, a commonly used proxy for brain volume that has been linked to later cognitive indices.
The study, published in Early Human Development, analyzed data from 225 newborns (100 males and 125 females). Both digit ratio and head circumference were measured immediately after birth.
The analysis revealed a statistically significant positive association among male neonates: higher 2D:4D ratios—indicative of greater prenatal estrogen exposure—were associated with larger head circumference. This pattern was not observed among female neonates.
According to Manning, these findings are evolutionarily relevant, as increased brain size in humans appears to coincide with skeletal features described as feminized. Previous research has linked higher 2D:4D ratios in men with elevated cardiovascular risk, reduced sperm quality, and greater susceptibility to schizophrenia. Nevertheless, evolutionary increases in brain size may offset some of these biological costs, suggesting a potential evolutionary trade-off between enhanced cognitive capacity and aspects of male physiological robustness.
Overall, the findings add to growing evidence that prenatal estrogen may have played a beneficial role in the evolutionary expansion of the human brain, albeit potentially accompanied by biological costs. Furthermore, Manning’s prior research has associated the 2D:4D ratio with various physiological and behavioral traits, including alcohol consumption, recovery from COVID-19 infection, and maximal oxygen uptake in soccer players. Collectively, these data support the view that finger length may serve as a non-invasive indicator of prenatal hormonal influences with long-term developmental implications.