In Catalonia, a new study from Swansea University adds fresh evidence to a long-running question about human brain evolution. Researchers found a link between the ratio of index to ring finger length in newborns and neonatal head circumference, which is used as an early indicator of brain size.
The study, published in Early Human Development, is titled Digit ratio (2D:4D) and neonatal head circumference: Evidence for oestrogenization in human brain evolution? It was written by Barış Özener, John T. Manning, Berna Ertuğrul, and Fatih Aydık. The researchers say the 2D:4D ratio, which compares the second finger with the fourth finger, may reflect prenatal exposure to oestrogen and testosterone during the first trimester of pregnancy.
To test the idea, the team analysed 225 newborns, 100 boys and 125 girls. They compared digit ratio with cranial circumference and found the clearest association in boys. Male newborns with a higher 2D:4D ratio, meaning a relatively longer index finger compared with the ring finger, tended to have a larger head circumference at birth. The same association was not significant in the girls in the sample.
Professor John Manning, from the Applied Sports, Technology, Exercise and Medicine team at Swansea University, said the result matters because increases in brain size during human evolution appear to have coincided with changes in the skeleton and the prenatal hormonal environment. He linked the finding to the so-called oestrogenised ape hypothesis, which suggests prenatal oestrogens may have played a greater role in brain development during human evolution.
The study does not claim that finger length can predict intelligence or individual brain traits. Instead, it suggests the index to ring finger ratio could act as a biological clue to processes that shaped human brain development. The authors also note that earlier research has linked high 2D:4D values in men with cardiovascular problems, poorer fertility, and a predisposition to schizophrenia, and Manning said the increase in brain size may have offset some of those disadvantages.
For readers following science coverage across Catalonia, this study adds another piece to the wider picture of human evolution. Read more science and research coverage on our news tag page, and see the study context through Swansea University and the journal Early Human Development.