The evolution of the human species is marked by an increase in brain size. Now new research suggests that could be partly dependent on increases in prenatal oestrogen - revealed by looking at the length of a person’s fingers.
Professor John Manning , of Swansea’s Applied Sports, Technology, Exercise and Med-icine (A-STEM) research team, is an expert in the field of digit ratio. His work examines the relationship between the length of index and ring fingers, known as the 2D:4D ratio, which is directly linked to the relative concentrations of oestrogen and testosterone a baby receives during the first trimester of pregnancy.
Individuals with high oestrogen relative to testosterone have long index fingers (2D) in relation to their ring fingers (4D) - that is their 2D:4D value is high.
Professor Manning’s latest research has seen him working with colleagues from Istanbul University’s Department of Anthropology and their findings have just been published in the journal Early Human Development.
Head circumference in newborn children is strongly associated with brain size and subsequent measures of IQ. For this research the team looked at 2D:4D and head circumference in 225 newborns - 100 boys and 125 girls.
This revealed that high 2D:4D (indicating high prenatal oestrogen) was related to large head circumference in boys but not girls.
Professor Manning said: “This finding is relevant to human evolution because increases in brain size are found alongside feminization of the skeleton, what is known as the oestrogenized ape hypothesis. High values of 2D:4D in males have been found to be related to elevated rates of heart problems, poor sperm counts and predisposition to schizophrenia.
“However, increases in brain size may offset these problems. Thus, the evolutionary drive for larger brains in humans may inevitably be linked to reductions in male viability including cardiovascular problems, infertility and rates of schizophrenia.”
The researchers believe their findings provide further evidence about the positive influence of prenatal oestrogen on human brain evolution.
Professor Manning’s previous research has examined how digit ratio may provide valuable information relating to alcohol consumption , outcomes after contracting Covid-19, as well as oxygen consumption in footballers.
Early Human Development
10.1016/j.earlhumdev.2026.106479
Observational study
People
Digit ratio (2D:4D) and neonatal head circumference: Evidence for oestrogenization in human brain evolution?
12-Jan-2026
We declare that we have no competing interests.