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The “digital ratio” is a visual indicator of exposure to estrogen and testosterone during pregnancy
Boys with greater prenatal exposure to estrogen have larger head sizes at birth, pointing to a possible hormonal stimulus behind human brain growth — and the so-called “digital ratio” is a surprising visible clue to this exposure.
One of the most striking features of human evolution is the continuous expansion of the brain. New results suggest that this growth could be, in part, linked to levels higher estrogen levels before birth.
Surprisingly, there appears to be a visible clue of this exhibition: the relationship between the length of a person’s fingers, explains .
Professor John Manning, from the Applied Sports, Technology, Exercise and Medicine (A-STEM) research team at Swansea University, specializes in studying the so-called digital ratio. This measurement compares the length of the finger index finger with the ring finger and is known as ratio 2D:4D.
This ratio reflects the balance between estrogen and testosterone to which the fetus is exposed during the first trimester of pregnancy.
People who, before they were born, were subject to a higher proportion of estrogen in relation to testosterone tend to have their index finger (2D) longer than their ring finger (4D), which results in a higher 2D:4D value.
How the cranial perimeter of newborns is closely associated with brain size and later IQ measurementsthe researchers analyzed, in 225 babies, both the proportions of the fingers and the measurements of the head — 100 boys and 125 girls.
The results of the , conducted by Manning in collaboration with researchers at Istanbul University, were presented in an article published in the journal Early Human Development.
The analysis found a clear association in boys: Higher 2D:4D values, which indicate elevated prenatal estrogen, were linked to greater cranial perimeter. In girls, the same pattern was not observed.
Professor Manning explained the broader implications of these results: “This finding is relevant to human evolution because increased brain size arises side by side with the feminization of the skeletonin what is known as the estrogenic monkey hypothesis“.
“High values of 2D:4D in men have been associated with higher rates of heart problems, low sperm counts and predisposition to schizophrenia”, adds the researcher.
“However, the Increased brain size may compensate these problems. Thus, the evolutionary drive for larger brains in humans may inevitably be linked to reduced male viabilityincluding cardiovascular problems, infertility and schizophrenia rates”.
The research team says these results reinforce the growing evidence that prenatal estrogen may have played a beneficial role in evolutionary expansion of the human braineven if this change had biological costs.
Previous studies by Manning have also explored how the digital ratio relates to other characteristics and outcomes.
His work analyzed links to alcohol consumption, recovery after COVID-19 infection and use of oxygen in football players.
Taken together, this body of research suggests that finger length may offer clues about strong hormonal influences that shape development even before birth.
