New research reported by ScienceDaily suggests that exposure to estrogen before birth may have influenced the evolution of the human brain, and that a clue to that process may be reflected in the relative lengths of the fingers.
The study was led by Professor John Manning of Swansea University and researchers from Istanbul University. Their findings were published in the journal Early Human Development and focused on digit ratio, or the 2D:4D measurement that compares the index finger with the ring finger.
To examine the possible connection, the team studied 225 newborns, including 100 boys and 125 girls. They compared each baby’s digit ratio with head circumference, which is often used as a rough indicator of brain size in newborns.
According to the researchers, boys with a higher 2D:4D ratio also tended to have larger head circumferences. That relationship was not seen in girls. The team says the findings may support the idea known as the estrogenized ape hypothesis, while also noting that digit ratio is only an indirect marker of prenatal hormone exposure and does not directly determine brain size.
