Human evolution has long been tied to growing brain size, and new research suggests prenatal hormones may have played a surprising role. By studying the relative lengths of the index and ring fingers, a marker of prenatal exposure to oestrogen and testosterone, researchers found that higher prenatal oestrogen exposure was associated with larger head size in newborn boys.

Human evolution is closely tied to a steady increase in brain size. New research suggests that this expansion may be partly influenced by pre-birth oestrogen levels, with an unexpected indicator in the length of our fingers.