The researchers examined the relative length of the index and ring fingers as an indicator of exposure to estrogen and testosterone in utero. Human evolution has long been associated with increasing brain size. New research suggests a role for prenatal hormones in this process. A study found a link between higher prenatal estrogen and larger head size in newborns. This finger ratio serves as a key to hormonal exposure during pregnancy. The research thus provides a new insight into the development of the human brain.