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 estrogen—revealed by looking at the length of a person’s fingers. Professor John Manning, of Swansea’s Applied Sports, Technology, Exercise and Medicine (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 estrogen and testosterone a baby receives during the first trimester of pregnancy.