The division of the human species into male and female is such a basic fact that it has rarely required explanation. One need not search far, however, to find accounts of the differences between the sexes. St Thomas described the female as but a male manqué, biologically, intellectually and, of course, spiritually less than the male. Today we may dismiss medieval scholasticism for not being scientific but what have we learned from the scientific study of the sexes? The scientific study of the sexes has a brief history. Biologists only began to understand the contribution of both egg and sperm in reproduction towards the end of the nineteenth century when the microscope came into general use.