A large amount of research has been done on biological sex-based differences in various kinds of intelligence. Sociobiology is a tricky field, because it is difficult to separate the effects of environment and genetics on individuals. As this report has shown, environmental differences can be immense. Many studies in this field have been flawed by the lack of adequate controls. For example, some studies of differential mathematical ability have failed to take into account that the male subjects had taken more math courses than the females [Petersen 1980, page 33,] or used different types of toys as children. Researchers have generally found that men tend to have superior spatial ability, while women have superior verbal ability, with both differences developing at puberty [Petersen 1980, pages 31--33,]; however, the interplay between biological and social influences is not yet understood [Petersen 1980,Kramer et al 1990]. As discussed earlier in the report, boys are given more toys that would encourage development of spatial skills, while girls are spoken to more by adults. The argument for biological differences, however, was recently bolstered when Doreen Kimura found ``that hormonal levels can affect people's performance on certain verbal and spatial tests'' [Holloway 1990, page 40,].