Box 2 | Evolutionary explanations for sex differences in the brain

What evolutionary explanations might be offered to account for widespread sex influences on brain function? In some cases, they seem obvious. For example, Kazuhito Tomizawa and his colleagues⁹¹ recently found that oxytocin, a hormone that is necessary for mammalian labour and lactation, improves both spatial memory and memory-related neurochemistry in the hippocampus of female mice that have had litters. The improved spatial memory has clear advantages, allowing a mother to wander further afield to find and recall locations of food and water and thereby better ensure the development and survival of her offspring.

In more general terms, the best developed idea concerns sexual selection, a concept originally proposed by Charles Darwin and developed more recently by David Geary⁹². Sexual selection refers to the competition for mates that occurs both within and between sexes. Extensive evidence from many species makes it clear that males and females have evolved different behavioural strategies to optimize their chances of successful mating. Females tend to compete with other females more subtly, in ways that may depend more heavily on the processing of finer details; for example, of social cues. Such evolutionary accounts may help to explain the heightened recall of detailed information in females found in several studies of human memory so far⁹³.

Regardless of the ultimate evolutionary explanations, it seems incontrovertible that males and females evolved under some similar, and some very different pressures. We should therefore expect a priori that their brain organization will be both similar in some respects, and markedly different in others. This is precisely the situation suggested by the sex difference literature.