Hon, han och hjärnan review

For any one species, the body of knowledge in each area is awesomely complex. Heilig extracts a wide range of disparate data sets spanning withdrawal reflexes in sea slugs, mating routines in fruit flies, memory-specific neuronal networks in rat brains and tribal behaviours in early Homo sapiens that might be relevant to gender expectations in the species’ more recent versions.

Heilig is very good on the core story of his book: the scientific evidence for sex-specific differences in the organisation of the brains (or head ganglia) of species with biologically well-defined male and female individuals. Some early and fairly stable structures are, unsurprisingly, linked to reproductive functions. Later in development, groups of interconnected nerve cells appear that tend to show more marked male-female variations in the extent and direction of connectivity rather than in the organisation of the groups of cells.

The point is elaborated in one of the few illustrations: a stunning computer reconstruction of bundles of nerve fibres sweeping through the human brain in different directions. But, just as at some other points in the organisational analysis, the careful reader might end up being baffled. For instance: if, as the caption says, different connections have been coloured in warmer and cooler shades that reflect the ‘strength of [nerve cell] connections’, how come many change colour abruptly on their way from A to B? Why are some of the thin strands a warm pink and some thick bundles a cool blue?

Hon, han och hjärnan is, at one level, a demonstration of some of the pitfalls that await the specialist who tries to ‘reach out’ to non-specialists. Heilig is too smart to be caught out often, yet, now and then, he errs by being either slightly too flip or too laborious.

Almost all the sex-specific findings, though interesting in themselves, beg new questions. To what extent is brain organisation directly relevant to human maleness and femaleness? Where does experience, encoded in memories, come in? To what extent do basic biological sex differences really matter in modern societies, where technology has eliminated so many of the old biological imperatives? Heilig repeatedly refers to biological variation, arguing that many gender characteristics conform to a continuous distribution so that males and females differ significantly only at the extreme ends of the range of variability. Does that variability contain an evolutionary promise (or threat) of an androgynous option?

In the concluding sections, grouped under the title ‘Know Yourself’, Heilig tries to present socially acceptable arguments that will fit in with – or, at least, not contradict – the neurobiological evidence he has assembled in his book. After a well-meaning if slightly awkward discussion of ‘equality’ and the desirability of ‘fairness’, given that men and women often make different choices, he ends with an open-ended question: ‘[To me] the big question is not how we are to induce our daughters and sons to make the same life choices at group level. They must be allowed to choose independently. But for how much longer do we have to live with their choices being valued in such a systematically unjust way?’