Summers resigned as President of Harvard the following year, a prominent casualty in the culture wars surrounding sex differences. Finally, if some sex differences are inherited, did the genes involved arise by natural selection in our ancestors, with the differences enhancing reproduction? In A Billion Years of Sex Differences, Steve Stewart-Williams argues that it’s time to engage these questions with data. In the end, it’s obvious that the more we understand the basis of sex differences, the better equipped we are to address them. Some of these are obvious but not often recognised as innate sex differences.