Testosterone has been claimed as one of the most important drivers of human life - through the agency of sex and aggression. In the 19th century, Charles-Eduoard Brown-Séquard injected himself with extracts from ground-up animal testicles, and made startling claims for its rejuvenating properties and its ability to enhance virility. But the amount of testosterone derived from the injection was actually so small that it could only have been a placebo effect. Today synthesised testosterone is increasingly prescribed for the so-called 'male menopause'; it's also regularly used for trans men as they transition, as well as for some women with low libido. In 'How Much Testosterone Makes You a Man', Naomi Alderman explores how testosterone had been used and abused in the past. She considers the credits and deficits of its story, and asks what it can tell us about identity and masculinity.