Who survives in the financial markets? Efficient market theorists tell us only the rational survive, but traders need more than a calculating mind. They must also possess confidence, an appetite for risk, skill in visual scanning and quick reactions. Financial markets may therefore select not just by rationality, but also by biological traits.

These traits may derive in part from a trader’s prenatal exposure to steroid hormones. Testosterone levels surge between the ninth and 18th week of gestation, exerting powerful organising effects on the developing body and brain. These levels, researchers believe, can be estimated because they leave telltale traces on the adult body, some of them odd, like auto-acoustic emissions (strange sonar-like clicks in the inner ear) or fingerprint asymmetry. Estimates of prenatal androgens are often used by paediatricians to gauge whether a newborn has been exposed to abnormal levels of steroid. The most convenient is the ratio of the index to the ring finger (2D:4D), a lower ratio – ie a longer fourth finger – indicating higher prenatal testosterone. Men typically have lower ratios than women.