During the dot-com boom, he says, he was stunned to see male traders "displaying classic symptoms of mania," with symptoms of omnipotence, raging thoughts and diminished need for sleep.
Quitting finance and heading for Cambridge, Coates explored a hunch with Joe Herbert, a professor at the Cambridge Centre for Brain Repair.
They took saliva swabs from 17 male traders at a London stock-dealing firm twice a day and measured the samples for two hormones.
These were testosterone, which is associated with male aggressiveness and sexual behaviour, and cortisol, which is summoned by the body to deal with "fight or flight" emergencies.
When the traders were in profit, their testosterone levels surged. But when they were in loss, or in fluctuation, it was their cortisol that rose sharply.
Testosterone encourages confidence and risk-taking, and has an accumulative effect, which could explain winning streaks in sports teams, for instance.
But research in animals suggests that, over the long term, high doses of the hormone impair judgement and encourage excessive risks.