He told BBC News Online that the concept of seasonal variation in the risk of Crohn's disease had been studied on many occasions since the mid 1960s.

BBC: Seasonal variation in Crohn's risk

Previous research had shown, for instance, that 29% of the variation in whether or not people invest in stocks depends on their DNA. Studies of professional traders had demonstrated that approximately 25% of individual variation in portfolio risk is due to genetics.

WSJ: Jonah Lehrer on Genetics, Dopamine and Wall Street | Head Case