Tomatoes don't agree with John. He is sick within an hour of eating them and becomes sweaty and panicky. But worse than this, they also make him irritable and aggressive and liable to commit violent crimes. Jason has a similar reaction to bread. He has always loved doorsteps smothered in butter for breakfast. But it gives him diarrhoea and a weird kind of depressed 'hangover'. This makes him crave the heroin that once put his life on the skids. It may sound implausible, but a controversial theory is gathering momentum: that one explanation for crime may be found on our...