Consider the following story line for the ultimate video nasty. Single man meets radical male masochist on the Internet. On their first date, the masochist offers up his penis as main course in a romantic dinner for two. After some teething problems over the best way to prepare the food, the two men enjoy a meal of garnished genitals. Satiated, and feeling woozy, the masochist is led upstairs to the bathroom, where he is left to bleed to death. Hours later, our host pops in to see how his date is doing, and finishes him off with a knife to the throat. He then butchers the body and barbecues the meat.

Even as fiction, this extreme tale of human weirdness would be difficult to stomach. So how do we respond when two middle-aged computer engineers turn this incredulous plot into jaw-dropping fact? Two words: shock and awe. Just when you thought you’d heard it all, along comes a German cannibal, Armin Meiwes, and his willing victim, Bernd-Juergen Brandes, to rewrite the book of bizarre human behavior. Rarely has a criminal investigation aroused such ghoulish curiosity or raised such difficult questions about the dark places that the human mind can go.

Amid the media scramble surrounding the recent courtroom drama, there has been a clamour to understand and to explain this behavior, which, incidentally, is not even illegal under either German or British law. In desperation, we turn to science for answers. What can rational objectivity tell us about such irrational acts of violence and mutilation? Perhaps not very much. But with little else to go on, we must be content with what morsels of knowledge we can find.