A Christmas story: many years ago, when my age was still in the single digits, I was watching TV at a friend's house. A Christmas-themed advert came on for a toy that was typical of the offerings foisted upon children in 1980s New York – Barbie's plastic surgeon's office, maybe, or a Cabbage Patch Doll nuclear power station, or perhaps a Monopoly set that came with fake piles of cocaine. My friend turned to me proudly: "I've already put that on my Christmas list." For what would not be the first time in my childhood, I mused that, sometimes, being Jewish sucks.

I am happy to say I no longer feel that way (l'chaim!), partly because I am now old enough to buy my own Cabbage Patch Doll nuclearshown that birds prefer fresh fruits to rotten ones.

But in 2006, four scientists tried to build Janzen's theory into a mathematical model and ran into trouble. They found that if microbes put extra work into spoiling the food for large animals, they would be outcompeted by others that did not bother. Janzen's idea sounded plausible but was unlikely to be true, they concluded. The disgusting taste of a rotten apple, in their view, was more likely an accidental byproduct of its breakdown by microbes.

Now, some of the researchers who built that earlier model have revisited the topic, but they changed a key assumption: "In our original model pretty much any microbe could arrive anywhere," says David Wilkinson, an ecologist at the Liverpool John Moores University in the United