You can blame your Grandad. If you are caught in a moment of sexual infidelity it may be down to your genes. German scientists working with zebra finches have discovered that this normally faithful species has aberrant individuals who like to stray.
Darwin, whose theories of natural selection were sparked by the finches of the Galapagos, explained why the occasional male finch might do this: it increases his chances of spreading his genetic material. But why do some females also have a roving beak? It turns out that the daughters of promiscuous finches are more likely to play away, too. And the offspring of these females inherit the same wanderlust.
Scientists in America last year suggested a possible cause: a dopamine receptor gene called DRD4. It is found in people, too. The gene is linked to thrill-seeking, potentially addictive behaviour like alcoholism, gambling and even watching horror films. So it could be your DNA that makes you more prone to, well, getting prone. There is one catch. When the news about DRD4 broke last year, it was accompanied by headlines suggesting: "It might not be your fault that you're a cheating piece of trash." Whatever the genes say, society still presumes we should be held to account as beings possessed of free will. Life is sometimes more complicated than science allows.