BP Zietsch, a psychologist in Queensland, Australia, has a theory to explain why some people are more likely to practice infidelity than others.
There are plenty of social as well as biochemical factors that account for infidelity in human relationships, and they are all too familiar to story-tellers and historians.
Zietsch thinks there may also be a link between promiscuity and specific variants of vasopressin (below, left) and oxytocin (below, right) receptor genes.
Vasopressin is a hormone that has powerful effects on social behaviors like trust, empathy and sexual bonding in humans and other animals. Mutations in the vasopressin receptor gene may change human sexual behaviour.