We might have sexually transmitted infections to thank for our modern-day monogamous society, according to a new study from Canada this week. Between ten and fifteen thousand years ago, as agriculture was established and humans swapped a hunter gatherer lifestyle for life in larger group settlements, our ancestors also appear to have embraced monogamy - having a single partner, rather than multiple wives. Mathematician Chris Bauch has designed a computer simulation that suggests that, as populations increase in size, the threat of sexual disease drives the switch to monogamy, and the desire to punish those that don't toe the line, as he explained to Chris Smith