Why do humans kiss each other when most animals don't?

When you think about it, kissing is strange and a bit icky. You share saliva with someone, sometimes for a prolonged period of time. One kiss could pass on 80 million bacteria, not all of them good.

Yet everyone surely remembers their first kiss, in all its embarrassing or delightful detail, and kissing continues to play a big role in new romances.

At least, it does in some societies. People in western societies may assume that romantic kissing is a universal human behaviour, but a new analysis suggests that less than half of all cultures actually do it. Kissing is also extremely rare in the animal kingdom.

So what's really behind this odd behaviour? If it is useful, why don't all animals do it - and all humans too? It turns out that the very fact that most animals don't kiss helps explain why some do.

According to a new study of kissing preferences, which looked at 168 cultures from around the world, only 46 per cent of cultures kiss in the romantic sense.

Previous estimates had put the figure at 90 per cent. The new study excluded parents kissing their children, and focused solely on romantic lip-on-lip action between couples.

Many hunter-gatherer groups showed no evidence of kissing or desire to do so. Some even considered it revolting. The Mehinaku tribe in Brazil reportedly said it was "gross". Given that hunter-gatherer groups are the closest modern humans get to living our ancestral lifestyle, our ancestors may not have been kissing either.

The study overturns the belief that romantic kissing is a near-universal human behaviour, says lead author William Jankowiak of the University of Nevada in Las Vegas. Instead it seems to be a product of western societies, passed on from one generation to the next, he says.

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