If you can get in close enough, checking out someone’s smell is a valuable way of finding Mr or Ms Right. Despite our aversion to smell and our much reduced olfactory areas in the brain (at least compared to dogs and horses) we are in fact surprisingly sensitive to it. Newborn babies and their mothers can identify each other by smell alone within hours of the birth – which is one reason why we now like to make sure that the baby goes straight on the mother’s breast as soon as it is born. This is something that we share with most other mammals. In sheep and goats, the mother learns to recognise its newborn young by smell within 24 hours, and in the following days will allow only that lamb to suckle. And the lambs themselves learn to identify the right mother to suck from in the same way, though they are, perhaps understandably, a bit slower and it usually takes a couple of days’ exposure to the mother’s smell.In fact, smell provides one of the best markers of who you really are. The reason for this is that your smell is determined by the same set of genes, the major histocompatibility complex genes (MHC), as your immune system. It is part of who you are, your personal chemical signature. The MHC gene complex is particularly susceptible to mutation, producing new immune complexes with each new generation. This is probably just as well, as these are our first line of defence against bacteria and viruses which are themselves undergoing constant genetic change.Our immune-system genes have evolved to be almost as changeable as virus genes in an effort to track the ever-changing biological threats that we face from them. So smell may be one way of checking out who’s a good bet and who’s not, but it’s not the only function of smell in this context. Female moths famously dribble molecules of an incredibly powerful scent into the air. Male moths can detect these scents in the tiniest quantities from hundreds of yards away and find them quite irresistible. These sexual attraction scents are known as pheromones and occur widely in the animal kingdom, including monkeys. There has been some debate as to whether or not they occur in humans, but, in fact, there is considerable evidence to suggest that they do.There do appear to be significant differences between the sexes in their respective sensitivity to odour: women are much more sensitive than men. There is now quite a lot of evidence that women in particular are quite good at identifying their children and their lovers by scent alone. However, we are by no means perfect at this, it must be said, and it is probably just as well that we don’t manage our social world by smell rather than by vision – we would be likely to make an inordinate number of embarrassing mistakes if we did. However, it seems that, having identified the right person, smell plays a very important role in sexual arousal for women in a way it doesn’t for men. Perhaps as a result, women rate smell as more important in mate choice than men do, whereas men rely much more on visual cues, reflecting the fact that men tend to make up their minds about a prospective mate from further away than women do. Women need to get up close and personal.The Guardian