Sense of smellLust is based on someone's genes being a good match for yours, say scientists. 'You're looking at another person and figuring out whether you want your children to carry that person's genes,' says Devendra Singh, professor of psychology at the University of Texas. So if you fancy someone, you've decided without realising that their genes will combine with yours to produce healthy kids.
But how does this happen? According to Rob Elder of Emory University, Atlanta, we literally sniff out our date's genetic fitness. 'Scientists have known for some time that pheromones can set off large sexual responses in animals,' says Elder. 'But until recently they'd assumed that humans had lost that ability.'
Then in 1985, researchers found pheromone sensors in human nostrils, which connect directly to the part of the brain responsible for emotions, such as helpless lust for the bloke sitting opposite you in the pub.
Scientists then decided to research pheromones and found that women preferred the pheromones of men with different immune systems to their own.