While there is debate over whether humans release ‘pheromones’ like many insects do to attract a mate (we don’t have the vomeronasal organ to detect them for starters), there is a wealth of evidence that smell does in fact play a part in our choice of mate.
In experiments where women were asked to sniff t-shirts worn by different men and rate how attractive they were, they preferred the smell of men whose major histocompatability complex (MHC) genes differed from their own. High variation in MHC genes means a more effective immune system, so the studies suggest that women may choose a mate based on whether they will produce healthier children.
And it turns out women can also sniff out genetic quality in a male. In a similar t-shirt experiment, women preferred the smell of more symmetrical men. Body symmetry is thought to be a marker of a high quality genome, with fewer deleterious mutations – once again good for producing healthy kids. This could potentially explain why women are more likely to cheat on their spouse or partner with more symmetrical men – the health of potential offspring taking precedence over social traditions against cheating.