Your eyes meet across a crowded room. You know you are being obvious but you cannot stop staring. When you finally speak, butterflies flutter in your stomach. But what is it that makes that person so irresistible?
Cheesy romance aside, there is growing evidence that sexual attraction, a fundamental part of who we are, is governed by biology. Take looks for example; it seems obvious that we are attracted to others based on their physical appearance, even if we try and deny it. And although they say that beauty is in the eye of the beholder, there is support for the idea that we all look for certain qualities – symmetry, averageness and secondary sexual characteristics – that offer clues on potential benefits of choosing a person as a mate, either directly to us or to our potential offspring.
Yes, as boring as it sounds, average is what we actually look for, a face that resembles the majority of faces in a population and that lacks any extreme features. From a genetic point of view, averageness is thought to signal genetic diversity, a lack of potentially harmful genes (which may cause such extreme characteristics) and general health. Average faces also tend to be the most symmetrical, another feature we are attracted to as it also acts as a signal for health status and genetic fitness. Indeed, numerous studies have shown that digitally manipulating a face to make it more symmetrical is enough to increase its attractiveness.
There is also a link between attractiveness and the presence of secondary sexual characteristics: feminine female faces and masculine male faces. Once again, this is thought to be because such features ‘advertise’ the individual’s reproductive health (for example, a strong jaw is a sign of high levels of testosterone) and genetic fitness.
Of course the story does not end with looks; our sense of smell is also thought to have a key role. In a classic study by Wedekind and colleagues, women were asked to rate the odour of T-shirts worn by male participants for two consecutive days. They found that the women were more attracted to the odour of men with MHC genes different to their own. These genes code for proteins that have a key role in the immune response, and so selecting a mate with a distinct set of genes would ensure diversity in the immune genes of their offspring. Although the exact role of MHC genes in mate selection remains controversial (another study found that women were attracted to the smell of men with MHC genes most similar to their fathers), it is clear that humans can discriminate at such a fine genetic level based on odour alone and use this information to help select a mate.
Interestingly, hormones can also influence attraction, both at the level of visual and olfactory cues. For example, women tend to be more attracted to more masculine and symmetrical faces during ovulation, when they are at their most fertile. There is also some evidence that women in relationships with men sharing similar MHC genes are more likely to be attracted to other men during their fertile days. Furthermore, men seem to be more attracted to women who are ovulating, possibly owing to physical changes (face, odour, voice pitch).
The complication here is that the contraceptive pill, used by numerous women around the world, can toy with these preferences. For example, women on the contraceptive pill show weaker preferences for masculine and symmetrical faces, and a recent study found that stopping the pill can influence relationship contentment, with satisfaction being linked to the husband’s attractiveness.
Of course these are not hard and fast rules and can vary from person to person; for instance, our own self-perception of attractiveness can guide our preferences. But what they tell us is that attraction, governed by the quest for a mate whose genes will ensure the survival of our offspring (and thus our own genes), seems to be quite a self-centred process, at least from an evolutionary perspective.
Rachel David is studying for an MSc in Science Communication