There are some good, bad and ugly factors involved in assessing the merits of sexual reproduction, researchers at the Perception Lab in St Andrew's University in Scotland have discovered, writes CLAIRE O'CONNELL

WITH VALENTINE’S Day around the corner, love is in the air. But have you ever wondered what attracts us to a potential partner? And why do we even go to the effort of finding a mate in the first place? Scientists have wondered. And while it might not seem terribly romantic to dismantle the process of falling for someone, it does offer up some interesting insights.

For starters, what makes someone easy on the eye? Whether it’s Brad Pitt, Beyoncé, a cute co-worker or someone you just spot in passing on the train, it appears there are certain features that make us view a face as attractive.

Daniel Re is a graduate student at the Perception Lab in the University of St Andrews, Scotland ( perceptionlab.com), where researchers set up experiments to tease out what participants tend to like in faces. "The cues that we find attractive are attractive for certain reasons," he says. "It indicates what we call mate quality." A popular belief is that we tend to find symmetrical faces attractive, but Re argues that there's more to it than that.

“Studies show that facial symmetry has an effect on attractiveness, but that it is a small effect,” he says. “So it’s true that more symmetrical faces are perceived to be better looking, but that’s an effect that is [trumped] by other factors, like skin quality or how masculine or feminine a face looks.”

Heterosexual men, it appears, tend to like feminine faces, which Re describes as having bigger eyes, a pointier chin and higher cheekbones. “More feminine faces indicate a high level of oestrogen,” he says.

Women can vary in their preferences for male faces, and what’s perceived as attractive can change over the monthly hormonal cycle, according to Re. “For women who are naturally cycling, when they are ovulating they prefer men with much more masculine features,” he says. “She is going for masculine cues – masculine faces, deeper voices – but if you ask that woman which sort of man would she prefer for a long-term relationship, her preference for masculinity will go down.”

Interestingly, it seems hormonal contraception might change the landscape. “We find those effects don’t seem to happen so much with women on the birth-control pill,” says Re. “There’s a fairly consistent preference for more feminine features in general, they don’t get a strengthened preference for masculinity at times when they would be ovulating.”

It seems that lifestyle can also affect how healthy and attractive our faces appear, and Re describes how work is ongoing at the Perception Lab on the effects that exercise and eating fruits and vegetables can have on attractiveness in Caucasian, African, and Asian faces.

“We know that if you increase your cardiovascular exercise it increases blood flow, which makes your skin look more red, which makes you look more healthy and attractive looking,” he says.

“Also, if you eat a lot of fruits and vegetables you get to consume carotenoids that can make your skin a bit more of a healthy yellow colour. So by increasing either fruit and vegetable consumption or aerobic exercise you could make your face look quite a bit better.”

Picking up on all these cues – even subconsciously – as well as the more conscious efforts of courting and wooing seems like a lot of hard work. Wouldn’t it be easier for us as a species if we could just asexually produce copies of ourselves without involving a mate? From a genetic perspective there are some good reasons why humans reproduce sexually and we mix our genes in with those of another, says Aoife McLysaght, associate professor at the Smurfit School of Genetics in Trinity College Dublin.

“It is perplexing and confusing,” she says. “If you think of the ‘selfish gene’ idea, the whole point is to pass on your genes and to do it as well as possible, and if you pass on your genes better or more often or more vigorously then you win in terms of evolution. But for sexual organisms you are investing all of your energy in the offspring that only has half of your genes. Why do you go to all of this effort to have this elaborate mechanism of sexual reproduction and attraction so you spend all of this energy on something that only shares half of your genes?”

It’s not that “clonal reproduction” is impossible – several plant and animal species can do it – but the fact that sex has evolved independently in plants and animals means we can infer that there must be some advantages to it, explains McLysaght.

She describes how those might boil down to “the good, the bad and the ugly”. The “good” refers to when a beneficial mutation arises in a lineage – if we reproduced asexually, it would stay in an individual lineage and would need to arise again by chance in other lineages. “But sex allows recombination of the genes, so you get this new combination in the offspring that is better.” Likewise for “bad” mutations with a potentially negative effect, the recombination of the genome that sex allows means they can get left out when certain combinations don’t survive well.

And the “ugly”? That refers to sex as a defence against parasites, says McLysaght. “Sex allows recombination of immune-related genes, which means that there are greater varieties of immune system due to the many possible combinations. So from a genetic and evolutionary point of view, longer term, you are better off doing sexual reproduction for all these reasons.”

Love is the drug

If you’ve ever fallen in love, you’ll know that euphoric feeling of walking on air because you are with that special partner. But what is going on in your brain? The neurochemical dance that starts when we fall for someone romantically appears to involve dopamine, says Patricia Casey, professor of psychiatry at University College Dublin and a consultant psychiatrist at the Mater Misericordiae University Hospital.

“Dopamine is a substance that lights up when you are enjoying something – if you like someone, your dopamine receptors will signal that and you will do things that will help to reinforce that.”

She points to some recent research that illustrates the almost drug-like quality that love’s reward can have on the brain: a 2010 US study in PLoS One of 15 individuals who were in the first nine months of a new, romantic relationship, found that when they viewed images of their loved ones, neural-reward systems lit up in their brains and they reported less sensitivity to experimental pain.

But while we know some of the players involved in the in-love brain, there’s plenty left to unravel. “We don’t know exactly what all of the neural pathways are. We know some of the neurochemicals involved like dopamine and the hormone oxytocin [which is thought to promote bonding] but we don’t know all of the connections and routes,” she says. “While modern imaging tools can provide us with powerful images of the brain and of its activity at various stages of romance, I doubt if science will be able to identify the magical je ne sais quoi that bonds people in lifelong love.”