Scientists have discovered why we are attracted to certain people and the reasons are far from mysterious or romantic. Claire O'Connell reports
Birds, bees and even educated fleas do it, but the seemingly straightforward process of falling in love is much more complex than you might think.
Visual cues, smelly genes and chemical messengers in the brain are among the more subtle biological processes that send us tumbling head over heels when we find someone special.
Determined not to leave any element of romance to mystery, scientists have tested urine for hormones, thrust sweat samples under the noses of volunteers and even scanned the brains of the love-struck to figure out why and how we fall in love.
For example, researchers have recently looked at how hormones can make faces appear more attractive, how we are drawn to each other's immune systems and even how the euphoria of falling in love shares brain pathways with obsessive behaviour like stalking.
A study last year revealed that our looks can speak volumes about our health and fertility, showing that women with high levels of oestrogen, the female sex hormone, have more attractive looking faces.
Scientists at St Andrew's University measured oestrogen levels in the urine of 59 young women and photographed their faces. When a panel of men and women viewed the photos, they rated women with higher oestrogen levels as more attractive, feminine and healthier looking.
Oestrogen is an important factor in fertility, and the researchers believe that a woman's face can subtly advertise her potential to provide offspring. But the good news for the hormonally less well off is that the study found no link between attractiveness and oestrogen when the women wore make-up, which scrambles visual cues and masks tell-tale signs about health and fertility.
However, even if you deceive the eyes, potential mates may still turn up their noses. Scientists have long known that many animal species communicate through volatile chemicals called pheromones, but thought that humans lacked the apparatus to sniff out these subtle messages. More recently though, studies have found that pheromones can also act in humans, and that sex-related pheromones can light up different parts of the brain.
In addition, we may be able to literally smell whether we are genetically suited to a person by catching a whiff of their immune system.
A set of genes in our bodies called MHC help to regulate the immune system by making proteins that identify our own cells as belonging to us. They also contribute to our body odour and act as a promotional jingle for our genetic make-up.
The theory is that by sniffing out this signature and ensuring it is not too close to our own we may help to avoid inbreeding with someone who is genetically too similar. Once we have scanned and sniffed our way to a suitable partner, the fun really starts, namely the euphoria of early-stage romance.
But rather than leaving a tender moment alone, scientists have now scanned the brains of enamoured subjects to see what happens to our grey matter in this first flush of romance. Researchers in New Jersey carried out functional magnetic resonance imaging (fMRI) on the brains of besotted subjects who were looking at a photo of their beloved.
They found that gazing at a loved one lights up certain parts of the brain associated with reward and motivation and uses systems involving the chemical messenger dopamine. Intriguingly, these are the same reward pathways that become activated in stalking behaviour. How can this be?
It's all about the link between dopamine and pleasure, explains Patricia Casey, professor of psychiatry at University College Dublin and the Mater Misericordiae Hospital. "If you eat chocolate or if you drink a glass of wine and enjoy it, dopaminergic reward pathways will light up in the brain," she says. "And if it happens once, because it's pleasurable the behaviour is reinforced and you do it again."
Similarly, reward systems are activated in stalking because it's a source of pleasure, she says. "People stalk because they feel an affinity in some way with the person, it may be a bizarre affinity but to them it's an affinity and generally it gives them pleasure to see the person," she says.
Prof Casey says brain imaging is opening a new and exciting window on how the brain, the mind and emotions are all intertwined. "It's assumed that these things are purely in the mind and psychological and have nothing to do with physiology but there are neurophysiological pathways related to our emotions including love, forgiveness and guilt," she says.
And you thought falling in love was simple.