Blood, Sweat, and Eyes

The chemical basis of love is well-known. When you see the face of the person you're in love with, your brain floods with a surge of dopamine. Doses of this happy-drug can become addicting to the point that being apart from your beloved feels torturous, and induces symptoms of chemical withdrawal.

Dopamine addiction explains why people tend to stay in love once they've become so, but it doesn't account for why they fall in love in the first place - or who with. Those matters seem too multi-faceted to be fully addressed by MRI scans. In fact there are so many factors involved in the love plunge that we might never sort them out in a scientifically rigorous manner. Nevertheless, certain key pieces have been identified by psychologists and neuroscientists. Here are two particularly interesting findings.

We each have a set of genes, called the major histocompatibility (MHC) locus, that determines which type of immune system we have. To investigate how MHC genes correlate with sexual attraction in humans, Swiss zoologist Claus Wedekind conducted an experiment modeled on one done with mice. He had 44 men of varying immune system types each put his sweaty t-shirt in a box. 49 women with diverse MHC genes subsequently smelled the boxes of t-shirts and rated the sexual attractiveness of the sweat smells.

It was found that the more different a man's immune system genes were from a woman's, the more sexually attractive she found his smell. This makes a great deal of sense: our offspring are more likely to outlast diseases if they're pooling a diverse set of MHC genes from their parents. Furthermore, if one parent dies of a disease, it's best for the kid if the other one does not. Our senses of smell are attuned to who would be the best other parent.

In 1998, psychologist Art Aron conducted a study in which he had (straight?) participants of opposite sexes pair up and engage in various "icebreaker" activities together. One task required the couples to silently gaze into each others' eyes for 2 minutes. The couples reported that this made them feel extremely close; in fact one pair who did it went on to get married six months later.

Follow-up neuroscience studies on the effect of eye contact on attraction were done. It was found that more dopamine is released in the brain when we look at pictures of a person looking at us than when the person is looking away. (By the by, it's weird to think about how our brains can simultaneously know and not know that a photograph isn't a real person, don't you think?)

A participant's rating of the pictured person's attractiveness was correspondingly higher if he or she appeared to be making eye contact. The study's authors surmised that it was the feeling of being valued and attended to themselves that participants found so attractive in onlookers.

Though feeling attraction isn't falling in love, it often plays a significant part in it. And it's the part we're closest to making scientific.