Several years ago I fell into conversation with a man I’d never seen before and have never seen since. However, he told me a story that always stuck in my mind. We met at a bar not far from where I live. He said that one afternoon he walked into the bar. There was only one other customer -- a pretty girl. He started talking to her. She was receptive and friendly enough but no matter what topic he brought up, no matter how strenuously he attempted to engage her, she never seemed quite to warm to him. He could understand that not every attempt to get over succeeds but what happened next left him stupefied. Another man came into the bar. He went right over to her. Without a word, he undid his belt and cracked it on the bar. “Let’s go, he said and she did. Now the man relating this story had been struggling to make headway with her for the better part of an hour and this man comes from out of nowhere – “I swear she didn’t know him from before,” he assured me – takes one look at her and whips out his belt and she goes off with him. He might not have known her but he knew her just the same. She was giving off signals that the first man didn’t or maybe couldn’t pick up on. This kind of thing happens so often that it’s almost (but not quite) commonplace – why you see so many mismatched couples who themselves can’t figure out what they’re doing together but can’t seem to ever break apart for long, either. They seem on the contrary to be condemned to each other. There’s an obscure phenomenon called assortative mating, which may help explain if not all of these implausible, impossible pairings. I first learned about it when I was collaborating on a book about depression. The same question that can be asked about depression – why does it exist if evolution presumably weeds out inimical traits in a population? – can be asked about assortative mating.
First of all, assortative mating is the well-studied tendency for people with similar traits to get together more often than random chance would predict. Such traits could include physical type, for instance, or level of education. The same also applies to depression and personality disorders. But here’s the thing: these people usually don’t meet in hospitals or in therapists’ waiting rooms. They don’t sign up for an online dating site dedicated to matching singles with bipolar disorder or schizophrenia. These couples may even meet, go out, and marry without ever realizing that the love of their life is also being treated for a psychiatric illness – they may not find out until they reach the altar, maybe not even then. So it seems that these couples are attracted to each other by forces that they aren’t aware of. And I don’t mean the unconscious impulses that govern far more of our behavior and thoughts than we might care to admit. I’m referring to their genes, specifically genetic mutations, that may have more to do with whether someone responds favorably to a potential mate than all the attributes that they think they’re looking for (‘brooding lawyer in his forties, nonsmoker, loves to travel, likes long walk on the beach,’ ‘fun-loving, voluptuous brunette, likes Lucretius, rock-climbing and Pilates.’) And if a man and a woman, both with genetic mutations that influence mood disorders, have children then the chances are much greater that they, too, will be more predisposed to having a mood disorder. (A predisposition doesn’t necessarily mean that you will inherit an illness or a disorder like depression or alcoholism, only that you will be more vulnerable to it.) It’s as if the mutations are striving to perpetuate themselves by using sexual attraction to outwit people who should know better. When you’re drawn to that stranger across a crowded room it might not be love at first sight so much as complimentary genetic mutations calling to each other. And poor, besotted you are simply giving them a lift to their destination.