["On a chilly Ohio Saturday morning in 1979, some time after divorcing his first wife Linda, Jim Lewis awoke in his modest, middle-class home next to his second wife, Betty.
Jim - a romantic, affectionate type - was determined that this marriage would work and made a habit of leaving love notes to Betty around the house. As Jim lay in bed
he thought about others he had loved, including his son, James Alan, and his faithful dog Toy.
Having outfitted a workshop in a corner of his basement, Jim looked forward to spending some of the day's free time on his woodworking hobby. He had derived many hours of satisfaction from building furniture, picture frames, and an assortment of other items, including a circular white bench around a tree in his front yard. Jim also liked to spend free time driving his [light-blue] Chevy, watching stock-car racing, and drinking Miller Lite beer.
Jim was basically healthy. Having had a vasectomy, he was done having children. His blood pressure was a little high, perhaps related to his chain-smoking habit.
He chewed his fingernails to the nub. And he suffered occasional half-day migraine headaches – “like somebody’s hitting you with a two-by-four in the back of the neck.”
He had become overweight a while back but had shed some of the pounds. ] – from Exploring Psychology, by David G. Myers
All those same things were true of Jim Springer – his identical raised-apart twin, including the phrase about migraine.. They also vacationed on the same three-block-long beach area neat St. Petersburg. Both disliked baseball. Tests of intelligence, personality, heart rate, and brain waves were about as similar as the same person taking the test twice.
More generally, the Minnesota Twins study found that the majority of reared-apart twins were not just similar in IQ and personality. They tended to have very similar speech patterns, sense of humor, marriage patterns, careers, tastes in clothes, hobbies, etc.
Bouchard only found two pairs of male twins with at least one homosexual: one pair was concordant, the other somewhat ambiguous [one was clearly gay, the other had had a male boyfriend from 15-18, but not afterwards]. Larger studies have found a low concordance rate for homosexuality in males. A recent Swedish study found 7 concordant and 64 discordant MZ pairs, and 3 concordant and 50 discordant DZ pairs. Their estimate was that 19% of the variance was explained by genetic factors, 0.00 by shared environment, and 0.64 by unique environment – something that had happened to one twin but not to the other.
It’s worth remembering that practically everything you can measure is partly explained by genetics. For example, stealing from Jayman’s blog, let me list a few traits that are more influenced by genetics than homosexuality, often much more: IQ of course, ideology (liberal/conservative), interest in politics, social trust, racial attitudes, foreign policy preferences (!), women’s rights, schizophrenia, alcoholism, antisocial behavior, blah blah blah….
Nearly everything is influenced by genetics, but often genetics does not explain very much – in the sense of supplying a necessary (although often insufficient) cause. Many infectious diseases, such as tuberculosis, show far higher MZ twin concordances than homosexuality does. You could try to explain them genetically – and if you did a good job, you would probably conclude that genetic vulnerability to TB is a mix of ongoing deleterious mutations and selection for immunological variants that increase vulnerability to TB while decreasing vulnerability to something even worse, say smallpox or malaria. There’s not much you could do with that information, even if you had it.
But the necessary cause, the key causal link, of TB is Mycobacterium tuberculosis: you can have the world’s most screwed up immune system, but you still won’t get TB if you’re not exposed to the bug. And that’s useful knowledge.
Ulcers are far more heritable than homosexuality, and genetics matters: but you don’t get the ulcers without h. pylori.
Geneticists try to explain homosexuality with genetic models because that’s what they do – and because right now, ‘enlightened’ circles know that homosexuality ought to be genetic, even though nothing else ought to be. Strikes me that a lot of people are one Bo tree shy of a load.