What You Can Change _. And What You Can't - Martin E. Seligman [23]
The upshot is that there is unlikely to be a molecular biology of beauty. There will not be “beauty genes,” or there will be so many combinations of genes underlying beauty as to be scientifically unwieldy. But beauty will still be subject to natural selection and will still be inherited. So too with intelligence, aggression, and all complex traits.18 It follows that the notion of “aggressive genes” may not make much sense, but that the notion of the heritability of aggression makes good, scientific sense.
How the inheritance off personality is studied. Beauty, aggression, nervousness, depression, intelligence, and the ability to make up limericks all run in families. But if you can’t find the genes underlying a personality trait, how can you possibly find out if it is inherited rather than learned? This formidable question has a surprisingly simple answer: Study twins and study adopted children.
Identical twins are genetically identical. They always have, for instance, the same color eyes. Fraternal twins have, on average, half their genes in common. Sometimes one twin is green-eyed and the other blue-eyed. They are no more alike genetically than any other two siblings. When identical twins are more similar for some trait than fraternal twins, we say that the trait is heritable.
This is true for eye color, but how about more complex traits, like limerick composing? Even if identical twins have the same limerick-composing ability (or its lack), and fraternal twins are not as similar, this talent could still be the result of child rearing. Everyone knows that identical twins are raised more similarly: Their parents dress them the same, they share the same bedroom more often, they take the same classes, and so on. Identical twins reared apart have the same genes, but they grow up in vastly different environments. If they are similar for some personality trait, it must be heritable and not learned. The study of identical twins reared apart is the best way to untangle the effects of child rearing from the effects of genetics.19
Indeed, if you want to attach a number for degree of heritability, simply take the correlation for identical twins reared apart: When that correlation is 1.00, the trait is completely determined genetically; when it is lower, say .50, this means that the trait is half genetic and half nongenetic in origin.
Identical twins reared apart.
Tony and Roger were given up for adoption as infants. Tony grew up in a warm and effusive working-class Italian home in Philadelphia. Roger was raised in Florida by austere, highly educated Jewish parents. While in his twenties, Tony, a traveling salesman, was eating at a restaurant in New Jersey when he was accosted by a very insistent woman diner. “Roger, how have you been? You haven’t called.” With effort, Tony was able to convince her that he wasn’t Roger and that he knew no such Roger. But he was intrigued and tracked Roger down. When they traded birth dates, each discovered he had a twin brother.
The similarity was spooky. Of course, they looked and sounded identical. But their IQ was also exactly the same. They used the same toothpaste. They had both been atheists since grade school. Their school grades had been the same. They both smoked Lucky Strikes and wore Canoe after-shave. They had the same politics. They had similar jobs and liked similar types of women. On their next birthday, they sent each other surprise gifts by mail: identical sweater-and-tie sets.
Over the last twelve years, a diligent group of University of Minnesota psychologists led by Tom Bouchard, David Lykken, and Auke Tellegen has studied the psychological profiles of twins. The group started with the “Jim” twins (both were named Jim), a pair whose reunion was written about in the press in the 1970s. The project snowballed. People who knew they had a long-lost twin came to the University of Minnesota for help in finding their twins. Minnesota has now accumulated no pairs of identical twins