Reader Dorsa Amir called my attention to a story by Susan Dominus in today’s New York Times magazine, “The mixed-up brothers of Bogotá.” It tells a bizarre tale of swapped twins that gives clues about the genetic basis of human behavior.

It turns out that two pair of identical twins were born in Colombia at the same time, and one individual of each pair was accidentally swapped. Each pair was then raised as assumed fraternal twins. Only much later was the mistake noticed, discovered by a woman who knew one twin and encountered the other in working a butcher shop, assuming it was her friend who was playing a joke when he didn’t recognize her. After a period of trepidation, the real identical twins met each other and discovered that their similarity ran far deeper than appearance.

Here they are: each twin was raised with an individual from the other pair:

Identical twins separated at birth are precious resources for those interested in the genetic basis of human behavior and morphology, for their similarity gives us a clue to how much of human variation has a basis in genes versus environments—or the interaction between the two factors. We know from the photo above that morphology (in this case, both shape and facial features) are largely determined by genes: although the first and third twins from the left, as well as the second and fourth, were raised together, the pairs that look the same are #1 and #2, as well as #3 and #4.  Despite the identical twins having experienced different environments from the moment of birth, their much closer resemblance compared to the assumed fraternal twins (actually unrelated individuals) is clear . This, and other similar work with twins, shows us that the variation in appearance among humans is largely based on the variation in their genes rather than in their upbringing.

But we already knew that: offspring tend to resemble parents even when the offspring are separated when young from their parents, and reared in a different environment. But identical twins raised apart are one of the best tests of the genes-versus-environment—nature versus nurture—hypothesis (yes, the genes do interact with environment) for behavior as well. If identical twins raised apart, which share all of their genes, are more similar to each other for a given trait than are real fraternal twins reared together (which, like regular siblings, share half of their genes), that suggests that the genetic contribution to the trait’s variation is more important than the environmental contribution. For if similarities in behavior were caused entirely by the environment, then identical twins reared apart should be less similar in behavior than fraternal twins reared together: the latter were raised in a common environment (more or less), while the former weren’t.

In general, as the article notes, earlier studies have shown a remarkable influence of genes on human behavior: identical twins raised apart are often eerily similar:

Arguably the most intriguing branch of twins research involves a small and unusual class of research subjects: identical twins who were reared apart. Thomas Bouchard Jr., a psychologist at the University of Minnesota, began studying them in 1979, when he first learned of Jim and Jim, two Ohio men reunited that year at age 39. They not only looked remarkably similar, but had also vacationed on the same Florida beach, married women with the same first name, divorced those women and married second wives who also shared the same name, smoked the same brand of cigarette and built miniature furniture for fun. Similar in personality as well as in vocal intonation, they seemed to have been wholly formed from conception, impervious to the effects of parenting, siblings or geography. Bouchard went on to research more than 80 identical-­twin pairs reared apart, comparing them with identical twins reared together, fraternal twins reared together and fraternal twins reared apart. He found that in almost every instance, the identical twins, whether reared together or reared apart, were more similar to each other than their fraternal counterparts were for traits like personality and, more controversial, intelligence. One unexpected finding in his research suggested that the effect of a pair’s shared environment — say, their parents — had little bearing on personality. Genes and unique experiences — a semester abroad, an important friend — were more influential.

But, as Dominus notes, these studies aren’t perfect for several reasons: identical twins could be adopted into similar environments by the agencies; they might have known each other, so there’s the possibility of some shared environmental influences; there could be a form of self-selection, so that identical twins who share quirky behaviors are more likely to be found since the media reports on them more often or they’re more likely to be discovered. In the case of the Castro and Velasco “brothers,” most of these problems don’t exist: there was no adoption and no self-selection.

The separated twins were studied by researchers, and I’ll let you read the article to see the results. Let me just say that in general they align with the earlier studies of twins separated at birth: a strong genetic component to behavioral variation—and the behaviors involved are sometimes quirky. Here’s a snippet describing how the two pair of twins behaved after they hung out together before they were formally tested:

The four young men all knew one another well by then. Over the past six months, they had gone on outings and shared meals, talked about women, family, money, values. Even weeks in, each had stared, still unnerved and amazed, into the eyes of his identical brother. They had measured, assessed and inspected. They stood back to back, comparing height (those raised in the city were taller than those from the country [JAC: this shows that there is a big environmental influence on height, as we’ve known for some time that nutrition is important here]); Carlos had crushed Wilber in a food-­eating contest, William had vanquished them all when they arm-­wrestled. In the stands at a soccer match, Carlos watched, in fascination, as William’s hand reached down his jeans to scratch his backside: Jorge did the same thing, Carlos told Wilber. Over dinner one night, Jorge noted that Carlos and Wilber both leaned in at the same odd angle toward their plates.

But there were differences between the identical twins, as well: researchers showed that in some respects they were less similar than expected (though it’s not reported whether they were still more similar than were the unrelated twins who were reared together). And epigenetics is part of the story, too.

This is a fascinating tale of both human and scientific interest, and although it’s a bit light on the science for my taste, it still shows the kind of natural experiment we need to determine what proportion of human variation is based on genetic differences, what proportion on environmental differences, and what proportion on the interaction of genes and environment.  Ethics dictates that we can’t do the kind of experiments on humans that we can on flies and cows: separating individuals at birth and seeing how much difference in behavior and appearance can be created by rearing siblings (identical or fraternal) in different environments. Data so far show that a surprisingly large amount of variation in human behavior rests on variation in genes, but (as I noted) these studies aren’t perfect. Still, they should give pause to those who believe (often based on political ideology) that genes don’t play much of a role in the diversity of behavior among individuals in human population.