An amazing tale of identical twins (two pairs!) swapped and raised apart

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:

Screen shot 2015-07-11 at 8.18.29 AM

(From left) Jorge Enrique Bernal Castro, William Cañas Velasco, Carlos Alberto Bernal Castro and Wilber Cañas Velasco. Credit Stefan Ruiz for The New York Times

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.


  1. ronanon
    Posted July 12, 2015 at 1:08 pm | Permalink

    I wonder if their sitting positions for the photo was intended, or, like the scratching and leaning towards their plates when eating, wasn’t.

    It seems reasonable that similarities in bone and muscle in the twins would make some sitting positions more comfortable than others. But the two sitting positions sure stands out.

    • rickflick
      Posted July 12, 2015 at 1:46 pm | Permalink


    • Diane G.
      Posted July 13, 2015 at 6:35 pm | Permalink

      Agree. I wondered, too.

  2. Doris Fromage
    Posted July 12, 2015 at 4:00 pm | Permalink

    It’s well-established that adopted children are more likely to grow up into troubled individuals than children raised by their own biological parents. I understand the risk for substance abuse is twice as high for adoptees. I wonder if this effect was observed in the twins mis-assigned to the wrong mothers. It’s by no means 100%, of course, but it would be interesting to see if the wrongly assigned twins had more psychological difficulties than their twins who remained with the right mothers.

    • thh1859
      Posted July 12, 2015 at 5:23 pm | Permalink

      “It’s well-established that adopted children are more likely to grow up into troubled individuals than children raised by their own biological parents.”
      Really? Seems, on the face of it, most unlikely.

      Almost by definition, couples who adopt are more likely to make children the centre of their lives than the average couple.

      • Diane G.
        Posted July 13, 2015 at 6:37 pm | Permalink

        But if you include all adoptees, you include the ones from poverty, born to drug addicts, etc. They may come with “built-in” problems, nurturing notwithstanding.

  3. Gregory Kusnick
    Posted July 12, 2015 at 4:03 pm | Permalink

    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.

    I don’t doubt that genes influence behavior, but this sort of catalog of coincidences smells of cherry-picking. Where’s the control group of randomly chosen strangers who just happen to shop in the same stores, own the same color car, and play cards with a guy named Bob? We can’t evaluate the significance of these twin stories without knowing the baseline rate of such coincidences in the general population.

    • Sastra
      Posted July 12, 2015 at 5:27 pm | Permalink

      Agree. Instead of being used for genetics vs. environment illustrations, a lot of these identical-twins-reared-apart situations have eagerly been co-opted by paranormalists.

      Twins in general are supposed to have ESP. And when I was younger and “into” books on psychic powers, my recollection is that most of them proudly used trivial and random facts like “both of them married women with the same first name” as proof positive of deep psychic connections running behind materialistic reality.

      • John Taylor
        Posted July 13, 2015 at 5:41 am | Permalink

        I’m an identical twin and find this kind of stuff irksome. My brother and I share the same genes and we were raised in the same environment. We have a lot in common but a lot of differences also. The married woman with the same name stuff is really silly.

        • Diane G.
          Posted July 13, 2015 at 8:13 pm | Permalink

          Does your brother read WEIT? 😀

          • thh1859
            Posted July 16, 2015 at 2:43 pm | Permalink

            What a relevant and contextually apt question!

    • Posted July 12, 2015 at 5:49 pm | Permalink

      The scientific literature on twins goes beyond these coincidences to general measures of heritability. I agree that these quirks of behavior are at best anecdotal. But even putting those aside, the heritabilities calculated for behavioral traits from twin studies (even with their flaws) are surprisingly high.

    • Larry Cook
      Posted July 17, 2015 at 1:34 pm | Permalink

      I agree except for the building of miniature furniture. That can’t be a very common hobby unless I’ve been missing something. It’s a remarkable coincidence.

  4. colnago80
    Posted July 12, 2015 at 5:51 pm | Permalink

    On the issue of identical twins, one is reminded of the experiments with identical twins by Sir Cyril Burt which turned out to be fraudulent as the alleged subjects didn’t exist. Any number of subsequent papers cited Burt’s papers, which turned out to be very embarrassing after the fraud was discovered. See Arthur Jensen, a professor at the Un. of California, Berkeley whose paper on the How Much can we Boos IQ published in the Harvard Educational Review depended heavily on Burt’s non-work.

    • peepuk
      Posted July 13, 2015 at 12:50 pm | Permalink

      It seems to me that interpretation of “twin research” is always surrounded by politically motivated reasoning.

      According to some (f.i. W.D. Hamilton) it isn’t clear that Burt was fraudulent, and, more importantly, his findings seem to be in line with other twin studies (wikipedia).

      • thh1859
        Posted July 16, 2015 at 2:44 pm | Permalink

        Spot on, Peepuk

  5. Posted July 13, 2015 at 12:57 am | Permalink

    I discovered a doppelganger while working backstage as a civic theater volunteer. My unrelated twin is so close in appearance that his wife mistook me for him. We dress alike and somehow share a love of stagecraft. I am stopped in the street and in stores by his friends. He is from a different city.

    • Diane G.
      Posted July 13, 2015 at 6:40 pm | Permalink

      That’s rather spooky!

      But with >7 billion of us…

  6. Posted July 13, 2015 at 8:57 am | Permalink

    I’ve been looking into the genetic basis for religiosity, and twin studies seem to show that religiosity is about 50% genetic.

  7. Diane G.
    Posted July 13, 2015 at 8:11 pm | Permalink

    What a riveting read! And pretty slow, for that matter, trying to keep who’s who straight throughout.

    The difference in environments could hardly be more striking, though it does sound as if both families began in poverty. Oddly, little was made of the presence of a father in the one case, vs. the apparent distance in the other.

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