The Mixed-Up Brothers of Bogotá Part 2

PART 2

When Two Are Like One

Identical twins don’t make obvious evolutionary sense; fraternal twins at least have the benefit of genetic diversity, improving the odds that at least one might survive whatever misfortune comes their way. And yet, in their utter inexplicability, identical twins have helped elucidate our most basic understanding of why, and how, we become who we are. By studying the overlap of traits in fraternal twins (who share, on average, 50 percent of their genes) and the overlap of those traits in identical twins (who share 100 percent of their genes), scientists have, for more than a century, been trying to tease out how much variation within a population can be attributed to heredity and how much to environment. 

‘‘Twins have a special claim upon our attention,’’ wrote Sir Francis Galton, a British scientist who in the late 19th century was the first to compare twins who looked very much alike with those who did not (although science had not yet distinguished between identical and fraternal pairs). ‘‘It is, that their history affords means of distinguishing between the effects of tendencies received at birth, and those that were imposed by the special circumstances of their after lives.’’

Galton, who was Darwin’s cousin, is at least as well known for coining the term ‘‘eugenics’’ as he is for his innovative analysis of twins (having concluded, partly from his research, that healthy, intelligent people should be given incentives to breed more). His scientific successor, Hermann Werner Siemens, a German dermatologist, in the early 1920s conducted the first studies of twins that bear remarkable similarity to those still conducted today. But he also drew conclusions that for decades contaminated the strain of research he pioneered; he supported Hitler’s arguments in favor of ‘‘racial hygiene.’’ In seeking genetic origins for various traits they considered desirable or undesirable, these researchers seemed to be treading dangerously close to the pursuit of a master race.

‘So we were swapped,’ Wilber said, shrugging. ‘You’re my brother, and you’ll be my brother until the day I die.’

Despite periods of controversy, twins studies proliferated. Over the last 50 years, some 17,000 traits have been studied, according to a meta-­analysis led by Tinca Polderman, a Dutch researcher, and Beben Benyamin, an Australian, and published this year in the journal Nature Genetics. Researchers have claimed to divine a genetic influence in such varied traits as gun ownership, voting preferences, homosexuality, job satisfaction, coffee consumption, rule enforcement and insomnia. Virtually wherever researchers have looked, they have found that identical twins’ test results are more similar than those of fraternal twins. The studies point to the influence of genes on almost every aspect of our being (a conclusion so sweeping that it indicates, to some scientists, only that the methodology must be fatally flawed). ‘‘Everything is heritable,’’ says Eric Turkheimer, a behavioral geneticist at the University of Virginia. ‘‘The more genetically related a pair of people are, the more similar they are on any other outcome of interest’’ — whether it be personality, TV watching or political leaning. ‘‘But this can be true without there being some kind of specific mechanism that is driving it, some version of a Huntington’s-­disease gene. It is based on the complex combined effects of an unaccountable number of genes.’’

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.

As pure science, the study of twins reared apart has troubled some researchers. Those twins either self-­select and step forward or become known to researchers through media reports — which are less inclined to cover identical twins who do not look remarkably alike, who did not marry and divorce women of the same name or choose the same obscure hobby. Identical twins who do not look remarkably alike, of course, are also less likely to be spotted and reunited in the first place. And few studies of twins, whether reared apart or reared together, have included twins from extremely different backgrounds.

‘‘Every study will have its critics,’’ says Nancy Segal, a professor at California State University, Fullerton, who worked with Bouchard from 1982 to 1991. ‘‘But studying twins reared apart separates genetic and environmental effects on behavior better than any research design I know.’’

Segal has been studying Chinese twins (fraternal and identical pairs reared together and reared apart) since 2003. In several books about twins, Segal has merged science with human-­interest tales, walking readers through statistical evidence but also highlighting anecdotal details: the identical twins reared apart who each showed up for research wearing seven rings, or the reared-­apart sisters who rubbed their noses the same way and called it ‘‘squidging.’’

Last October, Yesika Montoya, a Colombian psychologist who is now a social worker at Columbia University, saw on Facebook a video clip from a Colombian newsmagazine program, Séptimo Día, which confirmed through DNA testing that the four young men were two sets of identical twins. She got in touch with Segal, whom she knew only by reputation. She then approached the young men, who agreed to be the subject of their research.

No matter how fascinating, the two sets of twins represent a sample of only two. But to Segal, the possibilities were dizzying, unique. In no other family she knew of were there so many kinds of twin pairings to analyze and compare: Jorge and Carlos, Jorge and William, Jorge and Wilber and so on. ‘‘It’s an experiment within an experiment,’’ she said, comparing it to one of those Russian dolls: Crack open one, and there would still be another, and another, and another.

The twins knew the research would require them to submit, over the course of a week in March, to several probing interviews, individually and in pairs, as well as hours holed up in a conference room filling out questionnaires. There would be questions about their homes, lives and education, as well as personality and intelligence tests. Segal told them that she was interested in writing a book about them (Montoya would later collaborate with her), and the young men were enthusiastic subjects.

William had only one condition for his participation: He insisted that Segal and Montoya visit the home in which he grew up in Santander. Without that, he thought, they could never really understand who he was. He did worry, however, that if he told Segal and Montoya how long it would take to get to Santander, they would never agree to go. So he dodged and evaded whenever the subject of travel time came up. It’s a four- or five-hour drive, William would say, and then add, almost as an afterthought, that when the road could get them no closer to their destination, they would get out and walk. For how long? A little while, William would say; it might be a little muddy. How muddy? Maybe, he would suggest, it would be easier if at that point Segal traveled by horse. Would she, by any chance, rather ride a horse? Segal, a woman in her early 60s who grew up in the Bronx, said no.

The Importance of Will

Around 9:30 a.m. on March 29, three cars pulled into La Paz, a dusty town whose few small streets offer sweeping views of the Andes. The group — Segal, Montoya, the two sets of twins, translators and assorted friends and family members — had already been on the road for six hours. They settled in for a traditional breakfast of bone broth and hot chocolate at a diner in town. Jorge and William sat next to each other along one side of a wooden table, while Carlos sat across the way. Wilber sat with Segal and Montoya. While everyone ate breakfast, Carlos took out his phone and called up a picture of him and Jorge. ‘‘I love my brother, even though I only show it when I’m drunk,’’ Carlos said.

‘‘See?’’ In the picture, Carlos was puckered up, giving Jorge a big kiss on the cheek.

William watched Carlos, feeling annoyed. Wilber, he had often thought, was the same way: He took William entirely for granted, showing his love only on very rare occasions — when, for example, he thought one of them could die. They served in the same platoon in the military, and when they entered a particularly dangerous zone, Wilber would say to him, white-­faced: ‘‘May God be with you, my brother. I love you.’’ William knew Wilber loved him; but both Jorge and William wished that the brothers they grew up with had been more supportive, more expressive — the way William and Jorge now were with each other. They often called each other right before they fell asleep, just to say good night.

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); 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. Jorge felt comfortable gently correcting his identical twin’s grammar; Carlos took seriously such brotherly responsibilities as instructing Wilber in how to approach an attractive Bogotá woman at a bar or how to down a shot of tequila. The twins from Santander were amazed that neither of their city counterparts had ever fired a gun, which they quickly remedied on a visit to the country.

Carlos did feel immediately at ease with his newfound twin, he had to admit. Wilber did not try to tell him what to do when he talked about his love life, the way Jorge did; he just listened and supported him. Yes, they understood each other: their manly pride around women, their furious response to their brothers’ incessant teasing. But Carlos was also unnerved by Wilber’s Carlosness. His twin’s very existence refuted a concept dear to him: his sense of his own uniqueness. Having grown up so different from his other family members, he had come to pride himself on his individualism; now, as an identical twin, he was part of a rare subset of humans whose replicability was embarrassingly on display. Once, Wilber posted on Facebook a picture of himself back in Santander, bare-­chested in a river, triumphantly holding two chickens he had just killed. With his hair wet and slicked down like Carlos’s, the campesino in the picture looked too much like Carlos for his comfort. ‘‘Take that thing down,’’ he told Wilber. ‘‘People will think it’s me.’’

Far from believing that he had found his perfect other half, Carlos felt lonelier than ever. For all of Jorge’s reassurances, he could feel Jorge drifting toward William. The two now wore the same sneakers, shaved their goatees the same way. On weekends, Jorge often went to William’s butcher shop and got behind the counter, waiting on customers, so he could spend time with his twin. He sometimes slept at Wilber and William’s tiny apartment, while Carlos slept at home. Sometimes Carlos told himself, with a strange twisted relief, that he was glad this had all happened after his mother died; the jealousy he would have felt had she embraced William as Jorge had would have been more than he could take.

Carlos knew that Jorge was attuned to his sadness, that he even wanted to help. But whenever they tried to talk about it, they fell into mutually irritating old patterns. Carlos felt as though Jorge dismissed his concerns; Jorge felt frustrated that nothing he said could assuage Carlos’s sense of isolation. But Jorge tried. Six weeks or so after the reunion, Jorge asked Carlos for a photo of himself. That Saturday, Jorge went to a tattoo parlor. He already had a tattoo of his mother over his heart. Now he sat in a chair for four painful hours as his favorite practitioner needled his brother’s image permanently into his flesh, just inches away from the image of his mother. He came home and lifted his shirt to show Carlos the work, his skin still bloody and swollen from the violence of the needle. It was, Carlos would later remark, with tears in his eyes, the best present anyone had ever given him. It brought him some measure of peace.

After learning of his identical twin, Jorge honored his fraternal twin, Carlos, by adding his portrait next to one of their mother. Credit Stefan Ruiz for The New York Times

At breakfast in La Paz, however, Carlos felt that Jorge was provoking him once again. Moments after Carlos pulled out that photo, Jorge turned to him and brought up a sensitive subject the two had already discussed in many late-night conversations: Who would Carlos have turned out to be had he been raised in Santander?

Come on, Carlos, Jorge said — look around. Do you really think that if you had been raised here you would have ended up an accountant or even a professional?

Carlos refused to concede Jorge’s point. Who was to say he wouldn’t have found a way to go to school, to get his degree, to be working in the very same firm where he had only just recently been promoted?

William said nothing, but his face took on a hard cast. Carlos had no idea, he thought, how far a strong will could or could not get you. William had that strong will, had tried to exert it in every way, desperate to get to that petty-­officer training. First, he had moved to Bogotá to study for his high-school degree. He managed to pass the test, but his score was low — eight months of part-time cramming could not make up for all those years of lost schooling. He made it only onto a waiting list for the petty-­officer training, but that did not deter him. He packed up, left Bogotá and took a long bus ride to the barracks where the leadership course was being offered. When William arrived at the barracks, a commanding officer recognized him. ‘‘Those who persevere succeed,’’ the officer told him. The commanding officer managed to pull some strings on William’s behalf, but as they were going through the paperwork, officials discovered that William had already been discharged and compensated by the military for a disease he contracted while serving. The compensation made him ineligible for re-­enlistment. There were no more strings to be pulled; he could never be a petty officer; it was over. He would have to go home. But hadn’t the commanding officer told him that those who persevere succeed? For five days, William stayed past his welcome, hiding and mingling among the groups of soldiers. He hoped that things would sort themselves out, but more than that, he could not bring himself to leave: Leaving meant he had given up. On the sixth day, a sympathetic but fully armed officer accompanied him to the bus station and personally put him on a bus back to Bogotá.

William knew that Carlos was unfamiliar with that part of his history. Carlos probably did not know that William, as a 6-year-old, used to walk with his mother to this very town, La Paz, for five hours each way, just to buy groceries; they would spend the night at a kind woman’s place in town and then walk home again, groceries on their backs. And Carlos could not know, could never really know, how many hours William had spent hacking sugar cane with a machete as a teenager, his skin crawling from the heat and the itchy scraps of stalk, carrying 50 pounds of cane at a time, mindless, painful, strenuous work. Carlos had spent those same years, William knew, flirting with girls at an excellent public high school, playing basketball with his friends, racking up points on some video game

Carlos was wrong, William felt certain. Sometimes, a will was not enough. Had he grown up in Santander, Carlos would not be an accountant on the rise right now. And Carlos’s insistence on that point felt, to William, like an insult to all he had endured — a life he had endured, no less, in Carlos’s place.

When City Meets Country

After breakfast, the cars left La Paz, driving on serpentine, stone-strewn roads with lush palm fronds and ferns closing in overhead. With the heat of the sun now strong, one driver kept mopping his sweating face with a bandanna he borrowed from one of the relatives in the car, as if he was physically exhausted from the stress of maneuvering the vehicle over riverbeds and around ditches. Finally, around 11:30 a.m., the caravan stopped near a large gazebo in a grassy field. Everyone piled out of the vehicles. It was time to walk.

Segal had brought a bright purple rolling suitcase, which held materials she hoped to use that day for interviews and research with William and Wilber’s family; their brother, Ancelmo, now lived in their childhood home, but their parents and other relatives would also be there to celebrate Ancelmo’s birthday and see the twins. It became clear that the grassy path would not be suitable for luggage rolling, so William, who had carried far heavier loads on this journey before, easily slung the purple suitcase onto his shoulders.

The group started making its way along the path, which briefly lurched uphill. William was moving at high speed, despite the suitcase. He called out that as strong as he was, Jorge was every bit as strong, although it seemed unlikely that that could possibly true. ‘‘But not Carlos,’’ William said. ‘‘Carlos is not as strong.’’ William took a few more steps, then turned around as if something had occurred to him. ‘‘Why shouldn’t Carlos carry it?’’ he said. He backtracked until he reached Carlos, pushed the suitcase at him and then quickly headed off.

The path tracked across a grassy meadow and then started a long, steep descent. Within minutes, the path was made of mud — rich, claylike mud that was two feet deep in some patches. Carlos, who was always impeccably dressed, stepped carefully. But his Adidas basketball sneakers were quickly soaked with oozing earth.

William and Wilber’s parents, Carmelo and Ana, at the home in Santander where they raised the boys. Credit Stefan Ruiz for The New York Times

Carlos was as uncomfortable emotionally as he was physically. He had visited Santander twice since the reunion — once for a birthday party for all four brothers in La Paz and once to visit his biological parents, José del Carmen Cañas (known as Carmelo) and Ana Delina Velasco, at the home where they now lived. But he felt ill at ease on both visits. He knew William thought he had behaved churlishly, resisting the friendly overtures of his extended family. But there were just too many people around — locals, cousins, every one of them, it had seemed, wanting a photograph or a hug or some other sign of a connection that he himself did not feel. How was he supposed to get to know his biological parents when there was always a crowd around? When he met Carmelo and Ana for the very first time at William and Wilber’s apartment, a camera crew for a Colombian newsmagazine show had been with them in the room. As he embraced his biological parents, they were weeping profusely. He was moved when he felt Carmelo’s arms around him — he had never really known his own father, who died not long after his mother did. But something about Ana’s tears left him feeling detached, calm. He had had a mother, and a very good one at that. ‘‘Don’t cry,’’ he said to Ana, wiping her tears. ‘‘These are God’s ways.’’

It was high noon in Santander. Carlos picked his way through the mud, which splattered and quickly baked hard onto his legs in the sun. Then Carlos — Carlos, who was so vain about his clothing, fussy about fit, who was always brushing at the cuff of his pants to rid it of some imaginary lint — let out a howl. His foot had sunk deep in the mud. Slowly, with the help of someone from the area who was walking alongside him, he began to extricate it. There was a loud suctioning sound: Sludge coated his bare leg well past his knee.

Carlos at the home in Santander where he would have grown up. Credit Stefan Ruiz for The New York Times

More than an hour later, sweaty, exhausted, filthy, Carlos arrived with the group at William and Wilber’s childhood home. It had no toilet, no drywall, no paint, just wooden sides and a wood-­burning stove with a pipe jutting out of the roof. Carlos approached Carmelo with a smile: The two hugged warmly. But then there was silence; neither seemed to know what to say. William was standing close by, watching Carlos and his father. William looked pristine, except for a little mud on his boots. He wore a striped purple button-­down shirt for the occasion; Carlos was dressed in a black baseball cap with a Batman symbol on it, a tank top and sunglasses. He had not even had a moment to catch his breath when William quickly batted his cap. ‘‘Take off your hat and sunglasses,’’ William told Carlos. ‘‘Try to really be here.’’

Carlos watched Jorge, who was moving easily through the crowd, ingratiating himself with William and Wilber’s family in a way that Carlos still could not. Carlos was still annoyed about the conversation they had at breakfast. Jorge seemed to want him to make some grand emotional statement about how lucky he had been in the swap, how much tougher his lot would have been had he, in fact, been raised in Santander. It was not as if he hadn’t thought hard, lying awake many nights, about what his fate would have been had he been raised with this biological family. Two of William and Wilber’s brothers had died very young, one in a gun accident and one in an ambush while serving in the military. 

He might not even be alive if he had grown up here. Maybe it was easy to be a good guy in Bogotá. Maybe if he had grown up in Santander, he would have joined the guerrillas, who were popular a decade earlier but also brutal. Far from believing in the inevitability of his professional success, he worried about whether his character, in that alternate life, would have withstood the forces around him.

But no, he was not going to say all of that at breakfast, in front of a bunch of people. That was not who he was.

PART 3

The Myth of Identical Twins

At the moment that a sperm penetrates an egg, that single-­cell zygote is what is known as totipotent: It is pure potential. It has in it the makings of an eyebrow’s curve, a heart’s thick muscle, a neuron’s electrochemical power; it has in it the finicky instructional manual that will direct the building of the body’s every fiber and the regulation of those fibers. But that one cell splits into two, and instantly, lights begin to go out, potential dims. In order for that one cell to become a tiny bit of flesh in a heart, and not the hair of an eyebrow, one or more of its genetic signaling pathways must shut down. The result is differentiation, a steady process of elimination that allows complex biological universes to be built. Every time a group of cells divides, each one becomes more like one thing, less like another.

By the time that embryo is five or six days old, which is when a majority of fateful twin splits occur, some of those cells, by chance, go to one twin and some to the other. This means that the expression of some genes in one of those future twins is already, in subtle ways, likely to be different from the expression of genes in the other future twin, theorizes Harvey Kliman, the director of the reproductive and placental research unit at the Yale School of Medicine. From the moment that most identical twins separate, they may well have different epigenetics, a term that refers to the way genes are read and expressed, depending on environment. They are already different products of their environment, the environment being whatever uterine conditions rendered them separate beings in the first place.

The casual observer is fascinated by how similar identical twins are, but some geneticists are more interested in identifying all the reasons they might differ, sometimes in significant ways. Why might one identical twin be gay or transgender and not the other? Why do identical twins, born with the same DNA, sometimes die of different diseases at different times in their lives? Their environments must be different, but which aspect of their environment is the one that took their biology in a different direction? Smoking, stress, obesity — those are some of the factors that researchers have been able to link to specific changes in the expression of specific genes. They expect, in time, to find hundreds, possibly thousands, of others.

The meta-­analysis published this spring in Nature Genetics, which examined 50 years of studies of twins, arrived at a conclusion about the impact of heredity and environment on human beings’ lives. On average, the researchers found, any particular trait or disease in an individual is about 50 percent influenced by environment and 50 percent influenced by genes. But that simple ratio does not capture our complicated systems of genetic circuitry, the way our genes steadily interact with the environment, switching on, switching off, depending on the stimulus, sometimes with lasting results that will continue on in our genome, passed to the next generation. How an individual’s genes respond to that environment — how they are expressed — creates what scientists call an epigenetic profile.

Before she left for Bogotá, Segal contacted Jeffrey Craig, who studies epigenetics at Murdoch Childrens Research Institute in Australia, to ask if he would analyze the epigenetics of Carlos, Jorge, Wilber and William, using saliva swabs she would obtain while she was there.

Craig has analyzed the epigenetic profiles of 34 identical and fraternal twins at birth, collecting swabs from their inner cheeks. To Craig, it was noteworthy that in some cases — not many, but some — the epigenetic profile of one newborn twin was more similar to an unrelated baby than to the identical twin with whom that baby shared a womb. Structural differences in the womb could possibly account for it, Craig says — a thicker umbilical cord for one than the other (there are, in fact, two cords) or an awkward site of connection for the umbilical cord on the placenta. But he recognizes that there could be additional factors still in the realm of guesswork. Perhaps one twin is farther from the sound of the mother’s heart, its reassuring steady beat, sending that child on a slightly different life course.

Segal and Craig were eager to see the epigenetic results for the Colombian twins. Whose epigenetic profile, they wondered, would look more alike? The biologically unrelated twins who shared an environment — Segal calls them virtual twins — or the ones whose DNA was the same?

A sample of four subjects could only raise questions, not answer them. But epigenetic testing on larger samples of twins reared apart could one day provide a valuable resource for epigenetic science, says Kelly Klump, who is the co-­director of the Michigan State University Twin Registry. ‘‘You can’t look at how the environment will change the function of the genome without holding constant the genome,’’ she says. ‘‘Identical twins allow you to do that.’’ Given how hard it is to find identical twins raised apart, twins researchers working in epigenetics have mostly been focusing on the identical twins who show difference. Tim Spector, a professor of genetic epidemiology at King’s College London, is generating a huge global registry for identical twins in which only one twin has, for example, diabetes or autism.

Bouchard was influential in convincing his fellow researchers, as well as the public, that some significant part of who we are is influenced by DNA, which was hardly a given when he started his work. Spector and Craig, by contrast, are trying to identify how, exactly, we change in response to the environment. Their essential question is different: How can science identify genes that have been flicked on or off, with potentially harmful results, so they can be switched back the other way? Traditional twin studies were perceived to be seeking the immutable; epigenetic twin studies try to clarify what, in us, is subject to change — and more specifically, what mechanisms make that change happen.

Falling Into a Hole

A local politician had accompanied the group on the hike to Santander. Along the way, he tried to persuade the group to visit a nearby attraction: the second-­biggest hole in Colombia, a cavernous pit 500 feet wide and 600 feet deep. Locals like to get on their bellies, inch their way up to its rim and peer down into the abyss.

The second-­biggest hole became a recurring joke among the brothers, but for Yesika Montoya, the Colombian psychologist, it also became something of a metaphor for the young men’s experience. She was trying to get them to identify their feelings about all they had gone through, partly by recalling the physical sensations that they felt at various stages. ‘‘It was vertigo,’’ Jorge told her, as he described waiting for William the first time they met. ‘‘I felt a pressure. Like when you go on the roller coaster and you’re falling.’’

Montoya imagined that feeling to be like ‘‘going down a hole and not being able to feel the bottom.’’ She added: ‘‘It never stops. And just when you have put a foot here or there, you keep going down.’’

The process of spending time with Segal and Montoya and sharing their life histories necessarily changed the young men’s experience of their reunion. Carlos seemed surprised at one point when Segal asked him to describe the ways in which he and Wilber differed. ‘‘Well, the thing is, we’ve always focused on what our similarities are,’’ Carlos said. ‘‘We haven’t actually talked about our differences.’’ He seemed pleased, at last, to be given the opportunity.

At the time, Carlos pointed out that he liked older women, while Wilber liked younger ones. But the answer was, of course, far more complicated. Carlos was like Wilber in large, sweeping ways, and unlike him in infinite small ways: the expressions that darted across his and his face alone, the thoughts and worries that filled his mind. Carlos was, for better or for worse, more cynical than Wilber, more suave; Wilber was more joyful around small children, quicker to laugh out loud.

Jorge and William, too, have obvious differences. Jorge is a dreamer, a restless traveler, an optimist who believes that ‘‘if you give your best to the world, it will give its best back.’’ William’s face, more narrow, more gaunt, reflects a far warier outlook. ‘‘Nothing in life is easy,’’ he remarked once, a sentiment that you could hardly imagine Jorge expressing.

Carlos could feel Jorge drifting toward his identical twin. It was making him feel lonelier than ever. 

Was every one of these differences learned? Did some reflect different epigenetics? Perhaps there might be some extra biological protection built in for Wilber and Jorge, who, unlike Carlos and William, had been raised in their biological mothers’ arms. The mother who raised Carlos loved him, he knew. But he was also aware that a cousin had moved in with them when they were babies, expressly so that each child could be the beneficiary of the form of attachment parenting the hospital was encouraging at the time. Their mother wore Jorge in a sling; it was the cousin who wore Carlos.

In May, Carlos told Wilber that he wanted to visit his biological family, but without crowds of relatives or psychologists or camera crews. And Wilber passed that on to William. It was becoming easier for William to accept that Carlos’s reserve on those excursions to Santander was not so much in reaction to his new family as it was in response to the public nature of the outings. On a weekend in June when Wilber unfortunately had to work, William, Jorge and Carlos took a bus to see Carmelo and Ana for a relaxed, private visit.

Carlos sat next to William on the bus on their way up and listened as William, who had become something of a local celebrity in Santander, talked about his plans to run for City Council in La Paz. Carlos did not think much of Colombian politicians, but he was impressed by William’s ambition; he liked that William was taking a class to learn Microsoft Word. He had discovered, from the questions Segal and Montoya asked, that Wilber had no intention of returning to school. That disappointed him; he wanted to talk to Wilber about more than women. He wanted more for Wilber — wanted more from Wilber, but he was starting to think he might not get it.

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

Carlos knew Wilber wanted the two of them to spend more time together. But he also knew that Wilber, at some level, understood that Carlos was a solitary soul. Wilber, at any rate, had a life of his own and a new girlfriend, who had two young children whose photos he showed off, with admiration, to anyone who would look. The whole experience was less complicated for Wilber than for the other three brothers — simply because, as Wilber himself put it, he was not a very complicated person.

For Carlos, this fourth visit to Santander felt like a fresh start. The brothers arrived at Ana and Carmelo’s home early in the morning, after traveling through the night, but Carlos was enjoying the beauty of the countryside too much to go straight to sleep. Instead, he bathed in a water tank. He listened to the birds; he was a willing audience to the family parrot, Roberto, who had a talent for singing ranchera songs. Then, while his brothers dozed, he wandered into the kitchen, where Ana, a tiny woman — he had her giggle, he was told, although he never heard it that way himself — was cleaning a sheep’s head she would cook for dinner that night. He stood by the kitchen counter, keeping her company as she worked. He realized it was their first time alone.

They talked about her health, her aching joints, her back pain. ‘‘You know, you’ve worked so much your whole life,’’ Carlos told her. ‘‘It’s time for you to rest. Your children are so big already. Why do you work so hard for them?’’ The relationship with Ana felt more relaxed, but not necessarily closer. He told himself it would come in time. Jorge was always implying that there was something wrong with him for not feeling, instantly, that powerful, primal connection, that emotional force of biology and destiny, that William seemed to feel for the mother he never got to know. Carlos wondered whether he might have drawn closer to Ana had his own mother been alive to grant some kind of permission. But maybe it was simpler than that. Maybe he and William were just different that way.

Moving Forward

Before starting her research, Segal would not have been surprised if each young man tested similarly to his identical twin, despite their different environments. But her preliminary results, she said, show that on a number of traits, the identical twins were less alike than she initially anticipated. ‘‘I came away with a real respect for the effect of an extremely different environment,’’ Segal said.

Perhaps the results merely indicate that people raised in deeply rural environments, with little education, take tests in a wholly different manner from those who attended a university. William, who managed a small business with competence, at times seemed overwhelmed by the test. But Segal considered the young men’s story a case history that might provoke further research, inspiring others to seek out more examples of twins reared apart with significantly different upbringings, whatever they were.

Over the course of the week that the young men spent on Segal’s questionnaires, they looked back at the past that helped make them who they were. How many books did they have in their childhood homes? Did they ever smoke? Did they grow up in families in which people kept their feelings to themselves? For one week, they stepped out of time to look backward. But the moment Segal would leave, they would continue on their usual paths, speeding forward toward some unknown future, colliding with chance. They sometimes talked about all living together; as four, William liked to think, they were at their strongest. Like members of any family, they might drift and then regroup, or find themselves falling back on the deep comfort of their particular bonds. It is rare to grow up as a twin at all, part of a primal pair; now each young man had a second, rare pairing, a second chance at an unusual kind of closeness. What did that kind of entanglement — a double-­doubling — mean for whom they would each become or what they might achieve?

To celebrate the end of a week’s worth of research, Segal and Montoya decided to take the young men dancing one night at a popular Bogotá steak house with a big dance floor. Jorge and William took turns dancing with Segal; they smiled gamely and turned and twirled with only glancing attention to the rhythm. Carlos, in his element, showed Wilber a few steps; they danced in not-­quite-­synchrony, side by side, Carlos with sureness, Wilber staring down at his feet and concentrating. Occasionally he looked up, as if he was feeling it: He would get the hang of it soon enough, he knew. ‘‘Wilber has the goods,’’ Montoya said, watching from the table. ‘‘He just needs the experience.’’ When all the brothers stopped for more aguardiente, a sugar-­cane based liqueur, and sat at the table, they took turns flirting with a young woman who had joined the party.

Here at the club, Carlos was assured, poised, smooth. As the evening wore on and he drank more aguardiente, his moves got bigger, more daring, until he was showing off a maneuver that he and a friend had made up one night, a pivot from the waist that had him leaning so far back that his spine was practically parallel with the floor, his knees bent and nearly buckling. Carlos called that move ‘‘the Matrix,’’ after a similar backward dodge that the movie’s star, Keanu Reeves, executes while evading bullets in a parallel universe. As he leaned all the way back, Carlos looked as if he might lose his balance altogether. Wilber, William and Jorge quickly surrounded him, still dancing, a mixture of emotions on their faces: amusement, irritation, concern. But Carlos was not falling. It only looked that way, and he managed to right himself.

The dancing went on as before. The four men seemed to bounce off one another in different pairs and groupings, splitting off in search of young women, returning to compare notes before heading out onto the floor again. They were one, they were two, they were four, merging, dividing and merging again as the music played, long into the evening.