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.