EARLIER this month, I helped reunite the fraternal twins Ann Hunt and Liz Hamel, who had been apart for 78 years. Born in England in 1936, they were separated at birth when their unmarried mother decided she could not care for both. Ann, the “prettier” sister, was given away, while Liz, the “less adoptable” baby marked by scoliosis, remained at home. Ann Hunt was unaware that she was a twin until last year, when her daughter made the discovery while researching their ancestry.

Ms. Hunt and Ms. Hamel are the longest-separated twins on record. I learned about them a year ago, at which point they had spoken by phone and by Skype but had not met in person. (Ms. Hunt lives in England, Ms. Hamel in the United States.) As a psychological researcher who specializes in twin studies, I knew immediately that I wanted to reunite them and observe the impact of their meeting on their lives. I invited them to participate in a behavioral investigation at the Twin Studies Center that I direct at California State University, Fullerton, and they accepted.

Twins who have been reared apart present a fascinating natural experiment, since they share genes but differ in environment, allowing us to identify how much of our behavioral and physical traits are guided by each factor. (Identical twins share all their genes; fraternal twins share half their genes, on average.) But the complex, often intimate, sometimes uneasy associations between reunited twins fascinate me even more.

In the case of identical twins, research shows that they share an unusually close bond. My laboratory has found that identical twin children are more successful at joint puzzle completion than fraternal twins, and that identical twins do better at shared problem solving. More recently, I showed that identical twins who are aunts and uncles invest more in caring for the children of their twins (their “genetic” sons and daughters) than do fraternal twins. And in continuing studies, I have found that identical twins mourn the loss of their twin siblings more intensely than do fraternal twins.