By Michelle Bryner, published on September 1, 2005 - last reviewed on June 9, 2016
Because identical twins are genetic copies of each other, their children share half of their genes with both their parent and the parent's twin. Genetically speaking, identical twins are parents to each other's children.
To investigate whether these close genetic ties affect family relationships, Nancy Segal, director of the Twin Studies Center at California State University at Fullerton, and Jim Seghers, a graduate student, examined bonds between some 300 sets of identical and fraternal twins and their children.
Segal found that aunts and uncles who are identical twins say they feel closer to and more involved with their co-twin's daughters and sons than do fraternal twins. More than 60 percent of the identical twins in the study thought of their co-twin's kids as their own.
One pair of identical twins in the study, Jill Sigel-Greer and Judy Sigel-Freeman, said they feel like mothers to each other's children, sharing the same feelings of pride and concern for their nieces and nephews as they do for their own kids. Jill and Judy's children even call their aunts "Aunt Mom."
Why would Jill and Judy see themselves as mothers to each other's children when they feel like typical aunts to their non-twin sister Susie's children? Although it could be that identical twins are more alike and get along better, Segal suggests that it is the twins' 100 percent shared genetic makeup that causes the closer connection between twins and their nieces and nephews. Ordinary siblings and fraternal twins have only 50 percent of their genes in common.
"Evolutionary psychology suggests that we're predisposed to act in ways to get our genes into subsequent generations," Segal says. That means by caring for her twin's children, Jill is unconsciously working to get her genes passed on.
Judy says she isn't thinking about passing on her and her twin's genes. She says she simply feels like Jill's double when she is with her sister's family. "You add Jill's two kids to my three, and really and truly they feel like five kids, one mother, maybe two fathers. But definitely one mother," Judy says.
The study was presented at the annual meeting of the Human Behavior and Evolution Society.