Fraternal and, especially, identical twins are famous for sharing close relationships. Are those relationships closer than those of other siblings? It’s a relevant question when the number of twins being born in the U.S. has increased dramatically due to the widespread use of fertility drugs -- 3.2 out of 100 births result in multiple babies. Debbie Ganz, who co-wrote a book on twins with her twin sister Lisa, says, “It’s not a normal relationship.” The personal accounts of twins show that their relationship can be much deeper and more idiosyncratic than those of siblings who don’t share the exact same birth date and time.
Identical twins and tennis doubles champions Mike and Bob Bryan call their high level of connection “twinergy synergy," while their coach calls it “the twin ESP thing.” Twins like the Bryans often know each other so well that they seem to communicate telepathically, while other tennis doubles teams take more time to hash out their strategies verbally. A study done by Alan Mikkelson of Whitworth University states that while identical twins share 100 percent DNA similarity or relatedness, fraternal twins and full siblings share about 50 percent DNA similarity. This means that identical twins are more alike not just in looks, but in psychological makeup and personality, and thus actually have more “synergy” than most other siblings.
Identical twins are likely to give each other the most social support of any siblings, closely followed by fraternal twins and full siblings. Mikkelson’s study concluded that there is a high correlation between genetic relatedness and social support, because siblings who have more in common tend to like the same things, and therefore spend more time with one another in a supportive relationship. The siblings with the most in common are usually identical twins. Other siblings may have interests in common and develop a close relationship, but the relationships of identical twins will almost always be closer than relationships between other siblings due to the similarity in their genetic makeup.
Age differences between siblings can also affect their level of closeness. Since most twins are born just seconds or minutes apart, they have almost no age difference between them and often go through life perceived by others and themselves as a set of two people rather than just one person. Siblings who are close in age tend to have more in common with each other and remain more socially supportive of each other than siblings who have a wide age gap between them. This may be because siblings who are close in age grow up at relatively the same pace, go through school together and might participate in similar activities.
Society tends to expect of twins that they behave as an inseparable duo in childhood, but then grow up to become separate, individual people. According to Mike and Bob Bryan, this isn’t always the case. The adult twins still share a uniquely close relationship, spend the majority of their time with each other and even dress alike both on and off the court. It makes dating difficult, one ex-girlfriend said, because they already have what you might call a “soul mate.” The twins say that they want to date, and that ideally their girlfriends would be best friends with each other, but they realize that’s a tall order. Even in adulthood, then, twin relationships can be much closer than other sibling relationships.