I'm often asked why identical twins sometimes grow up to be different. Most twins share the same birth date and time. A small percentage are separated by an hour. Usually, identical twins are very similar. Some stories tell of long-lost twins who, upon reuniting as adults, discover their lives are remarkably similar—spouse similar, even suffering from the same illness on the same day. However, some twins who grow up together become quite different as adults. I think there are two reasons. First, even though they are separated by less than an hour, this creates a distinction of age, leading to differences in roles, mindsets, self-expectations, and even parental expectations. Long-term role-playing causes these differences. Second, the thought, "I'm not your backup plan!" can cause both partners to magnify even the smallest differences, deliberately drifting apart. This can create some space in a sometimes suffocating relationship. Doing different things together becomes more interesting and can also create a complementary effect.