DANIEL: Poset and Mehlman are like dozens of pairs of identical twins who psychologist Nancy L. Segal has studied over the years. Her research, she says, leads to a clear message.

One example of a poset consists of an integer and all its positive divisors (excluding 1). For instance, the positive divisors of 42 are 2, 3, 6, 7, 14, and 21.