Physically, siblings tended to differ somewhat, but they were a lot more similar on average when compared to children picked at random from the population. That's also true of cognitive abilities.
"The surprise," says Plomin, "is when you turn to personality."
Turns out that on tests that measure personality — stuff like how extroverted you are, how conscientious — siblings are practically like strangers.
"Children in the same family are more similar than children taken at random from the population," Plomin says, "but not much more."
In fact, in terms of personality, we are similar to our siblings only about 20 percent of the time. Given the fact that we share genes, homes, routines and parents, this makes no sense. What makes children in the same family so different?