Science tells us that we are shaped by two fundamental forces: our genes, which we inherit from our parents, and our environment, or the circumstance in which we are raised. Based on this explanation, you would expect siblings to be quite similar. Consider two brothers-let’s call them Paul and James-who are now adults. They have the same parents and they grew up in the same home. Yet, as adults, the two brothers could not be more different, according to their parents and to the men themselves. How can this happen?