One of the most studied and, at times, most misunderstood phenomena of biology (study of living organisms) and psychology (study of the mind) is how people develop mentally: how do people become who they are? How do individuals develop the capacity for learning, memory, intelligence, and personality? Why do two or more individuals, born to and reared by the same parents (and therefore possessing similar genetic makeup to their siblings), often turn out to be individuals with such different likes and dislikes, different strengths and weaknesses? Why do some people develop mental illness while their brothers or sisters do not?

As with most studies pertaining to the mind, there are not many hard and fast answers. Scientists, doctors, and mental health professionals have different theories as to how people develop and grow mentally. Some theorists believe that the environment in which a person is raised contributes...