“here is the weirdest thing in all of personality research: Studies of twins and adoptive children consistently show that shared environment effects are virtually zero. In other words, once you account for the effects of genes, the shared environmental effects that nearly everybody believes to be so important for the development of personality are vanishingly small—effectively nil in most studies. If my last two sentences do not surprise you, then you are not reading carefully enough (or else you took a course in personality psychology once upon a time and you have already wrapped your mind around these surprising findings). According to the research, the reason that identical (momozygotic [MZ]) twins are so similar to each other in personality traits is that they have all their genes in common. The fact that they happen to have grown up in the same family adds nothing to the similarity.”