In the previous chapter, I argued that, in contemplating most psychological problems, it is helpful to begin with some understanding of the innate psychological tendencies and constraints that we all share because they were adaptive to the simpler hunter-gatherer lifestyle in which our common ancestors evolved. I believe even more strongly that most psychological problems cannot be adequately understood without a clear appreciation of how much we differ still from one another psychologically or without realizing that these individual differences are determined largely by genetic differences among us. Like the previous one, this chapter is an excursion from my main theme, an essay on behavioral genetics in which the so-called nature-versus-nurture controversy is reviewed and evaluated in relation to recent twin and adoption research, much of it from my own university. Readers who are already comfortable with the neo-hereditarian position advocated here may wish to turn directly to the section titled "Nature Via Nurture," where I return to the main themes of this book.