To understand heredity–behavior relations it is important to first understand the mechanisms of heredity. The gains in knowledge about genetics in the twentieth century are stunning. An obscure scientific paper that was published in the late nineteenth century and rediscovered at the beginning of the twentieth laid the foundation for identifying the molecule of heredity about fifty years later, and for the project to map all human genes about fifty years after that. Basic principles of genetics are taught in middle schools today and it is generally accepted that genetic variation plays a role in individual differences in behavior. In this chapter we discuss some ideas about heredity that predate our modern understanding. We also examine the life of two important figures in the history of genetics and describe their contributions.

Have you ever drunk more alcohol than you intended? Or smoked a cigarette after telling yourself that you were quitting smoking? Have you ever gotten a headache because you did not drink your morning coffee? By the time we are adults, nearly all of us have used substances such as alcohol, tobacco, or caffeine. Most of us can control our use of substances, but a sizeable minority develop one or more disorders due to substance use at some point in their lives. You probably know someone who has problems that are a result of substance use.

The first six chapters of this book introduced you to behavior genetics as a field, refreshed your memory about Mendelian and molecular genetics, introduced the research methods of behavior genetics, and discussed how genetic variation can affect the brain structure and function. In the remaining chapters, we explore some of the most important areas of behavior genetic research, consider the future of behavior genetics, and examine ethical questions at the forefront of behavior genetics.