For evolution by natural selection to work, hereditary material must vary between individuals and such variation must cause individual differences in traits that are directly, or indirectly, involved with reproductive fitness. In other words, genetic differences between individuals are the raw material of evolution, but only if those differences produce differences in traits that are subject to natural selection. Theodosius Dobzhansky’s assertion that biology only makes sense in the context of evolution makes it clear that genetic variation is a cornerstone of the entire field of biology. It is certainly the basis of behavior genetics, where we strive to understand the role of genetic differences in producing individual differences in behavior.

Take a moment to think about cells. Which type of cell is your favorite? If you were to ask biologists, you would likely get a wide variety of responses. Some might have a soft spot in their hearts for red blood cells, or liver cells, or maybe even single-celled organisms like bacteria. However, if you were to ask psychologists which cells they prefer, you would likely learn that most of them favor neurons. Because it is with neural mechanisms that animals access, process, and act on information. Neural activity underlies all behavior, and therefore neurons occupy a privileged position in psychology.

Have you interacted with anyone today? For nearly everyone, the answer to that question is “yes.” We tend to live with or around other people, and our lives are greatly impacted by their behaviors. Our behaviors impact their lives too. Broadly speaking, animals act in ways that enhance their survival and reproduction. When those behaviors involve others of the same species, we define them as social behaviors. Social psychology is the subdiscipline of psychology that studies how people impact the thoughts, feelings, and behaviors of others. Of course, humans are not the only animals affected by the behaviors of others. Social behavior can be studied in many species and is sometimes even studied between species such as humans and dogs.

Behavior genetics addresses questions of fundamental importance to humanity. We seek to understand what makes us unique individuals, but also what we share with our families and with other species. We seek to explain why we tend to act in certain ways while others tend to act differently. We also strive to contribute to the development of treatments for behavioral problems. We ask these and many other questions with a limited set of tools in the human behavior genetic toolbox. Ethical and technological constraints on human behavior genetic research combined with the inherent complexity of pathways from genes to behavior mean that a firm grasp of causality may forever remain out of reach. Even in cases where causality can be known, moral and ethical constraints limit potential applications of human behavior genetic findings. Any such applications to human beings are fraught with concerns about human rights.

Gregor Mendel provided the framework for understanding genetics by systematically observing phenotypes in the context of breeding experiments even though he did not understand the underlying molecular biology. His theoretical model used the abstract notion of “factors” that we have since come to call genes. Quantitative genetics extends Mendel’s theoretical model to include many genes with small effects, called polygenes, to investigate phenotypic resemblance for those with known familial relatedness. Quantitative genetics does not require knowledge of the underlying molecular processes. In this chapter, we begin to focus on the core molecular mechanisms involved in heredity. We describe the molecules of heredity, how they are passed from one generation to the next, and how they participate in a variety of biological systems and functions. By understanding genetics at the molecular level, we will be better prepared to understand the mechanisms by which familial resemblance is achieved.

Mendel’s theoretical model provided a powerful framework for understanding inheritance. However, traits that do not fit neatly into categories, but instead vary along some dimension (e.g., height), present a challenge to the Mendelian genetic model. This chapter discusses how this conundrum was resolved and how the solution led to the establishment of the field of quantitative genetics. Most behavioral traits vary along a dimension, so this issue is particularly relevant to behavior genetics. Next, the chapter introduces the concept of variance partitioning and discusses approaches commonly used in behavior genetics to investigate the roles of genetic and environmental variation on individual differences in behavior. Finally, the chapter discusses what has been one of the most important, and sometimes misunderstood, statistics in behavior genetics: heritability.

If you have read the preceding chapters of this book, you know more about the field of behavior genetics than the average person on the street. But you may be wondering if the body of behavior genetic knowledge has any applications in real life. To be sure, most research in behavior genetics is basic research. In other words, the research is focused on better describing, predicting, and explaining heredity–behavior relations. It is not typically applied research where the focus is on solving practical problems, such as changing behavior.

Have you ever tried to lose weight? Many of us would like to weigh less than we do. On the other hand, some people struggle to gain weight. The topic of weight control is a stark reminder of the complex interplay between biology, psychology, and environment that has major implications for well-being. We may intend to change our weight by altering our behavior, but someone brings donuts, and we feel too tired to exercise after a long day of sitting at a desk. For many of us, it is all too easy to gain weight, and as we age it becomes increasingly difficult to lose it.

Imagine that you are about to give a presentation in front of an auditorium full of people. As you walk to the podium, what physical sensations do you feel? Is your heart pounding? Are your palms sweaty? Do you feel a bit nauseous? What are you thinking about? Are you concerned that you might forget what you are planning to say, or that people will evaluate you negatively? Although it is very common to have such feelings and thoughts at the prospect of speaking in public, there are individual differences such that some people experience few if any negative feelings and thoughts, while others may experience so many that they find it difficult to speak even to a few people. Most of us are somewhere in between (Figure 10.1).

Have you experienced any emotions today? Of course, you have. Perhaps you were irritated when your alarm woke you from a peaceful slumber thereby reminding you to go to class this morning. Maybe your roommate had already made the coffee, which cheered you up slightly. Such feelings are an integral part of human experience. When your emotions are functioning appropriately, they can help you navigate life by giving you a way to assess how the current situation aligns with your goals and to help prepare you to face new challenges. When your emotions are not functioning properly, you may find it difficult to deal with stressful situations, which may lead to social or occupational problems over time.

You probably learned a lot more about individual differences in grade school than you realize. Grade school was the first place where many of us regularly interacted with a relatively large number of human beings. You were able to observe a rich diversity of individual differences, although you probably were not thinking about it in such terms, at the time. Children of the same age can differ in physical traits, such as height, weight, eye color, hair color, and skin color. They can also differ in behavior traits such as coordination, running speed, strength, and stamina. Children can also differ on psychological traits such as anxiety, aggression, extraversion, and impulsivity. Of course, you also probably noticed that grade school students can differ on their success in the classroom.