"Behavioral genetics" does not describe a single field with a single set of methodological tools, nor does it describe a single explanatory project. Rather, different researchers are interested in answering different questions about the relationship(s) between genes, behaviors, and development, and they use different methodologies to answer their questions. The same diversity holds for human behavioral genetics: different researchers are interested in different questions, and in attempting to answer those questions they use different approaches.
At the broadest level, one can distinguish between (1) research into the differences in behaviors between different individuals and (2) research into behaviors shared by (most) individuals. It is obvious that some traits vary between people. Different people tend to act differently--when, for example, someone is said to be shy, it follows that, in general, they act differently at parties than people who are said to be gregarious. It is equally obvious that some traits do not vary much between people--although different people may speak different languages, all normal human adults (unlike other animals) use some complex language and learn that language while growing up.
Researchers interested in the differences within a population will focus on the variation within that population. For example, within normal human populations, some people are taller than others, some people score higher on standardized intelligence tests than others, and some are more prone to violent behavior than others. Researchers interested in such differences attempt to discover how these differences are associated with the presence or absence of particular genes or environments. In other words, are particular genes associated with being more (rather than less) prone to violence? Do particular environments in which children grow up result in their being more (rather than less) likely to score highly on intelligence quotient (IQ) tests?
More generally, such research focuses on particular differences in the resources used in organismal development. Humans, for example, develop over time from a single fertilized egg to an adult capable of a variety of complex behaviors, behaviors that require a body consisting of an astonishingly complex organization of many different types of cells. The development of any complex organism requires a variety of resources. Some of these resources are genetic (the genetic material inherited from the parents), some are environmental (from the prenatal environment of the mother, to the provision of food, and so forth), and some are hard to classify (the complex subcellular systems that, in conjunction with genes, make proteins, etc.). The outcome of this development is a complex organism that differs from (and, of course, resembles) other organisms in the population in a variety of ways. The goal of research focused on differences is to find ways to associate different phenotypes with differences in how the organisms developed--whether different phenotypes had, for example, different genes or experienced different environments. (1) In these projects, the hope is that researchers will be able to explain how differences in available resources produce different outcomes.
More specifically, human behavioral genetics research that is focused on variation in human behavioral tendencies tries to associate different behavioral tendencies with genetic differences. It asks, for instance, if people who are more prone to violent behavior are also more likely to have certain genes, or if people who tend to score highly on standardized intelligence tests also share particular genetic traits.
On the other hand, researchers interested in behaviors that do not vary significantly within a population have other goals. In the study of behaviors shared by (most) humans, the purpose is to figure out how particular traits are produced in normal development. …