Although there is essentially an infinite number of phenotypes that a person might be interested in, as a field organized around a common set of methods we’ve noticed over the years that, regardless of the phenotype, some consistencies tend to emerge, in both basic findings and new questions/theories that emerge. In this course, we will refer to these as Themes in Behavior Genetics, and we will use these Themes to organize the many topics (and many, MANY readings and other source materials) that we will cover.

We will refer to these extensively throughout the semester, so keep this list handy.

  • Everything is at least a little bit heritable (that is, more closely genetically related individuals are more similar phenotypically).

  • Nothing is 100% heritable (even identical twins are a little different from one another on anything we can measure).

  • Heritability is caused by many genes of small effect (there is no “gene for” in psychology, human behavior, or indeed ANY trait that commonly varies between people).

  • Correlations between phenotypes are partly due to correlated genetic/heritable influences.

  • Heritability increases through development (at least through middle adulthood).

  • Trait stability or consistency across time within an individual is due to stable genetic influences.

  • Even things we think of as “the environment” show non-zero heritability (example: divorce).

  • Correlations between phenotypes and environments are partly due to correlated genetic/heritable influences.

  • Most environments are not shared between people, even siblings or twins raised together.

  • Abnormal is normal (influences on extremes that we label as “disorders” versus normal-range traits are not different in kind, just amount).

  • Interpretation of results means understanding the strengths and weaknesses of choices made in the design of a study (for example: how to measure a phenotype, how participants were selected or excluded).

  • Phenotypes (means, variances, and presentations) change over development - we cannot use the same measures or interpretations without knowing the developmental context.

  • The development of an individual is not necessarily the same as the average developmental course.

  • Gene-environment interaction (GxE) potentially explains why people exposed to the same environments respond differently.

  • Most behavior genetic research necessarily assumes (because of data limitations) that genetic and environmental influences are independent (that is, uncorrelated).

  • Gene-environment correlation (rGE; the tendency of individuals to be selected or select into environments partly on the basis of their heritable characteristics) exists and, if not accounted for, causes biases in our estimation of heritability and genetic effects.

  • Mating is non-random (although random mating is an assumption of many of our models, again because of data limitations).

  • Population structure, or genetic ancestry, is a major confound in models because individuals are systematically exposed to different environments as a result of their perceived ancestry.

  • Correlation is not sufficient to demonstrate either causation or mechanism.

  • No influences exist in a vacuum: simultaneous gene-environment interplay (rGE & GxE) is almost certainly happening all the time, to all of us, in parallel, in sequence, reciprocally. The True Model (if such a thing exists) is almost certainly more complex than we could ever gather data for and test.

It is not my expectation that you memorize or understand these this first week. The goal now is awareness - these themes will pop up over and over again, throughout the course, across topics both in class and in your paper. Keep them handy as a reference, and know that you’re going to learn so much about all of them by the end of the semester.

Next: 1.3. Additional Readings and Sources

Previous: 1.1. What Is Behavior Genetics?

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