Among our similarities, the most important—the behavioral hallmark of our species—is our enormous adaptive capacity. Some human traits, such as having two eyes, develop the same in virtually every environment. But other traits are expressed only in particular environments. Go barefoot for a summer and you will develop toughened, callused feet—a biological adaptation to friction. Meanwhile, your shod neighbor will remain a tenderfoot. The difference between the two of you is an effect of environment. But it is also the product of a biological mechanism—adaptation. Our shared biology enables our developed diversity (Buss, 1991). Thus, to say that genes and experience are both important is true. But more precisely, they interact. Just how our genes and our experiences interact to form us as unique individuals is one of the hottest topics in psychology today.

11-5 How is molecular genetics research changing our understanding of the effects of nature and nurture?

Behavior geneticists have progressed beyond asking “Do genes influence behavior?” The new frontier of behavior-genetic research draws on “bottom-up” molecular genetics, which studies the molecular structure and function of genes.

Searching for Specific Genes Influencing BehaviorMost human traits are influenced by teams of genes. For example, twin and adoption studies tell us that heredity influences body weight, but there is no single “obesity gene.” More likely, some genes influence how quickly the stomach tells the brain, “I’m full.” Others might dictate how much fuel the muscles need, how many calories are burned off by fidgeting, and how efficiently the body converts extra calories into fat (Vogel, 1999). Genes typically are not solo players. So, one goal of molecular behavior genetics is to find some of the many genes that together orchestrate complex traits such as body weight, sexual orientation, and impulsivity (Derringer et al., 2010; Holden, 2008; Tsankova et al., 2007).

Genetic tests can now reveal at-risk populations for dozens of diseases, and the search continues. (For another aspect of genetic testing, see Thinking Critically About: Prenatal Testing to Predict Future Traits.)

Searching for Triggers That Switch Genes On and OffGenes can be either active (expressed, as hot water activates the tea bag) or inactive. Epigenetics (meaning “in addition to” or “above and beyond” genetics), studies the molecular mechanisms by which environments can trigger or block genetic expression. Genes are self-regulating. Rather than acting as blueprints that lead to the same result no matter the context, genes react. An African butterfly that is green in summer turns brown in fall, thanks to a temperature-controlled genetic switch. The same genes that produced green in one situation will produce brown in another.

Our experiences also lay down epigenetic marks, which are often organic methyl molecules attached to part of a DNA strand (FIGURE 11.3). If a mark instructs the cell to ignore any gene present in that DNA segment, those genes will be “turned off”—they will prevent the DNA from producing the proteins normally coded by that gene. As one geneticist said, “Things written in pen you can’t change. That’s DNA. Things written in pencil you can. That’s epigenetics” (Reed, 2012).

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Environmental factors such as diet, drugs, and stress can affect the epigenetic molecules that regulate gene expression. Mother rats normally lick their infants. Deprived of this licking, infant rats in one experiment had more epigenetic molecules blocking access to their brain’s “on” switch for developing stress hormone receptors. When stressed, those animals had above-average levels of free-floating stress hormones and were more stressed out (Champagne et al., 2003; Champagne & Mashoodh, 2009).

Researchers now wonder if epigenetics might help solve some scientific mysteries, such as why only one member of an identical twin pair may develop a genetically influenced mental disorder, and how childhood abuse leaves its fingerprints in a person’s brain (Spector, 2012). Epigenetics can also help explain why identical twins may look slightly different. Researchers studying mice have found that in utero exposure to certain chemicals can cause genetically identical twins to have different-colored fur (Dolinoy et al., 2007). These discoveries will be made easier by efforts such as the National Institute of Health-funded Roadmap Epigenetics Project, a massive undertaking aimed at making epigenetic data publicly available.

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