Gene-Environment Interaction
Among our similarities, the most important—the behavioral hallmark of our species—is our enormous adaptive capacity. Some human traits 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. Gene-environment interaction studies are revealing, for example, who is most at risk of permanent harm from stress or abuse and who is most likely to benefit from interventions (Manuck & McCaffery, 2014).
“ Men’s natures are alike; it is their habits that carry them far apart.”
Confucius, Analects, 500 B.C.E.
“ Heredity deals the cards; environment plays the hand.”
Psychologist Charles L. Brewer (1990)
Molecular Behavior Genetics
Behavior geneticists have progressed beyond asking “Do genes influence behavior?” The newest 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 Behavior
Most 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.” Some genes influence how quickly the stomach tells the brain, “I’m full” (Adetunji, 2014). 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.
Epigenetics: Triggers That Switch Genes On and Off
Genes can be either active (expressed, as in the example of hot water activating a 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.
Gene-environment interaction Biological appearances have environmental consequences. Basketball star Lebron James (right) looked like a basketball player from an early age, and those around him probably encouraged his interest and hard work in the sport. For popular comedian Kevin Hart (left), his shorter stature meant that he was less likely to be encouraged toward basketball, but his clever style and winning smile were rewarded on the comedy stage.
Our experiences also create epigenetic marks, which are often organic methyl molecules attached to part of a DNA strand. 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 (Figure 14.3). As one geneticist explained, “Things written in pen you can’t change. That’s DNA. Things written in pencil you can. That’s epigenetics” (Reed, 2012).
Figure 14.3 Epigenetics influences gene expression
Beginning in the womb, life experiences lay down epigenetic marks—often organic methyl molecules—that can affect the expression of any gene in the DNA segment they affect.
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 in experiments, infant rats 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 reactive (Champagne et al., 2003; Champagne & Mashoodh, 2009).
Epigenetics provides a mechanism by which the effects of childhood trauma, poverty, or malnutrition may last a lifetime (Nugent et al., 2016; Peter et al., 2016; Swartz et al., 2016). Our life events can mark us at the deepest level. Child abuse may leave its fingerprints in a person’s genome. Moreover, it now appears that some epigenetic changes are passed down to future generations. In one experiment, mice whose grandparents learned to associate the smell of orange blossoms with electric shock were themselves startled when first exposed to the scent (Dias & Ressler, 2014). Conceivably, your health and well-being could be affected by stresses or pollutants that your parent or even grandparent experienced (McCarrey, 2015; Skinner, 2014; Yehuda et al., 2016). In one study, Holocaust trauma survivors shared epigenetic alterations with their offspring (Yehuda et al., 2016).
Epigenetics research may solve some scientific mysteries, such as why only one member of an identical twin pair may develop a genetically influenced mental disorder (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).