Thus far, we primarily have examined research on psychological characteristics: personality, psychological disorders, social attitudes, intelligence. Results reveal three facts about nature, nurture, and the psychology of people:

If you’re thinking like a scientist, the question you’re asking is “why?” If we shift from a psychological to a biological level of analysis, can we explain why people’s psychological qualities are affected by nature, nurture, and nature–nurture interactions?

Yes, we can, thanks to research on genes and gene expression.

Your body’s cells are, in some ways, like books. Books contain a chemical substance: ink. The ink, thanks to its arrangement on the page, conveys information—stories, facts, instructions. Your body’s cells contain a chemical substance called deoxyribonucleic acid, or DNA. The DNA, thanks to its physical arrangement, contains information—the information needed to build the various parts of your body.

DNA AND GENES. The main ingredient needed to build body parts is protein molecules. Your body assembles protein molecules by following instructions encoded into DNA. This information is contained in the long sequence of chemical elements that make up the DNA molecule.

Any section of DNA that provides the full information needed to produce a protein is called a gene. Genes thus are the basic units of inheritance: the instructions used to build the body’s parts. The entire set of genes—the complete hereditary information—possessed by an organism is called the genome.

In the cells of the body, genes are contained within chromosomes, biological structures that consist of highly organized, tightly packed strings of molecules (Figure 4.7). The cells of your body contain 46 chromosomes, which are located in the nucleus of the cells. Chromosomes are so small that 46 of them easily fit within a cell’s nucleus. Yet they are so tightly packed that if you unwound any one chromosome and stretched it end to end, it would be roughly 6 feet long (National Institutes of General Medical Sciences, 2006).

We truly are a single human family; the genome is extremely similar from one person to the next. More than 99% of the sequences of biological molecules in DNA are the same in all persons, in all cultures of the world (National Institutes of General Medical Sciences, 2006). Yet there are variations. A given gene can come in different varieties, known as alleles. Different alleles produce different versions of a characteristic. For example, a gene that produces a chemical affecting bodily coloration comes in different alleles that are responsible for blue versus brown eye color (Sturm et al., 2008). Allelic variations in other genes are responsible for different human blood types.

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GENE EXPRESSION. There is a second way in which your body’s cells are like books. The information in a book has no effect unless the book is opened, read, and acted upon. A cookbook with a cake recipe will not, by itself, produce any cakes; for that, ingredients must be assembled according to the recipe’s instructions. Similarly, when the body wants to build a protein, the “recipe”—the information in DNA—cannot, by itself, do the job of producing protein molecules. The information in DNA needs to be read, ingredients must be gathered, and the ingredients must be assembled according to the DNA’s instructions (Slavich & Cole, 2013).

This assembly process—the set of steps from DNA recipe to the production of a protein molecule—is known as gene expression (Vogel & Motulsky, 1997). Here’s how it works:

What does the biology of gene expression have to do with the psychology of nature and nurture? Quite a lot! The timing of gene expression—that is, when the gene expression process occurs—is determined, in part, by the environment. Nature and nurture, then, directly interact. Environmental experiences (nurture) affect the biology of the organism (its biological nature) by affecting gene expression. These nature–nurture interactions are explored in a relatively new field called epigenetics.

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EPIGENETICS. In the field of epigenetics, researchers study how environmental experience “switches on” genes, making them active in the production of proteins (Masterpasqua, 2009). The switching-on process occurs in two steps:

As a result of this two-step process, nurture (environmental experience) affects biological nature (the biological structure of the organism).

Epigenetic principles have a fascinating implication: People with identical DNA may differ biologically if they experience environments that prompt different types of gene expression. You can see this in the accompanying photo. The two men differ physically; one is markedly taller and heavier. The difference might lead you to guess they are unrelated. But, in reality, they’re identical twins. After being separated at birth, the man on the right was raised in a normal caring environment, whereas the man on the left was raised by “a neurotic and cruel relative” (Tanner, 1978, p. 121) who failed to provide adequate nutrition. The result was biological differences despite identical genes.

This example should remind you of HT1, HT2, and HM, the triplets described in this chapter’s opening story. HM differed in behavioral style and sexual orientation from his brothers. Epigenetic processes could have caused the differences. Although HM had the same DNA as his brothers, environmental experiences could have “switched on” his genes in a way that differed from gene activation in HT1 and HT2.

The lesson? DNA is not destiny. Genes alone do not determine biological outcomes. Instead, in the development of the organism, genetic and environmental influences combine through epigenetic processes. Let’s see some examples of epigenetics in action.

Epigenetic research has identified numerous factors that influence gene expression. For instance, stress, poor nutrition, loneliness, and parenting styles can affect whether, and when, cells use the information in DNA to produce proteins (Cole, 2009; Gottlieb, 1998; McEwen et al., 2012). One major focus of such research has been social relationships and physical health.

Researchers have long known that social factors influence physical health (Uchino, Cacioppo, & Kiecolt-Glaser, 1996). Social isolation impairs health. Conversely, people with many close personal relationships tend to have fewer diseases and to live longer. Why? To find out, researchers (Cole et al., 2007) obtained two key pieces of information from a group of research participants: (1) level of social isolation, as measured by a self-report survey in which people indicated whether they felt isolated from others and lacking in companionship, and (2) gene expression, as measured through a molecular genetic analysis of biological samples obtained from each participant. Data analysis revealed a strong association between social isolation and gene expression. Specifically, socially isolated individuals differed from others in the activity of genes that influence the immune system, the body’s inner biological defense system; social isolation impaired the functioning of their immune systems, putting them at risk for poorer health.

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Research on parenting (Champagne & Curley, 2009) provides another example of how experiences can affect gene expression and biological structure. The parents studied in this work were not human but rodent; laboratory rats, like people, have offspring that require care, and basic principles discovered in one species may generalize to another. When starting their work, the researchers knew that not all rat mothers are alike. Some attend closely to their infant pups, frequently licking and grooming them, whereas others spend much less time physically interacting with their offspring. The researchers also knew that the mothers’ actions could predict the rat pups’ later behavior; offspring of mothers who do a lot of licking and grooming tend to develop into adult rats that are relatively less fearful than others. Through what process, the researchers asked, could licking and grooming in infancy affect emotions and behaviors in adulthood? The answer turned out to be gene expression (Weaver, Meaney, & Szyf, 2006). Frequent grooming and licking during the first week of life altered chemical processes in cells. These alterations affected gene expression. The altered levels of gene expression, in turn, affected the biological and behavioral development of the baby rats.

RESEARCH TOOLKIT

GENE–ENVIRONMENT INTERACTION AND DEPRESSION. Since you’re probably more interested in people than rat pups, let’s see how interactions among genes and the environment affect psychological development in our species. Evidence comes from a remarkable, long-term investigation involving more than a thousand people in New Zealand who were studied from early childhood through early adulthood (Caspi et al., 2003). A main branch of this research project explored nature, nurture, and the development of depression.

The researchers measured two factors that might predict the development of depression, one genetic and the other environmental. The genetic factor was variation in—that is, different alleles of—a gene that affects the activity of the brain chemical serotonin. Serotonin is one of the many biochemicals that brain cells use to communicate with one another (see Chapter 3). The environmental factor involved life stress. The researchers measured the number of highly stressful life events (e.g., breakup of a significant relationship; loss of a job; homelessness) each research participant experienced.

Key to this study was that, as in other studies reviewed earlier in this chapter, the researchers did not ask the simple question, “Nature versus nurture: Which is more important?” Instead, they explored how genes and environments work together in helping people avoid depression or, alternatively, in contributing to the development of depressive symptoms.

Figure 4.8 shows how genetic and environmental effects combined to predict the development of one symptom of severe depression, suicidal behaviors and thoughts (Caspi et al., 2003). Were genes influential? Sometimes yes, and sometimes no. Among people who experienced few stressful life events, genetic variations hardly made any difference. In low-stress environments, few people contemplate suicide. Was the environment influential? Again, sometimes yes, and sometimes no. People with one type of genetic makeup rarely contemplated suicide, no matter how much stress they experienced; their genetic makeup seemed to protect them against developing depression. What was most influential was a combination of genetic background and environmental experience. Specifically, people whose (1) biological nature included a particular genetic allele (the short/short allele) and whose (2) nurturing included larger numbers of stressful events were far more likely than others to become depressed and to consider suicide (Caspi et al., 2003).

The findings, which have been replicated by others (Karg et al., 2011), show how nature and nurture combine in the development of depression. People who experience high levels of environmental stress and who, due to genes that affect serotonin levels, are biologically inclined to react strongly to stressors are at greatest risk.

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In summary, contemporary research shows not only that genes are important, but also that their importance, and basic workings, are best understood by looking at how genetic and environmental factors combine in the psychological development of the individual.

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