4-1 What are chromosomes, DNA, genes, and the human genome? How do behavior geneticists explain our individual differences?

If Chelsea Clinton, daughter of former President Bill Clinton and former Secretary of State Hillary Rodham Clinton, becomes a politician, should we attribute her superior political talent to her “White House genes”? To growing up in a politically savvy environment? To high expectations? Such questions intrigue behavior geneticists, who study our differences and weigh the effects and the interplay of heredity and environment.

Behind the story of our body and of our brain—surely the most awesome thing on our little planet—is the heredity that interacts with our experience to create both our universal nature and our individual and social diversity. Barely more than a century ago, few would have guessed that every cell nucleus in your body contains the genetic master code for your entire body. It’s as if every room in Dubai’s Burj Khalifa (the world’s tallest building) contained a book detailing the architect’s plans for the entire structure. The plans for your own book of life run to 46 chapters—23 donated by your mother’s egg and 23 by your father’s sperm. Each of these 46 chapters, called a chromosome, is composed of a coiled chain of the molecule DNA (deoxyribonucleic acid). Genes, small segments of the giant DNA molecules, form the words of those chapters (FIGURE 4.1). All told, you have 20,000 to 25,000 genes, which are either active (expressed) or inactive. Environmental events “turn on” genes, rather like hot water enabling a tea bag to express its flavor. When turned on, genes provide the code for creating protein molecules, our body’s building blocks.

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Genetically speaking, every other human is nearly your identical twin. Human genome researchers have discovered the common sequence within human DNA. This shared genetic profile makes us humans, rather than tulips, bananas, or chimpanzees.

We aren’t all that different from our chimpanzee cousins. At a genetic level, humans and chimpanzees are 96 percent identical (Mikkelsen et al., 2005). At “functionally important” DNA sites, this number reaches 99.4 percent (Wildman et al., 2003)! Yet that wee 0.6 percent difference matters. Shakespeare intricately wove 17,677 words into his literary masterpieces. Despite some remarkable abilities, chimpanzees do not compose sonnets.

Small differences matter among other species, too. Common chimpanzees and bonobos resemble each other in many ways. They should—their genomes differ by much less than 1 percent. But they display markedly differing behaviors. Chimpanzees are aggressive and male dominated. Bonobos are peaceable and female led.

The occasional variations found at particular gene sites in human DNA fascinate geneticists and psychologists. Slight person-to-person variations from the common pattern give clues to our uniqueness—why one person has a disease that another does not, why one person is short and another tall, why one is anxious and another calm.

Most of our traits have complex genetic roots. How tall you are, for example, reflects the size of your face, vertebrae, leg bones, and so forth—each of which may be influenced by different genes interacting with your specific environment. Traits such as intelligence, happiness, and aggressiveness are similarly influenced by groups of genes. Thus, our genes help explain both our shared human nature and our human diversity. But knowing our heredity tells only part of our story. To form us, environmental influences interact with our genetic predispositions.

4-2 How do twin and adoption studies help us understand the effects and interactions of nature and nurture?

To scientifically tease apart the influences of heredity and environment, behavior geneticists could wish for two types of experiments. The first would control heredity while varying the home environment. The second would control the home environment while varying heredity. Although such experiments with human infants would be unethical, nature has done this work for us.

Identical (monozygotic) twins develop from a single fertilized egg that splits in two. Thus they are genetically identical—nature’s own human clones (FIGURE 4.2). Indeed, they are clones who share not only the same genes but the same conception and uterus, and usually the same birth date and cultural history. Two slight qualifications:

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Fraternal (dizygotic) twins develop from two separate fertilized eggs. As womb-mates, they share a prenatal environment, but they are genetically no more similar than ordinary brothers and sisters.

Shared genes can translate into shared experiences. A person whose identical twin has autism spectrum disorder, for example, has about a 3 in 4 risk of being similarly diagnosed. If the affected twin is fraternal, the co-twin has about a 1 in 3 risk (Ronald & Hoekstra, 2011). To study the effects of genes and environments, hundreds of researchers have studied some 800,000 identical and fraternal twin pairs (Johnson et al., 2009).

Are identical twins also behaviorally more similar than fraternal twins? Studies of thousands of twin pairs in Germany, Australia, and the United States have found that on the personality traits of extraversion (outgoingness) and neuroticism (emotional instability) identical twins report much greater similarity than do fraternal twins (Kandler et al., 2011; Laceulle et al., 2011; Loehlin, 2012). Genes also influence many specific behaviors. For example, compared with rates for fraternal twins, drinking and driving convictions are 12 times greater among those who have an identical twin with such a conviction (Beaver & Barnes, 2012). As twins grow older, their behaviors remain similar (McGue & Christensen, 2013).

Identical twins, more than fraternal twins, also report being treated alike. So, do their experiences rather than their genes account for their similarities? No. Studies have shown that identical twins whose parents treated them alike (for example, dressing them identically) were not psychologically more alike than identical twins who were treated less similarly (Kendler et al., 1994; Loehlin & Nichols, 1976). In explaining individual differences, genes matter.

Imagine the following science fiction experiment: A mad scientist decides to separate identical twins at birth, then raise them in differing environments. Better yet, consider a true story:

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On a chilly February morning in 1979, some time after divorcing his first wife, Linda, Jim Lewis awoke in his modest home next to his second wife, Betty. Determined that this marriage would work, Jim made a habit of leaving love notes to Betty around the house. As he lay in bed he thought about others he had loved, including his son, James Alan, and his faithful dog, Toy.

Jim looked forward to spending part of the day in his basement woodworking shop, where he enjoyed building furniture, picture frames, and other items, including a white bench now circling a tree in his front yard. Jim also liked to spend free time driving his Chevy, watching stock-car racing, and drinking Miller Lite beer.

Jim was basically healthy, except for occasional half-day migraine headaches and blood pressure that was a little high, perhaps related to his chain-smoking habit. He had become overweight a while back but had shed some of the pounds. Having undergone a vasectomy, he was done having children.

What was extraordinary about Jim Lewis, however, was that at that same moment (we are not making this up) there existed another man—also named Jim—for whom all these things (right down to the dog’s name) were also true.¹ This other Jim—Jim Springer—just happened, 38 years earlier, to have been his fetal partner. Thirty-seven days after their birth, these genetically identical twins were separated, adopted by blue-collar families, and raised with no contact or knowledge of each other’s whereabouts until the day Jim Lewis received a call from his genetic clone (who, having been told he had a twin, set out to find him).

One month later, the brothers became the first of many separated twin pairs tested by University of Minnesota psychologist Thomas Bouchard and his colleagues (Miller, 2012). The brothers’ voice intonations and inflections were so similar that, hearing a playback of an earlier interview, Jim Springer guessed “That’s me.” Wrong—it was Jim Lewis. Given tests measuring their personality, intelligence, heart rate, and brain waves, the Jim twins—despite 38 years of separation—were virtually as alike as the same person tested twice. Both married women named Dorothy Jane Scheckelburger. Okay, the last item is a joke. But as Judith Rich Harris (2006) has noted, it is hardly weirder than some other reported similarities.

Aided by publicity in magazine and newspaper stories, Bouchard (2009) and his colleagues located and studied 74 pairs of identical twins raised apart. They continued to find similarities not only of tastes and physical attributes but also of personality (characteristic patterns of thinking, feeling, and acting), abilities, attitudes, interests, and even fears.

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In Sweden, researchers identified 99 separated identical twin pairs and more than 200 separated fraternal twin pairs (Pedersen et al., 1988). Compared with equivalent samples of identical twins raised together, the separated identical twins had somewhat less identical personalities. Still, separated twins were more alike if genetically identical than if fraternal. And separation shortly after birth (rather than, say, at age 8) did not amplify their personality differences.

Stories of startling twin similarities have not impressed critics, who remind us that “The plural of anecdote is not data.” They have noted that if any two strangers were to spend hours comparing their behaviors and life histories, they would probably discover many coincidental similarities. If researchers created a control group of biologically unrelated pairs of the same age, sex, and ethnicity, who had not grown up together but who were as similar to one another in economic and cultural background as are many of the separated twin pairs, wouldn’t these pairs also exhibit striking similarities (Joseph, 2001)? Twin researchers have replied that separated fraternal twins do not exhibit similarities comparable to those of separated identical twins.

Even the impressive data from personality assessments are clouded by the reunion of many of the separated twins some years before they were tested. Moreover, identical twins share an appearance, and the responses it evokes. Adoption agencies also tend to place separated twins in similar homes. Despite these criticisms, the striking twin-study results helped shift scientific thinking toward a greater appreciation of genetic influences.

For behavior geneticists, nature’s second real-life experiment—adoption—creates two groups: genetic relatives (biological parents and siblings) and environmental relatives (adoptive parents and siblings). For personality or any other given trait, we can therefore ask whether adopted children are more like their biological parents, who contributed their genes, or their adoptive parents, who contribute a home environment. While sharing that home environment, do adopted siblings also come to share traits?

The stunning finding from studies of hundreds of adoptive families is that people who grow up together, whether biologically related or not, do not much resemble one another in personality (McGue & Bouchard, 1998; Plomin, 2011; Rowe, 1990). In personality traits such as extraversion and agreeableness, people who have been adopted are more similar to their biological parents than to their caregiving adoptive parents.

The finding is important enough to bear repeating: The environment shared by a family’s children has virtually no discernible impact on their personalities. Two adopted children raised in the same home are no more likely to share personality traits with each other than with the child down the block. Heredity shapes other primates’ personalities, too. Macaque monkeys raised by foster mothers exhibited social behaviors that resemble their biological, rather than foster, mothers (Maestripieri, 2003). Add in the similarity of identical twins, whether they grow up together or apart, and the effect of a shared raising environment seems shockingly modest.

What we have here is perhaps “the most important puzzle in the history of psychology,” contended Steven Pinker (2002): Why are children in the same family so different? Why does shared family environment have so little effect on children’s personalities? Is it because each sibling experiences unique peer influences and life events? Because sibling relationships ricochet off each other, amplifying their differences? Because siblings—despite sharing half their genes—have very different combinations of genes and may evoke very different kinds of parenting? Such questions fuel behavior geneticists’ curiosity.

The genetic leash may limit the family environment’s influence on personality, but it does not mean that adoptive parenting is a fruitless venture. Parents do influence their children’s attitudes, values, manners, politics, and faith (Reifman & Cleveland, 2007). Religious involvement is genetically influenced (Steger et al., 2011). But a pair of adopted children or identical twins will, especially during adolescence, have more similar religious beliefs if raised together (Koenig et al., 2005). Parenting matters!

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Moreover, in adoptive homes, child neglect and abuse and even parental divorce are rare. (Adoptive parents are carefully screened; natural parents are not.) So it is not surprising that studies have shown that, despite a somewhat greater risk of psychological disorder, most adopted children thrive, especially when adopted as infants (Loehlin et al., 2007; van IJzendoorn & Juffer, 2006; Wierzbicki, 1993). Seven in eight adopted children have reported feeling strongly attached to one or both adoptive parents. As children of self-giving parents, they have grown up to be more self-giving and altruistic than average (Sharma et al., 1998). Many scored higher than their biological parents on intelligence tests, and most grew into happier and more stable adults. In one Swedish study, children adopted as infants grew up with fewer problems than were experienced by children whose biological mothers initially registered them for adoption but then decided to raise the children themselves (Bohman & Sigvardsson, 1990). Regardless of personality differences between adoptive family members, most adopted children benefit from adoption.

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4-3 What have psychologists learned about temperament?

As most parents will tell you after having their second child, babies differ even before gulping their first breath. One aspect of personality—temperament (emotional reactivity and excitability) is quickly apparent, and it is genetically influenced (Kandler et al., 2013; Rothbart, 2007). From their first weeks of life, some infants are noticeably difficult—irritable, intense, fidgety, and unpredictable. Others are easy—cheerful and relaxed, feeding and sleeping on predictable schedules. Still others tend to be slow to warm up, resisting or withdrawing from new people and situations (Chess & Thomas, 1987; Thomas & Chess, 1977).

Temperament differences typically persist. Consider:

The genetic effect appears in physiological differences. Anxious, inhibited infants have high and variable heart rates and a reactive nervous system. When facing new or strange situations, they become more physiologically aroused (Kagan & Snidman, 2004; Roque et al., 2012). One form of a gene that regulates the neurotransmitter serotonin predisposes a fearful temperament and, in combination with unsupportive caregiving, an emotionally reactive child (Raby et al., 2012).

4-4 What is heritability, and how does it relate to individuals and groups?

So our biology helps form our personality. Yet asking whether our personality is more a product of our genes or our environment is like asking whether a flat-screen TV’s size is more the result of its length or its width. We could, however, ask whether the different TV sizes are more the result of differences in their length or differences in their width. Similarly, we can ask whether person-to-person personality differences are influenced more by nature or by nurture.

Using twin and adoption studies, behavior geneticists can mathematically estimate the heritability of a trait—the extent to which variation among individuals can be attributed to their differing genes. By one estimate, the heritability of general intelligence is 66 percent (Haworth et al., 2010). This does not mean that your intelligence is 66 percent genetic. (The heritability of height is 90 percent, but this does not mean that a 60-inch-tall woman can credit her genes for 54 inches and her environment for the other 6 inches.) Rather, it means that genetic influence explains about 66 percent of the observed variation among people. This point is so often misunderstood that we repeat: We can never say what percentage of an individual’s personality or intelligence is inherited. It makes no sense to say that your personality is due x percent to your heredity and y percent to your environment. Heritability refers instead to the extent to which differences among people are due to genes.

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Even this conclusion must be qualified, because heritability can vary from study to study. Consider humorist Mark Twain’s (1835–1910) fictional idea of raising boys in barrels to age 12, feeding them through a hole. If we were to follow his suggestion, the boys would all emerge with lower-than-normal intelligence scores at age 12. Yet, given their equal environments, their test score differences could be explained only by their heredity. With the same environment, heritability—differences due to genes—would be near 100 percent.

As environments become more similar, heredity becomes the primary source of differences. If all schools were of uniform quality, all families equally loving, and all neighborhoods equally healthy, then heritability would increase (because differences due to environment would decrease). But consider the other extreme: If all people had similar heredities but were raised in drastically different environments (some in barrels, some in luxury homes), heritability would be much lower.

If genetic influences help explain variations in traits among individuals in a group, can the same be said of trait differences between groups? Not necessarily. As we have seen, height is 90 percent heritable, yet nutrition (an environmental factor) rather than genetic influences explains why, as a group, today’s adults are taller than those of a century ago. More available food has caused Americans to grow to greater heights (Floud et al., 2011). In 1850, the average American male stood 5 feet 7 inches; in the 1980s, his counterpart stood three inches taller. The two groups differ, but not because human genes have changed in a mere century’s eyeblink of time. And today’s South Koreans, with their better diets, average six inches taller than today’s North Koreans, who come from the same genetic stock (Johnson et al., 2009). Genes matter, but so does environment.

As with height and weight, so with personality and intelligence scores: Heritable individual differences need not imply heritable group differences. If some individuals are genetically disposed to be more aggressive than others, that needn’t explain why some groups are more aggressive than others. Putting people in a new social context can change their aggressiveness. Today’s peaceful Scandinavians carry many genes inherited from their Viking warrior ancestors.

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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.

4-5 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 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.” 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 Off Genes 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 4.3). 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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