6-1 What are chromosomes, DNA, genes, and the human genome? How do behavior geneticists explain our individual differences?

Our shared brain architecture predisposes some common behavioral tendencies. Whether we live in the Arctic or the tropics, we sense the world, develop language, and feel hunger through identical mechanisms. We prefer sweet tastes to sour. We divide the color spectrum into similar colors. And we feel drawn to behaviors that produce and protect offspring.

Our human family shares not only a common biological heritage—cut us and we bleed—but also common social behaviors. Whether named Gonzales, Nkomo, Smith, or Wong, we start fearing strangers at about eight months, and as adults we prefer the company of those with attitudes and attributes similar to our own. As members of one species, we affiliate, conform, return favors, punish offenses, organize hierarchies of status, and grieve a child’s death. A visitor from outer space could drop in anywhere and find humans dancing and feasting, singing and worshiping, playing sports and games, laughing and crying, living in families and forming groups. We are the leaves of one tree.

But in important ways, we also are each unique. We look different. We sound different. We have varying personalities, interests, and cultural and family backgrounds. What causes our striking diversity? How much of it is shaped by our differing genes, and how much by our environment—by every external influence, from maternal nutrition while in the womb to social support while nearing the tomb? How does our heredity interact with our experiences to create both our universal human nature and our individual and social diversity? Such questions intrigue behavior geneticists.

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 6.1). Altogether, you have 20,000 to 25,000 genes, which can be 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.

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.

67

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 tall and another short, 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.

6-2 How do twin and adoption studies help us understand the effects and interactions of nature and nurture?

To scientifically tease apart the influences of environment and heredity, 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 VERSUS FRATERNAL TWINS Identical (monozygotic) twins develop from a single fertilized egg that splits in two. Thus they are genetically identical—nature’s own human clones (FIGURE 6.2). 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.

68

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 genetically identical twins also behaviorally more similar than fraternal twins? Studies of thousands of twin pairs have found that identical twins are much more alike in extraversion (outgoingness) and neuroticism (emotional instability) than are fraternal twins (Kandler et al., 2011; Laceulle et al., 2011; Loehlin, 2012).

Identical twins, more than fraternal twins, look alike. So, do people’s responses to their looks account for their similarities? No. In one clever study, a researcher compared personality similarity between identical twins and unrelated look-alike pairs (Segal, 2013). Only the identical twins reported similar personalities. Other 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.

SEPARATED TWINS 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:

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 to make this marriage 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 would hardly be weirder than some other reported similarities.

69

Aided by media publicity, 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.

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 note 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 with those of separated identical twins.

The impressive data from personality assessments are clouded by the reunion of many of the separated twins some years before they were tested. And 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.

If genetic influences help explain individual differences, can the same be said of trait differences between groups? Not necessarily. Individual differences in height and weight, for example, are highly heritable; yet nutrition (an environmental factor) rather than genetic influences explains why, as a group, today’s adults are taller and heavier than those of a century ago. The two groups differ, but not because human genes have changed in a mere century’s eyeblink of time. Ditto aggressiveness, a genetically influenced trait. Today’s peaceful Scandinavians differ from their more aggressive Viking ancestors, despite carrying many of the same genes.

BIOLOGICAL VERSUS ADOPTIVE RELATIVES 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, with the exception of identical twins, people who grow up together 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.

70

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 resembled 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 environment seems shockingly modest.

The genetic leash may limit the family environment’s influence on personality, but it does not mean that adoptive parenting is a fruitless venture. As a new adoptive parent, I [ND] especially find it heartening to know that 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!

Moreover, child neglect and abuse and even parental divorce are rare in adoptive homes. (Adoptive parents are carefully screened; biological parents are not.) So it is not surprising that studies have shown that, despite a slightly 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 and raised-apart biological siblings on intelligence tests, and most grew into happier and more stable adults (Kendler et al., 2015; van IJzendoorn et al., 2005). 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.

71

6-3 How do heredity and environment work together?

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.

Genes and environment—nature and nurture—work together, like two hands clapping. 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 produce brown in another.

To say that genes and experience are both important is true. But more precisely, they interact. Imagine two babies, one genetically predisposed to be attractive, sociable, and easygoing, the other less so. Assume further that the first baby attracts more affectionate and stimulating care and so develops into a warmer and more outgoing person. As the two children grow older, the more naturally outgoing child may seek more activities and friends that encourage further social confidence.

72

What has caused their resulting personality differences? Neither heredity nor experience act alone. Environments trigger gene activity. And our genetically influenced traits evoke significant responses in others. Thus, a child’s impulsivity and aggression may evoke an angry response from a parent or teacher, who reacts warmly to well-behaved children in the family or classroom. In such cases, the child’s nature and the adult’s nurture interact. Gene and scene dance together.

Identical twins not only share the same genetic predispositions, they also seek and create similar experiences that express their shared genes (Kandler et al., 2012). Evocative interactions may help explain why identical twins raised in different families have recalled their parents’ warmth as remarkably similar—almost as similar as if they had been raised by the same parents (Plomin et al., 1988, 1991, 1994). Fraternal twins have more differing recollections of their early family life—even if raised in the same family! “Children experience us as different parents, depending on their own qualities,” noted Sandra Scarr (1990).

Recall that genes can be either active (expressed, as the 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. Our experiences create epigenetic marks, which are often organic methyl molecules attached to part of a DNA strand (FIGURE 6.3). 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 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” (Read, 2012).

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 stressed (Champagne et al., 2003; Champagne & Mashoodh, 2009). 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, 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). Such discoveries will be made easier by efforts such as the National Institutes of Health–funded Roadmap Epigenetics Project, a massive undertaking aimed at making epigenetic data publicly available.

So, if Beyoncé and Jay Z’s daughter, Blue Ivy, grows up to be a popular recording artist, should we attribute her musical talent to her “superstar genes”? To her growing up in a musically rich environment? To high expectations? The best answer seems to be “All of the above.” From conception onward, we are the product of a cascade of interactions between our genetic predispositions and our surrounding environments (McGue, 2010). Our genes affect how people react to and influence us. Forget nature versus nurture; think nature via nurture.

73