Why do people differ so much? Some are outgoing and others shy, some are intelligent and others less so, some are politically conservative and others liberal. One possible explanation is inheritance. In the field of behavior genetics (Kim, 2009), researchers try to determine the degree to which individual differences in the behavior of animals (including humans) are inherited—that is to say, are due to genes. Key to these efforts is the study of twins.

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Twins are just what the behavior geneticist needs: people whose degree of genetic overlap is known. Humans share most genes, yet we all differ from one another a little bit; you aren’t exactly the same, genetically, as your friends and neighbors. Finding out exactly how much your genes that can vary overlap with these other people would require a lot of work: time-consuming analysis of genetic material. With twins, however, the degree of overlap is known: 100% for MZ twins, and 50% for DZ twins (but see This Just In).

MZ and DZ twins’ varying genetic overlap results from differing biological events at the point of conception (Figure 4.5):

Knowing the degree of genetic overlap makes life easy for the scientist. Through a relatively simple procedure known as the twin method, researchers can compute the degree to which genes account for individual differences. Let’s see how the twin method works.

The study of twins is similar—not identical, but similar—to an experiment. In an experiment, one factor varies (the level of the independent variable in one condition versus another) and others are controlled. In the twin method:

In an experiment, the researcher manipulates the independent variable. In a twin study, the variation of course occurs naturally. But in both cases, a researcher can determine whether the varying factor influences the outcome of the study, that is, its dependent variable.

In a twin study, the key outcome is the degree to which the co-twins resemble one another psychologically. If MZ twins are more similar to one another than DZ twins, genes are the likely cause.

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In the twin method, researchers determine the degree to which genes contribute to individual differences by following a three-step procedure:

The difference between the MZ and DZ correlations indicates the degree of heritability of the psychological characteristic that was measured. Heritability is the degree to which individual differences in a characteristic—that is, the overall degree of person-to-person variation on that characteristic—are explained by genetic factors. If individual differences are caused entirely by genes, heritability is 100%. If genes have no effect—that is, if nurture, rather than nature, creates individual differences—then the psychological quality is not heritable, which is to say, heritability is 0%.

Let’s see what heritability actually turns out to be by examining some research results.

A great many psychological qualities are heritable. Twin studies commonly find that identical twins are more similar psychologically than are fraternal twins. Examples come from research on (1) personality traits, (2) psychological disorders, and (3) social and political attitudes.

PERSONALITY TRAITS. Genes contribute to individual differences in personality traits, that is, people’s typical styles of behavior and emotion (see Chapter 13). Within some populations and for some traits, heritability is about 50%; roughly half of the person-to-person variability in personality traits is explained by genetics (Goldsmith, 1983).

Consider a study with an extraordinarily large population: 12,898 pairs of twins. They were part of a national twin registry in the nation of Sweden (Floderus-Myrhed, Pederson, & Rasmuson, 1980). The twins filled out questionnaires measuring two common personality traits: (1) extraversion, the degree to which a person is outgoing and sociable, and (2) neuroticism, a person’s tendency to be anxious and to experience mood swings. When researchers executed the three-step twin method described above, they found that, for both extraversion and neuroticism, the MZ correlations were about twice as large as the DZ correlations. Overall, genetics accounted for half of the variation in personality.

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In this large twin study, virtually all of the twins were raised together in the same household. What happens if they are raised separately? Researchers at the University of Minnesota (Bouchard, Lykken, & McGue, 1990) have studied more than 100 sets of twins who, due to adoption, were raised by different parents. If genes had no effect on personality, these twins might turn out to be no more similar than unrelated people. But findings show that they are similar, even though they were not raised together. On various personality measures, the correlation between the scores of identical twins raised apart was approximately r = .5—about as large as the correlation for identical twins raised together (Bouchard et al., 1990). Some studies have found that twins raised together are more similar than those raised apart; nonetheless, even if raised in different households, identical twins are far more similar to one another than are unrelated individuals (Pederson et al., 1988).

The heritability of personality, then, is much greater than 0%. However, it’s also much less than 100%. Identical twins raised in the same household are far from psychologically identical; the correlation between their personalities rarely exceeds r = .5 (by comparison, the identical-twin correlation for height exceeds .9; Silventoinen et al., 2008). This means that, in addition to genes, environments also profoundly affect personality. Specifically, environmental factors can cause siblings in the same household to differ from one another (Plomin & Daniels, 1987).

Have you ever known two siblings who differ so much that you can hardly believe they are from the same family? Such differences are common. Different peer groups, different perceptions of the parents, and different parental behavior toward individual children all contribute to differences between siblings. For example, one study showed that mothers of identical twins do not always treat the twins identically; they sometimes respond in more negative, punitive ways to one twin than to the other. These differences in mothers’ style of punishment predict differences in the twins. Twins whose mothers are less punitive tend to experience more positive emotional states (Deater-Deckard et al., 2001).

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PSYCHOLOGICAL DISORDERS. Genes affect not only everyday personality traits. They also contribute to psychological disorders, that is, prolonged experiences of psychological distress that interfere with a person’s everyday life (see Chapter 15). Let’s consider a particularly severe disorder, schizophrenia.

Schizophrenia is a mental illness that creates severe disturbances in thinking and emotion (National Institute of Mental Health [NIMH], 2009). People with schizophrenia lose the normal capacity to understand the world of reality; they may experience visual hallucinations or hear voices. Often, their emotional life is disturbed (see Chapter 16).

Twin studies reveal that schizophrenia is highly heritable. More than 80% of the overall individual differences in schizophrenia (i.e., variation in whether or not a person is suffering from the disease) is caused by genetic factors (Pogue-Geile & Yokley, 2010). The effects of genes are seen by comparing the chances of developing schizophrenia among DZ and MZ twins. If one twin in a DZ twin pair has schizophrenia, there is only about a 1-in-6 chance that the other DZ twin will have the disease. But if one twin in an MZ twin pair suffers from schizophrenia, there is about a 1-in-2 chance—a 50% chance—that the other twin also will develop schizophrenia.

One way that genes are linked to schizophrenia is through working memory, a mental system used to store and manipulate information (Chapter 6). (If you multiply two numbers in your head, you are using your working memory system.) Genetic variations are related to variations in working memory ability (Goldberg et al., 2003). Low working memory ability, in turn, is common among people with schizophrenia.

SOCIAL AND POLITICAL ATTITUDES. Our third psychological characteristic, social and political attitudes, sounds like a product of nurture, not nature. Your attitudes are shaped by friends, neighbors, schools, religious institutions, community leaders, and even campaign ads. Does genetics have any influence?

It turns out that genetics does play a role. People who are more similar genetically tend to have more similar attitudes (Smith et al., 2012). In one study (Eaves et al., 1999), MZ and DZ twins reported their attitudes about social and political issues such as abortion, foreign aid, school prayer, and socialism. MZ twins were more similar than DZ twins; attitudes were heritable. Genes have less of an effect on attitudes than on personality or schizophrenia, and the environmental effects on attitudes are substantial (Eaves et al., 1999). Nonetheless, this study and others (Hatemi et al., 2009) reveal that attitudes are partly heritable.

In some ways, this result is puzzling. Most of the evolution of human genes occurred tens of thousands of years ago, before there were complex societies or political institutions. How could relatively ancient genes affect contemporary political and social attitudes?

The answer is that genes do not affect social attitudes directly. They do so through indirect routes, one of which involves physical sensitivity to threatening noises and images. Researchers (Oxley et al., 2008) compared participants with strong “conservative” political views (e.g., in favor of greater military spending and the use of the death penalty, and support for U.S. actions in the Iraq War) to those with “liberal” views (opposing the same policies). They measured participants’ physiological reactions while exposing them to loud noises and to pictures displaying physical threats (e.g., a picture of a large spider on someone’s face). People with different political views responded differently. Conservatives responded more strongly to the threatening images and noises than did liberals (Oxley et al., 2008).

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Many factors other than genetics also shape political attitudes. Nonetheless, these results help to resolve the puzzle of how genes have any influence. Genetic variations can create individual differences in neural systems in the brain that are active when people respond to threat. These brain-system differences, in turn, contribute to individual differences in emotional response to threat. The differences in emotional response then affect political attitudes; people who respond strongly to threats are more likely to take actions (e.g., going to war) to combat them and thus are more likely to express political views labeled as “conservative” (Figure 4.6).

You’ve just seen that a range of psychological qualities are at least partly heritable. People who are more similar genetically are more similar psychologically. But what, exactly, do these results mean—and not mean?

What heritability results do mean is that we can reject a “blank slate” argument (Pinker, 2002; see Chapter 1). The idea that, at birth, the mind has no inborn characteristics or tendencies is contradicted by twin-study results. Whether raised together or apart, identical twins resemble one another psychologically to a significant degree. Their identical genes contribute to their similar psychological styles.

However, here are three conclusions that cannot validly be drawn from twin-study results:

False Conclusion #1: Since psychological characteristics are heritable, they cannot change. A person’s characteristics can change enormously, even if individual differences are heritable. Heritability and changeability can go hand in hand—for example, weight is highly heritable. MZ twins are more similar in weight than are unrelated people. But if you diet and exercise—or sit on a couch and eat doughnuts—your weight changes.

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False Conclusion #2: 50% heritability for a trait means that half of each individual’s trait is inherited. Statements about heritability are statements about individual differences in the population. Heritability percentages do not describe individual people. For instance, personality is 50% heritable, but that does not mean “half of one’s personality is inherited.” The word “half” does not refer to anything in an individual person, but to differences among people in the population at large. In general, numbers that describe a population of people do not also describe each individual person in that population. For example, the average height of students in your intro psych class might be 5´6˝, but it’s possible that no individual person is actually 5´6˝ tall.

False Conclusion #3: If heritability within two groups is high, and the groups differ, then the differences between the groups are due to genes. Heritability describes the influence of genes within a group. Even if heritability is high, differences between groups may be due to the environment, not inherited biology. Consider physical height among citizens of South Korea and North Korea. Within each population, 80 to 90% of the variation in height is heritable. People in one population (South Korean) are much taller than in the other. But the difference between the populations is not caused by genes. It can’t be; no fundamental genetic difference exists between the populations, both of which are of Korean ancestry (Johnson et al., 2009). Group differences are caused by an environmental factor: Nutrition and overall living conditions are poor in North Korea compared with South Korea.

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