In personality psychology, a trait is a person’s typical style of behavior and emotion. (“Trait” contrasts with the concept of psychological state— a person’s current thoughts and feelings, which may vary from moment to moment.) Trait theories are theoretical approaches that try to identify, describe, and measure people’s personality traits. A key word in that last sentence was “measure” (discussed in Chapter 2); reliable measurement is the foundation on which trait theory is built (Cattell, 1965).

Trait theorists recognize that good measurement benefits not only basic science, but also practical applications. Suppose you are hiring people to fill jobs at a company and want a sociable salesperson, an honest accountant, and a manager with a calm leadership style. There will be many applicants. Which would be best for each job? To find out, you need good measurement tools: efficient and reliable measures of the traits of interest. This is exactly what trait theory provides.

By way of background, trait theory has a long history. Early in the twentieth century, psychologists recognized that measurement principles used in the study of intelligence (see Chapter 8) could be applied to personality. By asking people a large number of questions and scoring their responses, personality could be measured. In the latter half of the twentieth century, research on genes and individual differences indicated that variations in personality trait scores were, to a significant degree, based on inherited biology (see Chapter 4). By the century’s close, the combination of quantitative measures and genetic findings made trait psychology a major force in the science of personality.

The major structure in personality trait theory is—no surprise here—the personality trait. As noted, a trait is a person’s typical style of behavior and emotion. More specifically, personality traits have three defining qualities that refer to consistent ways in which people differ in their average tendencies:

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In addition to these three defining qualities, the word “trait” has two extra meanings to most trait theorists. One is that it refers to a dimension, not a category. This means that there are gradual variations in levels of the trait. In this way, personality traits are like physical traits such as height. Although we sometimes talk about “tall people” and “short people,” we know that these are not distinct categories of people; height varies along a dimension. Similarly, although we may talk about “agreeable people” and “disagreement,” the trait of agreeableness varies dimensionally. For any given trait dimension, most people are near the middle of the dimension, and small numbers of people are at the very high or low ends (Figure 13.4).

Second, theorists see traits as causes of behavior (McCrae & Costa, 1995, 1996). Again, an analogy with height is useful. If someone buys shirts with extra-long sleeves and pants with extra-long legs, and he sometimes bumps his head on the top of doorways, the cause is his high standing on the physical dimension of height. Analogously, if someone helps people with errands, avoids getting into arguments, and cooperates cheerfully with team members on work projects, his behavior, according to trait theory, would be caused by his high standing on the personality dimension of agreeableness—an overall tendency to be pleasant, helpful, and accommodating (Graziano, Jensen-Campbell, & Finch, 1997; but see This Just In).

With the meaning of “trait” established, the next question to ask is what are the most important personality traits? There are two approaches to answering this question; we’ll look at both in the sections ahead. The first is to examine words in everyday language (Ashton & Lee, 2007; Goldberg, 1981). The lexical approach to trait theory presumes that people will notice, and invent words for, all significant differences among people, and thus that words in the language contain clues about the most significant personality traits. The other approach is to examine biology (Stelmack & Rammsayer, 2008). Neural or biochemical differences between people may produce variations in personality.

PERSONALITY TRAITS IN THE LANGUAGE. Imagine opening a dictionary and listing all the words that describe personality traits. You start with “abrasive,” move to “absent-minded” and “abstemious,” and—if you can keep yourself awake long enough—eventually arrive, appropriately enough, at “zonked.”

Incredibly, two personality psychologists once did just that: They listed every trait word in English—more than 4000 of them (Allport & Odbert, 1936).

The length of that list poses a problem: Measuring 4000 traits is impractical. It’s too much work to develop 4000 tests, and even if you did develop them, who would ever take them all? Trait theorists need to identify a small set of traits that correspond to the most significant individual differences in personality. They do so by using factor analysis.

Factor analysis is a statistical technique that identifies patterns in a large set of correlations (correlations are described in Chapter 2). Suppose you measure numerous personality characteristics—for instance, reliable, emotional, insecure, generous, hardworking, nervous, punctual, self-conscious, ambitious, and creative—by asking many people to rate, on a 7-point scale, whether each characteristic describes them. For any two characteristics (e.g., insecure and nervous), the correlation between the two measures indicates how strongly the characteristics go together (i.e., whether people who say they are insecure also tend to say they are nervous).

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Now suppose you want to know how, in general, the traits correlated with one another. You suddenly are faced with a lot of numbers: If 10 traits were measured, there are 45 pairs of traits and thus 45 correlations. If 100 traits are measured, there are 4950 correlations—way too many to keep track of without a mathematical tool. Factor analysis is that tool. It enables psychologists to sort through the correlations to find groups of measures that are correlated strongly. Trait psychologists reason that each of these strongly correlated groups represents one trait. In our example, the group including reliable, hardworking, punctual, and ambitious would represent the trait of conscientiousness.

In the study of personality traits, factor analysis usually yields a simple result. Only five or six traits are needed to describe the main individual differences in personality. One group of five traits is found so commonly that it is called the “Big Five” (Goldberg, 1993; John, Naumann, & Soto, 2008). The Big Five personality traits are found consistently when people’s descriptions of personality are analyzed with factor analysis. The same five traits are found when analyzing people’s descriptions of their own personality and people’s descriptions of others’ personalities (e.g., close friends or one’s spouse; McCrae & Costa, 1987). The Big Five traits—extraversion, agreeableness, conscientiousness, neuroticism, and openness to experience —are described in Table 13.4. (A hint for remembering them is that the first letters of the traits can be arranged to spell “ocean.”)

The Big Five can be assessed easily. Table 13.5 contains a simple 10-item measure of the five personality factors (Gosling, Rentfrow, & Swann, 2003).

In this measure of the Big Five trait variables, two items tap each of the factors: Extraversion (1, 6*), Agreeableness (2*, 7); Conscientiousness (3, 8*), Emotional Stability (4*, 9), and Openness to Experiences: (5, 10*). The asterisk indicates that the item is scored in reverse; strongly agreeing with the item gives one a lower score on the trait.

Some trait theorists think that the Big Five trait model is missing something—a sixth trait called honesty/humility (Ashton & Lee, 2007). People with high scores on this trait display sincerity and a lack of pretentiousness. Evidence of this sixth factor came from a multinational study. Native speakers of each of seven different languages—Dutch, French, German, Hungarian, Italian, Korean, and Polish—rated themselves on a list of about 400 personality trait adjectives, some of which had not been included in prior Big Five studies. In each language, factor analysis revealed that six, not five, factors were needed to describe individual differences, the sixth being honesty/humility (Ashton et al., 2004).

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In sum, the lexical approach summarizes personality traits found in everyday language. The big question, though, is whether that is good enough. Ultimately, psychologists don’t just want to know about the language of personality; they want to know the “flesh-and-bones” of personality: the biological structures underlying differences between people.

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THIS JUST IN

PERSONALITY TRAITS IN THE NERVOUS SYSTEM. People differ biologically in many ways (height, weight, blood pressure, bone density, etc.). Might some of the biological differences produce individual differences in personality traits? Many trait psychologists have pursued this possibility (Stelmack & Rammsayer, 2008). One of the first was the British psychologist Hans Eysenck.

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Eysenck (1970) searched for the biological basis of extraversion. He hypothesized that extraverts (high-energy people who like lots of activity and social interaction) and introverts (reserved individuals who are happy sitting quietly, reading a book) should differ in brain functioning. Specifically, Eysenck predicted they would differ in cortical arousal, that is, activation in the surface regions of the brain (the cortex; see Chapter 3). People with naturally low levels of arousal should compensate for this by pursuing arousing activities that raise their arousal level. People with high levels of cortical arousal should avoid simulating environments, which make them overly aroused. Arousal level therefore causes people to become extraverts (low arousal) or introverts (high arousal).

Some evidence supports Eysenck’s idea. In electroencephalographic (EEG) research (see Chapter 2), investigators record the brain activity of introverts and extraverts while presenting simple stimuli, such as flashes of light or moderately loud noises. Just as Eysenck predicted, the brains of introverts react more strongly than those of extraverts to this physical stimulation (Stelmack & Rammsayer, 2008). Outside of the lab, in everyday life, the differences in brain reactivity may create differences in introverted versus extraverted behavior.

Eysenck also sought the biological basis of neuroticism, that is, the tendency to feel anxious and worried. Here, he predicted that individual differences in neuroticism would be linked to variations in the reactivity of the autonomic nervous system (ANS), which responds to environmental threats (see Chapter 3). People with high scores on neuroticism should have “jumpy” nervous systems. This prediction, however, has received less support (Stelmack & Rammsayer, 2008). Individual differences in self-reported neuroticism and ANS activity are related only weakly. Factors not anticipated by Eysenck, such as people’s subjective interpretations of events, contribute to anxious, neurotic response styles (M. Eysenck, 2013). This weak result has motivated some trait psychologists to seek an alternative to Eysenck’s theory.

One alternative was advanced by the British psychologist Jeffrey Gray and his colleagues (Corr, 2004; Gray & McNaughton, 2000; Pickering & Corr, 2008). Gray identified a problem in Eysenck’s theory of neuroticism: It failed to distinguish between two fundamentally different emotions, namely, fear and anxiety. In Gray’s theory of personality and motivation, different brain systems are said to underlie these different emotional responses.

Traits are the personality structures of trait theory. What about personality processes?

Trait theorists have devoted less attention to dynamically shifting personality processes than to stable personality structures (Hampson, 2012; McCrae & Costa, 1996). The lexical approach to personality traits, for example, analyzes personality structure exclusively.

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Biological research, however, does shed light on personality processes. Consider again the work of Eysenck. His analysis of cortical arousal and extraversion identifies brain processes that come into play as people encounter situations featuring low and high levels of stimulation. Other researchers have extended Eysenck’s analyses by showing how extraversion may be linked to bodily arousal, which, in turn, affects thinking processes (Humphreys & Revelle, 1984). Nonetheless, the analysis of personality processes is not the strong point of trait theories.

We’ve seen that analysis of personality processes is not a strength of trait theory. Personality assessment, however, is. Trait theorists have developed tests that reliably measure individual differences in personality traits.

COMPREHENSIVE SELF-REPORT QUESTIONNAIRES. The most common personality tests are self-report questionnaires in which people indicate their personal preferences and tendencies. Some of these tests aim for comprehensiveness; test creators seek to measure all the major personality traits, to provide a complete psychological portrait of the individual. Let’s look at one such test in detail.

The NEO-Personality Inventory (NEO-PI-R; Costa & McCrae, 1992; McCrae & Costa, 2010) measures individual differences in each of the Big Five personality traits: neuroticism, extraversion, openness to experience, conscientiousness, and agreeableness. (Through a historical quirk, it gets its name, NEO, from only the first three of those traits. It exists in different forms, called the NEO-PI-R and NEO-PI-3.)

The NEO includes about 50 test items that measure each of the five traits. Why so many? No one question can measure a broad personality trait. But a large set of questions can. For example, conscientiousness (see Table 13.4) test items ask about keeping one’s possessions in neat order, paying bills on time, working efficiently, finishing jobs, and so on. Adding together responses across the items provides an accurate measure of the trait.

As noted earlier, there are shorter tests (Gosling, Rentfrow, & Swann, 2003; Rammstedt & John, 2007). Brevity sacrifices accuracy, however. When psychologists want highly accurate measures of Big Five traits, they prefer to use longer tests, such as the NEO.

PREDICTING BEHAVIOR. Do these personality tests predict people’s behavior? Often they do. Let’s see two examples, the first of which involves personality and personal space.

Most people have one or two personal spaces, that is, areas they decorate themselves. In your bedroom, for instance, you might arrange the furniture and choose art for the walls. Personality predicts decoration preferences. Researchers sent participants to individuals’ office spaces and bedrooms and asked them to judge the personality traits of the inhabitants. They made the judgments without ever meeting the occupants. Separately, the inhabitants of the rooms completed questionnaires measuring their personality traits. The ratings of conscientiousness, extraversion, and openness to experience that participants gave to occupants based on their rooms correlated significantly with the occupants’ self-ratings on those traits (Gosling et al., 2002).

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Personality traits also predict a more serious outcome: mortality. Researchers (Terracciano et al., 2008) measured personality traits in a large group of adults, and then followed their life course over decades. During this time, some research participants died. Remarkably, personality traits predicted length of life (Figure 13.8). People with more emotionally stable (low neuroticism) personalities, and those with higher levels of conscientiousness, lived longer (Terracciano et al., 2008).

Why would personality predict mortality? People with more emotional personalities may experience more life stress, which can impair health (Chapter 10). More conscientious people may maintain healthier lifestyles and have regular medical check-ups, which can prolong life.

Trait theories have three major strengths. They provide simple methods for assessing individual differences. They have spurred research into the connection between personality traits and biology. Their use of sophisticated statistical techniques brings scientific rigor that was sometimes lacking in prior theories of personality.

However, trait theories have limitations, too. One is that, like Rogers’s humanistic theory, they are insufficiently comprehensive. As we’ve noted, trait theory says little about personality processes—the dynamic interactions among thoughts and feelings that were highlighted, for example, by Freud. Trait theory also says little about inconsistencies in behavior. When asked to describe themselves, people often report behavior that is inconsistent; sometimes they act one way, sometimes another: “I am very shy, but I talk a lot once I get to know someone”; “I am organized for the most part, but I’m very rushed and messy at home”; “I have a very welcoming personality, but another aspect of my personality is my mean side” (Orom & Cervone, 2009). If you observe people’s behavior, you see these inconsistencies; most individuals sometimes look like they’re extraverts and other times look like they’re introverts (Fleeson, 2001). Because trait variables refer to average levels of behavior, they don’t explain such variations around the average. The psychologists who developed the fourth, and last, personality theory you’ll learn about have tried to overcome these two limitations of the trait approach.