Imagine writing a story about the people you know. To capture their special qualities, you might describe their traits: Meaghan is friendly, aggressive, and domineering; Amin is flaky, humorous, and superficial. With a dictionary and a free afternoon, you might even be able to describe William as perspicacious, flagitious, and callipygian. The trait approach to personality uses such trait terms to characterize differences among individuals. In attempting to create manageable and meaningful sets of descriptors, trait theorists face two significant challenges: narrowing down the almost infinite set of adjectives and answering the more basic question of why people have particular traits and whether they arise from biological or hereditary foundations.

One way to think about personality is as a combination of traits. This was the approach of Gordon Allport (1937), one of the first trait theorists, who believed people could be described in terms of traits just as an object could be described in terms of its properties. He saw a trait as a relatively stable disposition to behave in a particular and consistent way. For example, a person who keeps his books organized alphabetically in bookshelves, hangs his clothing neatly in the closet, knows the schedule for the local bus, keeps a clear agenda in a daily planner, and lists birthdays of friends and family in his calendar can be said to have the trait of orderliness. This trait consistently manifests itself in a variety of settings.

The orderliness trait describes a person but does not explain his or her behaviour. Why does the person behave in this way? There are two basic ways in which a trait might serve as an explanation: The trait may be a pre-existing disposition of the person that causes the person’s behaviour, or it may be a motivation that guides the person’s behaviour. Allport saw traits as pre-existing dispositions, causes of behaviour that reliably trigger the behaviour. The person’s orderliness, for example, is an inner property of the person that will cause the person to straighten things up and be tidy in a wide array of situations. Other personality theorists, such as Henry Murray (the creator of the TAT), suggested instead that traits reflect motives. Just as a hunger motive might explain someone’s many trips to the snack bar, a need for orderliness might explain the neat closet, organized calendar, and familiarity with the bus schedule (Murray & Kluckhohn, 1953). Researchers examining traits as causes have used personality inventories to measure them, whereas those examining traits as motives have more often used projective tests.

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Researchers have described and measured hundreds of different personality traits over the past several decades. Back in the 1940s, in the wake of World War II, psychologists were very interested in right-wing authoritarianism, or the tendency toward political conservatism, obedience to authority, and conformity. At that time, researchers were trying to understand what had made people support the rise of Nazi Germany and Fascism (Adorno et al., 1950). Although research on the personality traits that lead to authoritarianism continues (Perry & Sibley, 2012), the topic became less focal for researchers once World War II receded into history. Examples of other traits that have come into vogue over the years include cognitive complexity, defensiveness, sensation seeking, and optimism. As with television shows and hairstyles, fashions in trait dimensions come and go over time.

Picking a fashionable trait and studying it in depth does not get us very far in the search for the core of human character: the basic set of traits that defines how humans differ from one another. People may differ strongly in their choice of Coke versus Pepsi, or dogs versus cats, but are these differences important? How have researchers tried to discover the core personality traits?

The study of core traits began with an exploration of how personality is represented in the store of wisdom we call language. Generation after generation, people have described people with words, so early psychologists proposed that core traits could be discerned by finding the main themes in all the adjectives used to describe personality. In one such analysis, a painstaking count of relevant words in a dictionary of English resulted in a list of over 18 000 potential traits (Allport & Odbert, 1936)! Attempts to narrow down the list to a more manageable set depend on the idea that traits might be related in a hierarchical pattern (see FIGURE 12.3), with more general or abstract traits at higher levels than more specific or concrete traits. Perhaps the more abstract traits represent the core of personality.

To identify this core, researchers have used the computational procedure called factor analysis, described in the Intelligence chapter, which sorts trait terms or self-descriptions into a small number of underlying dimensions, or factors, based on how people use the traits to rate themselves. In a typical study using factor analysis, hundreds of people rate themselves on hundreds of adjectives, indicating how accurately each one describes their personality. The researcher then conducts calculations to determine similarities in the raters’ usage, for example, whether people who describe themselves as ambitious also describe themselves as active but not laid-back or contented. Factor analysis can also reveal which adjectives are unrelated. For example, if people who describe themselves as ambitious are neither more nor less likely to describe themselves as creative or innovative, the factor analysis would reveal that ambitiousness and creativity/innovativeness represent different factors. Each factor is typically presented as a continuum, ranging from one extreme trait (such as ambitious) to its opposite (in this case, laid-back).

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Different factor analysis techniques have yielded different views of personality structure. Cattell (1950) proposed a 16-factor theory of personality (way less than 18 000, but still a lot), whereas others have proposed theories with far fewer basic dimensions (John, Naumann, & Soto, 2008). Later, Hans Eysenck (1967) simplified things nicely with a model of personality with only two major traits (although he later expanded it to three). Eysenck’s two-factor analysis identified one dimension that distinguished people who are sociable and active (extraverts) from those who are more introspective and quiet (introverts). His analysis also identified a second dimension ranging from the tendency to be very neurotic or emotionally unstable to the tendency to be more emotionally stable. He believed that many behavioural tendencies could be understood in terms of their relation to the core traits of Extraversion and Neuroticism. The third factor he proposed was Psychoticism, which refers to the extent to which a person is impulsive or hostile. (Notably, the term “psychotic” nowadays refers to an abnormal mental state marked by detachment from reality. This is discussed further in the Psychological Disorders chapter.)

Today most researchers agree that personality is best captured by 5 factors rather than 2, 3, 16, or 18 000 (John & Srivastava, 1999; McCrae & Costa, 1999). The Big Five, as they are affectionately called, are the traits of the five-factor personality model: openness to experience, conscientiousness, extraversion, agreeableness, and neuroticism (see TABLE 12.2). (Remember them by the initials O.C.E.A.N.) The five-factor model, which overlaps with the pioneering work of Cattell and Eysenck, is now widely preferred for several reasons. First, modern factor analysis techniques confirm that this set of five factors strikes the right balance between accounting for as much variation in personality as possible while avoiding overlapping traits. Second, in a large number of studies using different kinds of data (people’s descriptions of their own personalities, other people’s descriptions of their personalities, interviewer checklists, and behavioural observation) the same five factors have emerged. Third, and perhaps most important, the basic five-factor structure seems to show up across a wide range of participants, including children, adults in other cultures, and even among those who use other languages, suggesting that the Big Five may be universal (John & Srivastava, 1999).

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In fact, the Big Five dimensions are so universal that they show up even when people are asked to evaluate the traits of complete strangers (Passini & Norman, 1966). This finding suggests that these dimensions of personality might reside in the eye of the beholder: categories that people use to evaluate others regardless of how well they know them. However, it is not all perception. The reality of these traits has been clearly established in research showing that self-reports on the Big Five are associated with predictable patterns of behaviour and social outcomes. People identified as high in extraversion, for example, tend to choose to spend time with lots of other people and are more likely than introverts to look people in the eye. People high in conscientiousness generally perform well at work and tend to live longer. People low in conscientiousness and low in agreeableness are more likely than average to be juvenile delinquents (John & Srivastava, 1999). It turns out that the Big Five personality traits also predict people’s online behaviour on social networking sites like Facebook (see the Hot Science box).

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Interestingly, research on the Big Five has shown that people’s personalities tend to remain fairly stable throughout their lifetime: Scores at one time in life correlate strongly with scores at later dates, even decades later (Caspi, Roberts, & Shiner, 2005). William James offered the opinion that “in most of us, by the age of thirty, the character has set like plaster, and will never soften again” (James, 1890, p. 121), but this turns out to be too strong a view. Some variability is typical in childhood, and though there is less in adolescence, some personality change can even occur in adulthood for some people (Srivistava et al., 2003).

Can we explain why a person has a stable set of personality traits? Many trait theorists have argued that unchangeable brain and biological processes produce the remarkable stability of traits over the life span. Allport viewed traits as characteristics of the brain that influence the way people respond to their environment. And, as you will see, Eysenck searched for a connection between his trait dimensions and specific individual differences in the workings of the brain.

Brain damage certainly can produce personality change, as the classic case of Phineas Gage so vividly demonstrates (see the Neuroscience and Behaviour chapter). You may recall that after the blasting accident that blew a steel rod through his frontal lobes, Gage showed a dramatic loss of social appropriateness and conscientiousness (Damasio, 1994). In fact, when someone experiences a profound change in personality, testing often reveals the presence of brain pathologies such as stroke, brain tumour, or changes consistent with dementia (Feinberg, 2001). The administration of antidepressant medication and other pharmaceutical treatments that change brain chemistry can also trigger personality changes, making people, for example, somewhat more extraverted and less neurotic (Bagby et al., 1999; Knutson et al., 1998).

Some of the most compelling evidence for the importance of biological factors in personality comes from the domain of behavioural genetics. Like researchers studying genetic influences on intelligence (see the Intelligence chapter), personality psychologists have looked at correlations between the traits in monozygotic, or identical, twins, who share the same genes, and dizygotic, or fraternal, twins, who on average share only half of their genes. The evidence has been generally consistent: In one review of studies involving over 24 000 twin pairs, for example, identical twins proved markedly more similar to each other in personality than did fraternal twins (Loehlin, 1992).

Simply put, the more genes you have in common with someone, the more similar your personalities are likely to be. Genes seem to influence most personality traits, and current estimates place the average genetic component of personality in the range of 0.40 to 0.60. These heritability coefficients, as you learned in the Intelligence chapter, indicate that roughly half the variability among individuals results from genetic factors (Bouchard & Loehlin, 2001). Of course, genetic factors do not account for everything and the remaining half of the variability in personality remains to be explained by differences in life experiences and other factors (see also The Real World box below). Studies of twins suggest that the extent to which the Big Five traits derive from genetic differences ranges from 0.35 to 0.49 (see TABLE 12.3).

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As in the study of intelligence, potential confounding factors must be ruled out to ensure that effects are truly due to genetics and not to environmental experiences. Are identical twins treated more similarly, and do they have a greater shared environment than fraternal twins? As children, were they dressed in the same snappy outfits and placed on the same soccer teams, and could this somehow have produced similarities in their personalities? Studies of identical twins reared far apart in adoptive families—an experience that pretty much eliminates the potential effect of shared environmental factors—suggest that shared environments have little impact: Reared-apart identical twins end up just as similar in personality as those who grow up together (McGue & Bouchard, 1998; Tellegen et al., 1988).

Indeed, one provocative, related finding is that such shared environmental factors as parental divorce or parenting style may have little direct impact on personality (Plomin & Caspi, 1999). According to these researchers, simply growing up in the same family does not make people very similar. In fact, when two siblings are similar, this is thought to be primarily due to genetic similarities.

Researchers also have assessed specific behavioural and attitude similarities in twins, and the evidence for heritability in these studies is often striking. One study that examined 3000 pairs of identical and fraternal twins found evidence for the genetic transmission of conservative views regarding topics such as socialism, church authority, the death penalty, and mixed-race marriage (Martin et al., 1986). It is very unlikely that a specific gene is directly responsible for a complex psychological outcome like beliefs about social or political issues. Rather, a set of genes (or, more likely, many sets of genes interacting) may produce specific characteristics or tendencies to think in a conservative versus liberal manner. One recent study examined the DNA of 13 000 people and measured the extent to which they reported conservative versus liberal attitudes and found associations between conservatism–liberalism and chromosomal regions linked to mental flexibility, or the extent to which a person changes his or her thinking in response to shifts in the environment, which could be one of the factors influencing our views on social and political issues (Hatemi et al., 2011). Current research by psychological scientists is aimed at better understanding how variations in our genetic code may contribute to the development of personality.

Another source of evidence for the biological basis of human personality comes from the study of nonhuman animals. Any dog owner, zookeeper, or cattle farmer can tell you that individual animals have characteristic patterns of behaviour. One Missouri woman who reportedly enjoyed raising chickens in her suburban home said that “the best part” was “knowing them as individuals” (Tucker, 2003). As far as we know, this pet owner did not give her feathered companions a personality test, though researcher Sam Gosling (1998) used this approach in a study of a group of spotted hyenas. Well, not exactly. He recruited four human observers to use personality scales to rate the different hyenas in the group. When he examined ratings on the scales, he found five dimensions; three closely resembled the Big Five traits of neuroticism (i.e., fearfulness, emotional reactivity), openness to experience (i.e., curiosity), and agreeableness (i.e., absence of aggression).

In similar studies of guppies and octopi, individual differences in traits resembling extraversion and neuroticism were reliably observed (Gosling & John, 1999). In each study, researchers identified particular behaviours that they felt reflected each trait based on their observation of the animals’ normal repertoire of activities. Octopi, for example, seldom get invited to parties, so they cannot be assessed for their socializing tendencies (“He was all hands!”), but they do vary in terms of whether they prefer to eat in the safety of their den or are willing to venture out at feeding time, and so a behaviour that corresponds to extraversion can reasonably be assessed (Gosling & John, 1999). Because different observers seem to agree on where an animal falls on a given dimension, the findings do not simply reflect a particular observer’s imagination or tendency to anthropomorphize (i.e., to attribute human characteristics to nonhuman animals). Such findings of cross-species commonality in behavioural styles help support the idea that there are biological mechanisms that underlie personality traits shared by many species.

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From an evolutionary perspective, differences in personality reflect alternative adaptations that species—human and nonhuman—have evolved to deal with the challenges of survival and reproduction. For example, if you were to hang around a bar for an evening or two, you would soon see that humans have evolved more than one way to attract and keep a mate. People who are extraverted would probably show off to attract attention, whereas you would be likely to see people high in agreeableness displaying affection and nurturance (Buss, 1996). Both approaches might work well to attract mates and reproduce successfully—depending on the environment. Through this process of natural selection, those characteristics that have proved successful in our evolutionary struggle for survival have been passed on to future generations.

What neurophysiological mechanisms might influence the development of personality traits? Much of the thinking on this topic has focused on the extraversion–introversion dimension. In his personality model, Eysenck (1967) speculated that extraversion and introversion might arise from individual differences in cortical arousal. Eysenck suggested that extraverts pursue stimulation because their reticular formation (the part of the brain that regulates arousal or alertness, as described in the Neuroscience and Behaviour chapter) is not easily stimulated. To achieve greater cortical arousal and feel fully alert, Eysenck argued, extraverts seek out social interaction, parties, and other activities to achieve mental stimulation. In contrast, introverts may prefer reading or quiet activities because their cortex is very easily stimulated to a point higher than optimal alertness.

Behavioural and physiological research generally supports Eysenck’s view. When introverts and extraverts are presented with a range of intense stimuli, introverts respond more strongly, including salivating more when a drop of lemon juice is placed on their tongues and reacting more negatively to electric shocks or loud noises (Bartol & Costello, 1976; Stelmack, 1990). This reactivity has an impact on the ability to concentrate: Extraverts tend to perform well at tasks that are done in a noisy, arousing context (such as bartending or teaching), whereas introverts are better at tasks that require concentration in tranquil contexts (such as the work of a librarian or nighttime security guard) (Geen, 1984; Lieberman & Rosenthal, 2001; Matthews & Gilliland, 1999).

In a refined version of Eysenck’s ideas about arousability, Jeffrey Gray (1970) proposed that the dimensions of extraversion–introversion and neuroticism reflect two basic brain systems. The behavioural activation system (BAS), essentially a “go” system, activates approach behaviour in response to the anticipation of reward. The extravert has a highly reactive BAS and will actively engage the environment, seeking social reinforcement and be on the go. The behavioural inhibition system (BIS), a “stop” system, inhibits behaviour in response to stimuli signalling punishment. The anxious person, in turn, has a highly reactive BIS and will focus on negative outcomes and be on the lookout for stop signs. Because these two systems operate independently, it is possible for someone to be both a go and a stop person (simultaneously activated and inhibited), caught in a constant conflict between these two traits. Studies of brain electrical activity (EEG) and functional brain imaging (fMRI) suggest that individual differences in activation and inhibition arise through the operation of distinct brain systems underlying these tendencies (DeYoung & Gray, 2009). Even more recently, studies have suggested that the core personality traits described above may arise from individual differences in the volume of the different brain regions associated with each trait. For instance, self-reported neuroticism is correlated with the volume of brain regions involved in sensitivity to threat; agreeableness with areas associated with processing information about the mental states of other people; conscientiousness with regions involved in self-regulation; and extraversion with areas associated with processing information about reward (DeYoung et al., 2010). Research aimed at understanding how the structure and activity of our brains can contribute to the formation of our personality traits is still in its early stages, but represents a growing area of the field that many believe holds great promise for helping us better understand how we each develop into the unique people that we are.

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