Investigating The Intersection of Personality and Intelligence

Traditionally, individual differences in intelligence and personality have been studied under separate paradigms. Intelligence is typically conceptualized and measured as “maximal performance” whereas personality is conceptualized and measured as “typical performance” (Goff & Ackerman, 1992). Intelligence is assessed using tests that are designed to elicit a person’s maximum mental capacity. The tests are usually given under strict administration procedures in order to create an environment of high situational strength that will limit environmental or contextual factors. An individual taking an intelligence test is instructed that there is a single correct answer to each question and the importance of giving their best effort is emphasized. In contrast, personality assessments are designed to measure behavioral tendencies across a wider variety of contexts. Personality traits are usually measured using self-report questionnaires in which the respondent is informed that there are no right or wrong answers and instructed that they should answer honestly. Items are not scored as “correct” or “incorrect”, but rather are scored on a bidirectional scale (e.g., Likert scale). These suggests that these constructs should be studied separately given their critical differences with regard to theory and measurement, an approach to studying personality-intelligence associations that has been labelled “the independence approach” (Chamorro-Premuzic, & Ackerman, 2011).

However, there have been substantial theoretical and empirical advancements in the study of the intersection between personality traits and intelligence (DeYoung, 2011). The expansion of the personality-intelligence association literature, which began with both broad investigations in explorations of trait correlations and progressed to specific theories hypothesizing the a small set of trait relations (Ackerman & Heggestad, 1997), eventually gave rise to intellectual “investment traits”. Investment traits are defined as “stable individual differences in the tendency to seek out, engage in, enjoy, and continuously pursue opportunities for effortful cognitive activity” (von Strumm, et al., 2011). Such traits are generally correlated more substantially with crystallized intelligence than fluid intelligence (Goff & Ackerman, 1992; Funmham. Monsen, & Ahmetoglu, 2009; Judge et al., 2007; von Strumm & Ackerman, 2013). This paper provides a historical review of the development of these investment traits. I begin by reviewing work that preceded the theoretical underpinning of investment traits but either: a) provided comments or theories that intelligence is not simply an inherited ability by acknowledging the role of non-ability factors, or b) included broad investigations of personality and ability associations. Next, I review two critical developmental theories that provide the theoretical backbone for investment traits and three of the most frequently used investment traits and their psychometric measurement scales. Finally, I conclude by discussing the burgeoning area of intellectual investment and the broad impact that investment traits have had on the field of psychology, both historically and currently.

Early Works That Led to Theoretical Development of Intellectual Investment Traits

Early empirical studies suggesting personality-intelligence associations were very broad in scope and that the literature later converged on a smaller number of traits that play an important role in intellectual development (Ackerman & Heggestad, 1997). The early part of the 20th century was characterized by such broad investigations, with one of the earliest being Edward Webb’s dissertation study (Webb, 1915). As a student if Charles Spearman, Webb supported Spearman’s theory of general intelligence (g) (Spearman, 1904). For his dissertation study, Webb designed a study in which he instructed experienced students to rate less advanced students in a teaching program on a wide variety of abilities and personality traits over the course of a semester and used factor analysis to isolate a general factor of intelligence and a general factor of “character” (i.e., non-ability factors). The results suggested two sides of mental processes: intelligence and character. While Webb considered these factors to be distinct and used an orthogonal rotation in his analysis, he observed that ratings of intelligence and character were correlated and attributed this to bias on the part of the raters toward students they viewed positively. The important aspect of this study for the purposes of this paper its broad nature – Webb reported that these two general factors appeared to be related (albeit, due to a ratings bias) but did not discuss a small number of specific traits that play a prominent role. This is a representative example of early studies of personality-intelligence associations that did not include the precision that studies later in history did.

The idea that personality traits are related to intellectual ability can also be traced back to the early history of differential psychology. Lewis Terman is well-known not only for the development of the Stanford-Binet intelligence test (Terman, 1916), but also for his longitudinal study of gifted children. Terman identified around 1500 children of substantial ability, who he termed “Termites” and selected them based on an IQ threshold of 140 (Terman, 1922; Terman & Fenton, 1921). He launched a large-scale longitudinal study in which he followed them throughout their lifetime and gathered a substantial amount of psychological measurements. While following up with the gifted children, Terman included an assessment of what he termed “intellectual traits” (Terman & Oden , 1959, as cited in von Strumm & Ackerman, 2013). The assessment asked parents and teachers to rate the four students on four different aspects of personality: Desire to know, general intelligence, originality, and common sense. While this was an early mention of personality traits related to intelligence, it is worth pointing out that Terman did not use a personality trait that was “specific to the investment personality space” (von Strumm & Ackerman, 2013).

There were two other early contributions that were important for laying the groundwork for theoretical developments that led to the development of intellectual investment traits. The first was presented in William McDougall’s book outlining his theory of dynamic psychology (McDougall, 1932). McDougall believed that natural instincts guided human behavior and that premise was the central theme of his book. He identified 18 different intrinsic “propensities” that motivate human behavior: food-seeking, disgust, sex, fear, curiosity, protective/parental, gregarious, self-assertive, submissive, anger, appeal, constructive, acquisitive, laugher, comfort, rest/sleep, migratory, and bodily needs. McDougall argued that no behavior is purely intellectual, but rather all forms of behavior rely on one or more of these instinctive propensities. He offered the hypothetical example of a hungry rat in a cage along with food that is tied to a string connected to a piece of lead on the other side of the cage, leaving the food suspended in mid-air. The rat obtains the food by pulling on the string. McDougall argued that while many psychologists consider this intellectual behavior, it is not purely intelligence because the food-seeking propensity drives the behavior. In other words, animal and human behavior is “both instinctive and intelligent” (McDougall, 1932, pg. 67). While McDougall’s argument did not specifically involve personality traits, his theory acknowledged the role of tendencies outside of the ability domain that impact behavior and this idea was expanded on by later theories that were impactful for the intellectual investment trait literature.

The second contribution that led psychologists toward theories that suggested the influence of personality traits on intelligence came from Keith Hayes (1962). Similar to McDougall, Hayes argued that intelligence was related not only to natural abilities, but to natural propensities in behavior. However, Hayes took the idea a step further by asserting that intelligence is solely determined by genetic differences in “experience producing drives” that in turn lead to differences in knowledge that individuals accumulate. Similar to McDougall, Hayes offered a hypothetical example to illustrate his point. If two fraternal twins who grow up in the same environment differ in interests such that one twin is oriented toward linguistic activities such as talking and reading and the other is oriented toward physical activities such as sports, it is a strong possibility that the twin oriented toward linguistic activity will score higher on the Stanford Binet as a teenager than the twin oriented toward physical activity simply because he has developed a larger vocabulary by reading and speaking more frequently. Hayes bases his proposal that intelligence is a result of experience producing drives on four connected arguments: 1) differences in motivation are genetically-based, 2) the differences in motivation lead to different experiences, 3) experiential differences lead to differences in abilities, and 4) differences in intelligence are nothing more than differences in acquired ability. Hayes uses a variety of evidence to support these arguments including learning and memory research, evidence form adults who experience brain damage, and a lack of statistical evidence of “higher mental functions.” In the end, he concludes that “innate intellectual potential consists of tendencies to engage in activities conducive to learning, rather than inherited capacities.” (Hayes, 1962). While Hayes does not mention specific personality traits, his theory suggested that intelligence is more than an inherited ability and that the drive to engage in different types of activities influences the knowledge that humans obtain throughout their life. Later theoretical contributions built on the work of both Hayes and McDougall to develop comprehensive theories of intellectual investment that set the stage for the development of scales that measure personality traits that drive the acquisition of knowledge and skills.

Theoretical Underpinnings of Investment Traits

While the work discussed in the previous section emphasized the idea that non-ability factors play a role in the depth and breadth of knowledge or skills that are acquired throughout one’s life and included broad empirical investigations that suggested personality-intelligence associations, none discussed specific theories that involve the role that personality traits play in the development of intelligence. In this section, I review two developmental theories that are critical for the conceptual understanding of investment traits. Raymond B. Cattell’s “investment theory of intelligence” may have been the most critical theory for the development of investment traits. Cattell famously introduced a hierarchical theory of intelligence with two factors at the broadest level: fluid intelligence and crystallized intelligence (Cattell, 1963). Fluid intelligence involves process related abilities such as novel problem solving, abstract thinking, memory, and flexible thinking that peak in one’s early twenties and then decline across the rest of the lifespan. Crystallized intelligence involves content related knowledge including factual knowledge, vocabulary, and comprehension that increases or is maintained through the lifespan (Horn & Cattell, 1966, 1967).

Cattell’s investment theory discusses the relationship between fluid and crystallized intelligence. The theory proposes that investing fluid intelligence eventually gives rise to crystallized intelligence as one uses knowledge or skills increasingly. Cattell made reference to a relationship between fluid abilities and accumulation of knowledge in a 1943 Psychological Bulletin paper when he said that “crystallized ability consists of discriminatory habits long established in a particular field, originally through the operation of fluid ability, but not longer requiring insightful perceptual for their successful operation” (Cattell, 1943). He later expanded this idea by proposing the formal theory in his book entitled Intelligence: Its Structure, Growth, and Action Cattell (1971/1987). In his book, Cattell offers an example that illustrates his proposed relationship between fluid and crystallized intelligence. A student learning to solve algebra problems uses their fluid intellectual abilities at first, but over time, as they become more familiar with the methods used to solve the problem, it becomes crystallized intelligence as they use procedural knowledge already acquired rather than engaging in novel problem-solving. If this premise holds, one’s current crystallized ability is a function of the “operation levels” of their fluid intelligence in the years prior.

According to Cattell, individual differences outside of the ability domain impact what people invest their fluid intelligence in and the extent to which they do so. He argues that this is why Spearman observes a general factor of intelligence. It is not because one’s vocabulary impacts their ability to solve math problems, but because people who invest their fluid intelligence to greater extents perform better on tests of each. This is particularly true in early educational contexts where there is a standardized curriculum. The correlations between different abilities and knowledge declines as one specializes in a particular subject or occupation. In a later chapter of his book, Cattell proposes a number of personality traits from the 16PF personality framework (Cattell, Eber, & Tatsuoka, 1970) that may stimulate the development of abilities. He suggests that higher levels of regression (i.e., neuroticism) and general inhibition are negatively related to abilities while higher levels of cortical alertness, temperamental independence, exuberance, and competitive ego strength are positively related to abilities.

In later years, Ackerman built on Cattell’s investment theory to propose a more specific theory regarding the role of personality in the development of intelligence (Ackerman, 1996). The theory, entitled the PPIK framework, identified four components of adult intellect: intelligence-as-process (P), personality (P), interests (I), and intelligence-as-knowledge (K). The framework specifies that the non-ability traits of personality and interests influence the subjects and activities that people invest their cognitive effort in, which then determines domain-specific knowledge (i.e., intelligence-as-knowledge). Evidence supporting the theory was provided by a meta-analysis conducted the next year that indicated that over half of the personality-intelligence correlations found in the literature significantly differed from zero and identified “four trait complexes” that described abilities and non-ability traits that generally fall together (Ackerman & Heggestad, 1997). As prominent examples, openness and ideational fluency typically fall in the same trait complex and conscientiousness and perceptual speed typically fall in another trait complex.

The PPIK framework built upon Cattell’s investment theories in two critical ways. First of all, PPIK is more specific than the investment theory of intelligence. Cattell’s theory was representative of early research on intelligence-personality associations in that it was relatively broad. Cattell clearly defined his idea individual differences such personality influence the extent to which people actively use their fluid intelligence what they choose to invest it in. However, specific traits were not a central part of his theory. He did propose a number of personality traits that might influence intelligence in a late chapter of this book, however it was a rather lengthy list and very few were proposed to influence specific types of abilities. Ackerman’s theory, on the other hand, proposes a specific role of a relatively parsimonious number of personality traits (and interests) that facilitate specific the development of specific types of abilities (Ackerman, 1996). Additionally, investment theory suggested a unidirectional relationship between individual differences and intelligence. The PPIK framework allows for a reciprocal relationship in which personality and interests influence the acquisition of knowledge and skills which then strengthens behavioral tendencies and interests (von Strumm & Ackerman, 2013). It is also relevant to note that because PPIK framework was proposed late in the 20th century, it did not stimulate the development of specific investment trait scales. The investment trait scales that are discussed in the next section preceded publication of the theory. Rather, it synthesized the accumulation evidence of personality-ability associations and proposed mechanisms by which the individual differences interact to give rise to knowledge and skills acquisition.