Human variation in psychological, physical, and behavioral characteristics is both an obvious and inescapable reality. Recognition of this universal phenomenon likely arose in step with the rise of human civilization. Formal philosophical treatment of individual differences in intellect, integrity, and motivation, for example, can easily be traced back at least to Plato, and evidence indicates that testing for such differences was practiced by the Ming dynasty in ancient China. The scientific study of how and why people differ in systematic ways is known as the psychology of individual differences. The psychology of individual differences seeks to understand how inter and intra-individual differences in psychological characteristics interact with environmental affordances and demands to produce differences in a variety of personal, work, educational, and social outcomes. The theories and psychometric methods developed by individual difference researchers are used by social scientists to understand, critique, and address practical problems in a variety of contexts such as education, selection, evaluation, and guidance.
Core Domains: Intellectual, Personality, And Conative
Individual differences have traditionally been studied as three broad domains: intellectual, personality, and conative. It is important to note that, although these are often treated as separate aspects for the purpose of investigation, they are intertwined to some degree.
Arguably, no area of psychological study has been more closely scrutinized than the science of mental abilities. Since its beginnings in the late 19th century, the scientific study of intellectual differences has amassed a wealth of empirical data that unarguably supports two conclusions. First, what people typically think of as “intelligence” is best described by a hierarchal model with numerous specific abilities occupying the lower levels, a small number of group factors at an intermediate level, and a single general cognitive ability factor, referred to as “g,” at the top. That is, although specific mental abilities (e.g., verbal ability, quantitative ability, visual-spatial ability, short-term memory) can be identified, people who are high (or low) on any individual specific ability tend to be relatively high (or low) on the others. Second, although the assessment of intra-individual differences in specific abilities may be especially useful in personnel classification, and academic and career counseling, it is the g factor that accounts for most of the variance in important academic, occupational, and social outcomes attributable to mental abilities. The g factor is formally defined as the “eduction of relations and correlations,” that is, the ability to infer or deduce meaningful principles and concepts from abstractness and novel situations.
Unfortunately, a number of misconceptions continue to plague the interpretation of this research. For example, although the popular media (and an occasional scientist) use the term “IQ” as a synonym for intelligence or cognitive ability, it is important to recognize that IQ is a score from a specific test, not a characteristic of a person. Just as the number associated with a given temperature will change if one switches from a Celsius thermometer to a Fahrenheit thermometer, a person’s IQ score may change if assessed with two different IQ tests. Nothing has changed about the person, but rather the scale used to generate the number has changed. IQ scores do tend be very useful, though, because they tend to reflect g to a large extent, as well as other factors. A similar misunderstanding arises from how IQ tests are scored. Individual differences in IQ tend to be stable across a person’s development because IQ tests are age-normed (i.e., scored relative to the typical level of development for that age group). Obviously, a person’s actual intellectual capabilities change over time; however, IQ tests are usually not designed to show this within-person development, but rather to assess the relative level of developed cognitive characteristics for a given stage of development. This is why IQ scores tend to appear stable despite notable changes in the population mean level of actual capacities across time. Another misconception is that, because the nature of human mental abilities is known and can be reliably measured, the source of individual differences in them is well-understood. This is not the case. Rather, an enormous amount remains to be understood about the nature of mental abilities, their development across the life span, and the specific genetic, biological, and environmental factors that either hinder or spur their development.
Personality is of interest to numerous disciplines, including the science of individual differences, which has given rise to a variety of theories. Despite their dissimilarities, most theories typically view personality as dispositional tendencies, or “a prepardedness,” to exhibit certain behavioral reactions to certain environmental affordances and demands. Within the individual difference tradition, most research has followed the “lexical approach” suggested by Galton. This approach assumes that important dimensions of human behavioral tendencies will be encoded in natural language. Using this approach to identify personality characteristics has led to a model of personality commonly referred to as the Big Five. The exact labels used to describe the dimensions have varied, but generally include (1) extraversion (includes surgency and positive emotionality factors), (2) neuroticism (includes anxiety and negative emotionality factors); (3) agreeableness, (4) conscientiousness, and (5) openness to experience. Though there is not a consensus regarding the appropriateness of this model, it is arguably the dominant model in individual difference research, and alternative models tend to be variations with two or more of the factors combined into broader factors or, alternatively, split into more narrow factors.
The use of personality tests for applied practices such as employee selection has had a long and rather tumultuous history. After a period of widespread use beginning in the early 1900s, industrial psychology placed a virtual moratorium on personality testing in the mid-1960s, primarily as a result of a failure to identify a clear pattern of criterion-related validity evidence. Applied personality testing was given new life in the mid-1980s, fueled by the emergence of the Big Five model and subsequent meta-analytic findings of meaningful, though moderate, criterion-related validities for some of these personality dimensions.
The importance of conative factors (or volition; e.g., interests and motives) is found in virtually all theories of the determinants of purposeful behavior. For example, John Campbell noted that one of the three major determinants of work performance, in addition to declarative knowledge and procedural skills, was motivation. Similarly, Richard Snow has long held that conative (as well as affective) factors are a critical component in the development of aptitudes, a contention supported by substantial research showing that interests, for example, are related to the acquisition of domain-specific knowledge and academic achievement. Likewise, vocational psychologists have long studied differences in interests to forecast vocational adjustment. For example, a widely used model of interest types developed by John Holland, known as the RIASEC model (which is an acronym based on names of the six interest types), has proven an effective framework for studying the impact of person environment fit on developmental, educational, and occupational outcomes.
Some individual difference scholars have begun using the term “affcon” to refer to this third domain, making salient the potential importance of affect (e.g., emotion, mood, and temperament) in conation. However, some researchers argue that so-called “affcon” factors, or at least the affective component, are actually part of personality. For example, some temperament theorists are emphasizing the role of early appearing differences in temperament as the basis for the development of the Big Five traits. Likewise, some personality researchers have shown evidence for trait-based motivational differences. Although such debate may initially seem to reflect confusion, it also appears to demonstrate the increasing effort to study the development of individual differences as constellations, rather than independent domains.
The Development Of Individual Differences: Nature Or Nurture?
Discussions of individual and group differences too often embrace a false dichotomy. The infamous “nature versus nurture” debate, though once central to the discourse of individual differences, is no longer a credible question asked by informed scientists. Differences in human characteristics are the result of a longitudinal interaction between genetic, biological, and environmental factors. To take a nonhuman example, the color of the hydrangea blossom will vary depending on the acidity of the soil during a certain period in its growth. Thus, the color of its blossom cannot be attributed solely to genetic or environmental factors; the color of the blossom is due to both genetic and environmental factors and the interaction between them. Likewise, to understand how human differences develop, it is necessary to understand (a) how differences in “nature” (i.e., genotypic differences) nurture environmental differences, and (b) how environmental differences (e.g., social, cultural, biological) nurture the expression of genotypic differences.
In addition to genetic–environment (g-e) interactions (e.g., differential reactivity to a given environment), that genetic and environmental factors have a symbiotic relationship is also made salient by the observation of three types of genotype-environment covariance. First, passive g-e covariance is reflected by circumstances where biological parents provide both the genetic code and the environment for a child. For example, parents who have high verbal abilities and like to read pass on both a genotype conducive to the development of verbal abilities and interest in reading, and a home environment filled with books and role models who read and express joy in reading. Second, evocative g-e covariance occurs because individual differences evoke different responses from the environment, and thus differences in experiences. For example, cooperative, bright students are more likely to receive positive responses from instructors than students who are argumentative and slow. Although a group of students is exposed to the same objective environment, differences in the experienced environment are evoked by manifest trait differences. Third, g-e covariance occurs because people actively select, attend to, and seek out specific opportunities and information from the myriad choices provided (often referred to as “niche-picking” or “niche-building”). To the degree that full and unimpeded access to a wide range of environments is available to all members of a group, genotypic and environmental differences are likely to become increasingly correlated as individuals seek out experiences that afford pleasantness (such as success and excitement) and withdraw from or avoid experiences that result in frustration, confusion, boredom, or consistent failure.
The Declaration of Independence proclaims, “We hold these truths to be self-evident, that all men are created equal, that they are endowed by their creator with certain unalienable rights, that among these are life, liberty, and the pursuit of happiness.” When Jefferson penned this phrase, he was neither denying nor ignorant of the omnipresent impact of human variability. Rather, his intent was to suggest the opposite; that is, despite undeniable individual differences, all humans should have equal rights and opportunities. “Created equal” means equality before the law, a concept that has found expression in parts of the U.S. Constitution, and has been given operational force in both legislation and judicial opinion. The study of individual differences embraces this philosophy by seeking to understand how and why humans differ from each other so that we can ensure each individual has an equal opportunity for optimal development.
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