Human Characteristics

Abstract

Human characteristics, which MeSH classifies under behavior and behavior mechanisms and indexes with the entry term human nature, are the dispositions and traits taken to characterize human beings. This article covers how psychology has studied them: personality trait structure across cultures and its proposed universality, the stability and change of traits across the life span, the heritability of human traits estimated from twin studies, an evolutionary account of a species-typical human nature, the social-cognitive skills that may distinguish human children from other great apes, lay concepts of what makes someone human, and the critique that much of the evidence comes from Western, educated, industrialized, rich and democratic samples. The central tension is between claims of a universal human nature and evidence of substantial variation across populations.

Keywords: human nature, personality traits, heritability, cross-cultural psychology, humanness

The concept of a universal human nature, based on a species-typical collection of complex psychological adaptations, has been defended as valid despite the existence of substantial genetic variation that makes each human genetically and biochemically unique (Tooby & Cosmides, 1990). Against that, a review of the comparative database across the behavioral sciences suggested both that there is substantial variability in experimental results across populations and that subjects from Western, educated, industrialized, rich and democratic societies are frequent outliers (Henrich et al., 2010).

What Human Characteristics Are

Psychology has approached the question of what characterizes humans in several ways. One is the study of individual differences: there appears to be a universal desire to understand individual differences, and this common desire exhibits both universal and culturally specific features (Heine & Buchtel, 2009). Another is the evolutionary account, in which an evolutionary approach to psychological variation reconceptualizes traits as either the output of species-typical, adaptively designed developmental and psychological mechanisms, or as the result of genetic noise creating perturbations in these mechanisms (Tooby & Cosmides, 1990). A third is the comparison with other species (Herrmann et al., 2007).

Figure 1

Four Questions in the Study of Human Characteristics

Personality Traits as Human Universals

Data from six translations of the Revised NEO Personality Inventory in German, Portuguese, Hebrew, Chinese, Korean and Japanese samples (N = 7,134) showed structures similar to the American factor structure, and because the samples represented languages from five distinct language families, the authors concluded that the data strongly suggest that personality trait structure is universal (McCrae & Costa, 1997). The Big Five Inventory, translated into 28 languages and administered to 17,837 individuals from 56 nations, showed a five-dimensional structure that was robust across major regions of the world (Schmitt et al., 2007).

The universality claim has limits. A review found that the five-factor model emerges quite consistently across cultures, with some key variations when the structure is drawn from indigenous traits in other languages; that the extent to which each trait is endorsed varies considerably across cultures, although there are methodological challenges with comparing personality traits across cultures; and that people in collectivistic cultures appear to rely on traits to a lesser degree when understanding themselves and others (Heine & Buchtel, 2009). The first test of the five-factor model in a largely illiterate, indigenous society, among Tsimane forager-horticulturalists of Bolivia, failed to find robust support for it; the authors argued that Tsimane personality variation displays two principal factors that may reflect socioecological characteristics common to small-scale societies (Gurven et al., 2013). In 29 face-to-face surveys of 94,751 respondents in 23 low- and middle-income countries, commonly used personality questions generally failed to measure the intended personality traits and showed low validity, in contrast with the much higher validity the same measures attained in internet surveys of self-selected respondents from the same countries; the authors concluded that although the five-factor model may well be universal, it appears hard to uncover this factor structure with survey data in contexts other than those for which it was developed (Laajaj et al., 2019). Trait levels also differ by region: people from South America and East Asia were significantly different in openness from those in other world regions (Schmitt et al., 2007).

A meta-analysis of 770 studies reporting 3,523 effects of personality traits on prosocial behavior in economic games found meta-analytic correlations ranging from -.18 to .26 (Thielmann et al., 2020).

Stability and Change

Human characteristics are not fixed across the life span. A meta-analysis of 92 samples found that people increase in measures of social dominance, conscientiousness and emotional stability, especially in young adulthood (age 20 to 40), and that 4 of 6 trait categories demonstrated significant change in middle and old age (Roberts et al., 2006). A later synthesis of longitudinal studies published after 2005 (rank-order stability: k = 189, N = 178,503; mean-level change: k = 276, N = 242,542) found that rank-order stability increased significantly throughout early life before reaching a plateau in young adulthood, found little evidence for increasing rank-order stability after age 25, and found that emotional stability increased consistently and more substantially across the life span than previously found (Bleidorn et al., 2022).

Heritability of Human Traits

Twin studies estimate how much of the variation in a trait within a population is associated with genetic variation. A meta-analysis of twin correlations and reported variance components for 17,804 traits from 2,748 publications, including 14,558,903 partly dependent twin pairs, found that across all traits the reported heritability is 49%, and that for a majority (69%) of traits the observed twin correlations are consistent with a simple model in which twin resemblance is solely due to additive genetic variation (Polderman et al., 2015). For intelligence, genome-wide association studies have identified inherited genome sequence differences that account for 20% of the 50% heritability of intelligence (Plomin & von Stumm, 2018).

According to Turkheimer (2000), behavior genetics has demonstrated that genetic variance is an important component of variation for all behavioral outcomes, but variation among families is not; Turkheimer also argued that genotype is a more systematic source of variability than environment for reasons that are methodological rather than substantive, and that twin studies do not show that genes are more fundamental than environments (Turkheimer, 2000).

In a sample of 7-year-old twins, a substantial proportion of them raised in families living near or below the poverty level, the proportions of IQ variance attributable to genes and environment varied nonlinearly with socioeconomic status: the models suggested that in impoverished families 60% of the variance in IQ is accounted for by the shared environment and the contribution of genes is close to zero, and that in affluent families the result is almost exactly the reverse (Turkheimer et al., 2003).

Evolution and Human Nature

The evolutionary account of Tooby and Cosmides proposes that a universal human nature can be reconciled with individual uniqueness. It holds that complex adaptations necessarily require many genes to regulate their development, and that sexual recombination makes it improbable that all the necessary genes for a complex adaptation would be together in the same individual if those genes varied substantially between individuals (Tooby & Cosmides, 1990). On this account, selection tends to impose relative uniformity at the functional level in complex adaptive designs, suggesting that most heritable psychological differences are not themselves likely to be complex psychological adaptations (Tooby & Cosmides, 1990).

A later evolutionary-genetic review assessed three mechanisms that could explain genetic variance in personality differences and concluded that selective neutrality is largely irrelevant, that mutation-selection balance seems best at explaining genetic variance in intelligence, and that balancing selection by environmental heterogeneity seems best at explaining genetic variance in personality traits (Penke et al., 2007).

What May Be Distinctively Human

Comparative studies have asked which cognitive characteristics separate humans from other great apes. In a comprehensive battery of cognitive tests given to chimpanzees, orangutans and 2.5-year-old children before literacy and schooling, the children and chimpanzees had very similar cognitive skills for dealing with the physical world, but the children had more sophisticated cognitive skills than either ape species for dealing with the social world, a result the authors took to support the cultural intelligence hypothesis (Herrmann et al., 2007).

A related proposal locates the difference in shared intentionality, the ability to participate with others in collaborative activities with shared goals and intentions (Tomasello et al., 2005). On that proposal, great apes understand the basics of intentional action but do not participate in activities involving joint intentions and attention, and human children's skills of shared intentionality develop gradually during the first 14 months of life (Tomasello et al., 2005). A review of experimental evidence on cooperation concluded that human altruism is a powerful force and is unique in the animal world, but that there is much individual heterogeneity, and that current gene-based evolutionary theories cannot explain important patterns of human altruism (Fehr & Fischbacher, 2003). Such comparisons have also been criticized: one review argued that most claims of human cognitive uniqueness are based on comparisons of White middle class Westerners with captive chimpanzees, and that systematic biases are too often introduced in experiment protocols when comparing humans with apes, which further cast doubts on cross-species comparisons (Boesch, 2007).

Lay Concepts of Humanness

Ordinary people also hold theories of what makes someone human. Studies of lay concepts demonstrated two distinct understandings of humanness, traits representing human nature and traits that are uniquely human, and showed that only the former are understood as inhering essences; participants rated themselves higher than their peers on human nature traits but not on uniquely human traits (Haslam et al., 2005). The same distinction was used to propose two forms of dehumanization: denying uniquely human attributes to others represents them as animal-like, and denying human nature represents them as objects or automata (Haslam, 2006).

The WEIRD Critique

Claims about human characteristics depend on who is sampled. An early content analysis found that, since 1960, published research in social psychology had overwhelmingly been based on college students tested in academic laboratories on academiclike tasks, and argued that the peculiarities of this narrow data base may have contributed to central elements of social psychology's portrait of human nature (Sears, 1986). Behavioral scientists routinely publish broad claims about human psychology based on samples drawn entirely from Western, educated, industrialized, rich and democratic (WEIRD) societies, and a review of the comparative database suggested that members of WEIRD societies, including young children, are among the least representative populations one could find for generalizing about humans (Henrich et al., 2010). A later tool for measuring psychological and cultural distance between societies, applied to the World Values Survey, found, in analyses of regional diversity, the relative homogeneity of the United States compared with China, India and the European Union and reported that cultural distance predicts various psychological outcomes (Muthukrishna et al., 2020).

Worked Example

The heritability demonstration above uses the classical twin-design estimates, with illustrative twin correlations rather than data. If identical twins correlate 0.50 on a trait and fraternal twins correlate 0.25, the additive genetic share is 2 × (0.50 - 0.25) = 0.50, the shared-environment share is 2 × 0.25 - 0.50 = 0, and the remainder, 1 - 0.50 = 0.50, is attributed to non-shared environment and error. That pattern, in which the identical-twin correlation is exactly twice the fraternal-twin correlation, is the one expected when twin resemblance is solely due to additive genetic variation, the model consistent with the twin correlations for most traits in the large meta-analysis, whose reported heritability across all traits was 49% (Polderman et al., 2015).

The intelligence figure can be decomposed the same way: if genome-wide association findings account for 20 percentage points of a 50% heritability (Plomin & von Stumm, 2018), they explain 0.20 / 0.50 = 40% of the heritable variance, leaving 60% of it not yet located in measured genetic variants.

Discussion

Three points follow. First, the evidence for universality is strongest for the structure of traits and weaker for their levels: five-factor structure replicated across language families (McCrae & Costa, 1997) and world regions (Schmitt et al., 2007), though it was not robustly supported among Tsimane forager-horticulturalists (Gurven et al., 2013) and was hard to uncover in face-to-face surveys in low- and middle-income countries (Laajaj et al., 2019), but the endorsement of traits varies considerably across cultures, although there are methodological challenges with comparing personality traits across cultures (Heine & Buchtel, 2009).

Second, heritability figures need careful interpretation. The reported average of 49% (Polderman et al., 2015) sits beside the argument that twin studies do not show that genes are more fundamental than environments (Turkheimer, 2000), and one evolutionary account suggests that most heritable psychological differences are not themselves likely to be complex psychological adaptations (Tooby & Cosmides, 1990).

Third, claims about human characteristics carry a sampling caveat. The comparative database suggests substantial variability across populations (Henrich et al., 2010), so a characteristic observed only in WEIRD samples is not thereby shown to be a characteristic of humans.

Current Directions

Large-sample synthesis has refined estimates of trait development, finding cumulative mean-level trait changes across the life span slightly smaller than previously estimated (Bleidorn et al., 2022). Genome-wide association findings were described as opening new avenues for research into the causes and consequences of intelligence using genome-wide polygenic scores that aggregate the effects of thousands of genetic variants (Plomin & von Stumm, 2018). Cross-cultural research now has a tool for measuring psychological and cultural distance between any two countries, intended for designing and justifying comparative psychological projects (Muthukrishna et al., 2020).

Common Misconceptions

High heritability shows that genes matter more than environments.

Twin studies offer a useful methodological shortcut, but do not show that genes are more fundamental than environments (Turkheimer, 2000).

Personality is fixed by adulthood.

People increase in social dominance, conscientiousness and emotional stability especially between ages 20 and 40, and four of six trait categories showed significant change in middle and old age (Roberts et al., 2006).

Findings from Western students describe people in general.

A review of the comparative database suggested that members of WEIRD societies, including young children, are among the least representative populations for generalizing about humans (Henrich et al., 2010).

Glossary

Additive genetic variation.

Genetic variation whose effects on a trait sum across variants; the source of twin resemblance in the simplest twin model.

Animalistic dehumanization.

Denying uniquely human attributes to others, which represents them as animal-like.

Big Five.

The five-factor model of personality: extraversion, agreeableness, conscientiousness, neuroticism and openness.

Cultural intelligence hypothesis.

The proposal that humans' distinctive cognitive skills are mainly due to species-specific social-cognitive skills for participating in cultural groups.

Heritability.

The share of variation in a trait associated with genetic variation, as estimated for example from twin studies.

Human nature traits.

In lay concepts of humanness, the traits understood as inhering essences, as distinct from uniquely human traits.

Mean-level change.

Change in the average level of a trait in a population over time.

Mechanistic dehumanization.

Denying human nature to others, which represents them as objects or automata.

Polygenic score.

A score that aggregates the effects of thousands of genetic variants.

Rank-order stability.

The degree to which individuals keep their relative positions on a trait over time.

Shared intentionality.

The ability to participate with others in collaborative activities with shared goals and intentions.

Twin study.

A design that compares the similarity of identical and fraternal twins to estimate genetic and environmental contributions to trait variation.

Uniquely human traits.

In lay concepts of humanness, the second sense of humanness, distinct from human nature traits and not understood as inhering essences.

WEIRD.

Western, educated, industrialized, rich and democratic, describing the societies on whose samples behavioral scientists routinely base broad claims about human psychology.

Key Researchers

Wiebke Bleidorn

. Professor of Differential Psychology and Psychological Assessment at the University of Zurich; led the meta-analysis of personality stability and change across the life span. ORCID - Wikidata - Wikipedia - Google Scholar - Faculty Page

Leda Cosmides

. Psychologist at the University of California, Santa Barbara; with John Tooby defended the concept of a universal human nature based on species-typical psychological adaptations. ORCID - Wikidata - Wikipedia - Google Scholar - Faculty Page

Nick Haslam

. Psychologist at the University of Melbourne; distinguished human nature from uniquely human traits in lay concepts of humanness and built a model of two forms of dehumanization on that distinction. Wikidata - Google Scholar - Faculty Page

Steven J. Heine

. Psychologist at the University of British Columbia; reviewed the universal and culturally specific features of personality and co-authored the analysis of WEIRD samples. ORCID - Wikipedia - Google Scholar - Faculty Page

Joseph Henrich

. Anthropologist at Harvard University; led the review arguing that people from WEIRD societies are among the least representative populations for generalizing about humans. ORCID - Wikipedia - Google Scholar

Robert R. McCrae

. Psychologist formerly at the National Institute on Aging; with Paul Costa tested the five-factor model across translated samples and argued that personality trait structure is universal. ORCID - Wikipedia

Michael Tomasello

(b. 1950). Psychologist at Duke University, formerly at the Max Planck Institute for Evolutionary Anthropology; proposed shared intentionality as the crucial difference between human cognition and that of other species. Wikidata - Wikipedia - Google Scholar - Faculty Page

Eric Turkheimer

. Psychologist at the University of Virginia; set out laws of behavior genetics and argued that twin studies do not show genes to be more fundamental than environments. ORCID - Wikipedia - Google Scholar

Frequently Asked Questions

What does human nature mean in psychology?

One evolutionary account defends the concept of a universal human nature based on a species-typical collection of complex psychological adaptations (Tooby & Cosmides, 1990).

Is personality structure the same in every culture?

The five-dimensional structure of the Big Five Inventory was robust across major regions of the world in a study of 56 nations (Schmitt et al., 2007), but the first test in a largely illiterate, indigenous society failed to find robust support for the five-factor model (Gurven et al., 2013).

Does personality change with age?

The rank-order stability of personality traits increased through early life before reaching a plateau in young adulthood (Bleidorn et al., 2022).

How heritable are human traits?

Across virtually all published twin studies of complex traits, the reported heritability was 49% (Polderman et al., 2015).

What makes human cognition different from that of apes?

In one comparison, 2.5-year-old children had more sophisticated cognitive skills than chimpanzees and orangutans for dealing with the social world, while the children and chimpanzees had very similar skills for the physical world (Herrmann et al., 2007).

What is dehumanization?

One model distinguishes denying uniquely human attributes, which represents others as animal-like, from denying human nature, which represents them as objects or automata (Haslam, 2006).

What is the WEIRD problem?

Behavioral scientists routinely publish broad claims about human psychology based on samples drawn entirely from Western, educated, industrialized, rich and democratic societies, whose members a review suggested are among the least representative populations for generalizing about humans (Henrich et al., 2010).

Do people see themselves as more human than others?

Participants rated themselves higher than their peers on human nature traits but not on uniquely human traits (Haslam et al., 2005).

References