Genes, evolution, and personality.

There is abundant evidence, some of it reviewed in this paper, that personality traits are substantially influenced by the genes. Much remains to be understood about how and why this is the case. We argue that placing the behavior genetics of personality in the context of epidemiology, evolutionary psychology, and neighboring psychological domains such as interests and attitudes should help lead to new insights. We suggest that important methodological advances, such as measuring traits from multiple viewpoints, using large samples, and analyzing data by modern multivariate techniques, have already led to major changes in our view of such perennial puzzles as the role of "unshared environment" in personality. In the long run, but not yet, approaches via molecular genetics and brain physiology may also make decisive contributions to understanding the heritability of personality traits. We conclude that the behavior genetics of personality is alive and flourishing but that there remains ample scope for new growth and that much social science research is seriously compromised if it does not incorporate genetic variation in its explanatory models.

No heritability of temporomandibular joint signs and symptoms.

The causes of temporomandibular joint (TMJ)-related signs and symptoms are largely unknown. We tested the hypotheses that these signs and symptoms, as well as oral parafunctional habits, are substantially heritable. Questionnaire and clinical data were collected from 494 twins, including pairs of reared-apart and reared-together monozygotic (MZ) and dizygotic (DZ) twins. A history of joint-area pain, joint noises, and clenching and grinding habits was scored as present or absent. Twenty-nine percent of the population experienced at least one sign or symptom. Nearly one-quarter of subjects clenched or ground their teeth, and 8.7% reported a history of joint-area pain. Pain was associated with clenching, grinding, and joint noises. MZ twins were no more similar than DZ twins for any outcome, suggesting that genetic factors do not influence these traits in the population. Reared-together MZ twins were no more similar than reared-apart MZ twins, suggesting a negligible effect of the family environment on these outcomes. Environmental factors unique to each twin appeared to be the major determinants of variation in this population.

Heritability of MMPI Harris-Lingoes and Subtle-Obvious subscales in twins reared apart.

A large sample of identical and fraternal twins who had been reared apart was used to examine the genetic and environmental architecture of the MMPI Subtle-Obvious and Harris-Lingoes subscales. Univariate genetic analyses indicated significant heritability for all 28 of the Harris-Lingoes subscales (estimates ranged from.23 to.61), all five Obvious subscales (estimates ranged from.37 to.56) and four of the five Subtle subscales (estimates ranged from.27 to.35). Two randomly constructed scales were analyzed as controls; neither of these scales showed significant heritability. Exploratory correlational findings suggested that three of the Wiener-Harmon Subtle subscales may tap aspects of psychological health, naivete, or repression. Ma-S may come closest to Wiener and Harmon s intent. Although they apparently diverge from their original purpose, it may be too early to abandon the low face valid items of the Subtle subscales.

Intrinsic and extrinsic religiousness: genetic and environmental influences and personality correlates.

This report presents findings for the Intrinsic (IR) and Extrinsic (ER) religiousness scales from the Minnesota Study of Twins Reared Apart. The scales were shown to be internally consistent, sufficiently distinct from the scales of the California Psychological Inventory and the Multidimensional Personality Questionnaire and unrelated to a number of measures of response style to justify treating them as distinct traits. The I scales also showed considerable evidence of construct validity in its correlations with religious fundamentalism and authoritarianism as assessed by the MMPI and Altemeyer's Right-Wing Authoritarianism scale. Data on IR and ER from 35 pairs of monozygotic twins reared apart (MZA) and 37 pairs of dizygotic twins reared apart (DZA) were fitted to a biometric model and demonstrated significant heritability (0.43 and 0.39), with a model containing genetic plus environmental factors fitting significantly better than a model containing only an environmental component. Twin similarity could not be explained by placement on a self-reported measure of family Moral Religious Emphasis as measured by the Family Environment Scale.

Periodontal bacteria in adult twins.

BACKGROUND: Both environmental and genetic factors are known to influence clinical measures of periodontal disease. The purpose of this study was to determine whether genetic factors similarly influence the presence of specific periodontal bacteria in subgingival plaque. METHODS: Reared-together and reared-apart monozygous (MZ) and dizygous (DZ) adult twins were examined clinically. Demographic and behavioral information was obtained from each subject by questionnaire. Subgingival plaque samples were obtained from the index teeth, and the presence of P. intermedia, P. gingivalis, A. actinomycetemcomitans, E. corrodens, and F. nucleatum was determined using an immunoassay. RESULTS: Microbiological and clinical data were available for 169 twin pairs. The subject-based prevalences of the bacteria in the twin groups ranged from 11% for Porphyromonas gingivalis to 40% for F. nucleatum. For all species examined, the concordance rates were not significantly different (P > 0.05) between MZ and DZ twin groups. These findings were apparent despite similar smoking histories, self-reported oral hygiene practices, and antibiotic use in the twin groups. Furthermore, MZ twins reared together were not more similar than MZ reared-apart twins with respect to any bacterial species examined. CONCLUSIONS: These findings suggest that in a population with access to routine dental care, any effects that host genes and the early family environment have on the presence of specific bacteria in subgingival plaque are not apparent in adulthood. Most twins with disease in this study had early periodontitis. Results from this study may not necessarily be extrapolated to more advanced disease states.

Genetic and environmental influences on self-reported diet: a reared-apart twin study.

A sample of 66 monozyogtic twins reared apart (MZA) and 51 dizygotic twins reared apart (DZA), and 101 nontwin individuals (mostly spouses of the twins) who participated in the Minnesota Study of Twins Reared Apart (MISTRA) from 1979 to 1995 completed a self-report food frequency questionnaire. Intraclass-correlations and model-fitting analyses indicated that approximately 30% of the variance in the self-report of diet was attributable to genetic factors, with random environmental factors and measurement error responsible for the remaining variance. Spouse correlations were moderate. To investigate the effects of living together during marriage, the absolute differences between husband and wife on the dietary variables with years of marriage were correlated. None of the correlations were significant. Hierarchical multiple regression analyses also indicated that no convergence occurred during marriage. These results suggest that sharing a current family environment exerts minimal influence on individual differences in self-reported diet.

Migraine in twins raised together and apart.

History of headache and migraine diagnosed by structured interview utilizing International Headache Society criteria was obtained in two samples of female twin pairs--154 raised together and 43 raised apart since infancy. Zygosity was determined by standard methods. Probandwise concordance rates were determined. Assuming that liability to migraine has a multifactorial etiology involving a number of genetic and environmental risk factors acting additively, tetrachoric correlations in the four groups of twins (monozygotic raised together, monozygotic raised apart, dizygotic raised together, and dizygotic raised apart) were then calculated. The genetic and environmental influences in the liability to migraine were estimated using biometrical model-fitting methods. Tetrachoric correlations of migraine were consistently higher in monozygotic than in dizygotic twins, for both reared together and reared apart samples, with a heritability estimate of 52%, Model fitting also suggested that approximately 50% of the variance in liability to migraine was attributable to genetic factors with nonshared environmental factors and measurement errors responsible for the remaining variance.

Individual differences in adult ego development: sources of influence in twins reared apart.

A behavior genetic analysis of the personality dimension of ego development, as assessed by the Washington University Sentence Completion Test, was carried out on a sample of 45 pairs of monozygotic twins and 28 pairs of same-sex dizygotic twins. The twins ranged in age from 16 to 70 years at the time of assessment, had been separated during infancy, and had been raised apart in adoptive homes before being reunited in adulthood. Analyses indicated that reared-apart twins were similar in trait levels of ego development and that such similarity could not solely be attributed to measured similarity in cognitive ability, including verbal reasoning. Estimates of broad similarity for ego development (46%) were comparable to values reported for other major personality dimensions. Phenotypic variance in adult levels of ego development appears to have substantial genetic and environmental sources.

Genetic and environmental influences on human behavioral differences.

Human behavioral genetic research aimed at characterizing the existence and nature of genetic and environmental influences on individual differences in cognitive ability, personality and interests, and psychopathology is reviewed. Twin and adoption studies indicate that most behavioral characteristics are heritable. Nonetheless, efforts to identify the genes influencing behavior have produced a limited number of confirmed linkages or associations. Behavioral genetic research also documents the importance of environmental factors, but contrary to the expectations of many behavioral scientists, the relevant environmental factors appear to be those that are not shared by reared together relatives. The observation of genotype-environment correlational processes and the hypothesized existence of genotype-environment interaction effects serve to distinguish behavioral traits from the medical and physiological phenotypes studied by human geneticists. Behavioral genetic research supports the heritability, not the genetic determination, of behavior.

Genetic and environmental influences on adult intelligence and special mental abilities.

I review representative biometric studies of adult twins and adoptees that have been used to generate estimates of genetic and environmental influence on intelligence (IQ) and special mental abilities. The various studies converge on a heritability estimate between 0.60 and 0.80 for IQ. Estimates of common environmental influence from the same studies are near zero. Studies of twins reared together and studies of unrelated individuals reared together yield sizable estimates of common family environmental influence in childhood but also demonstrate that this influence dissipates with age and approaches zero in adulthood. Twin studies of the major special mental abilities (verbal, spatial, perceptual speed and accuracy, memory) yield heritability estimates of about 0.50 and modest estimates of common environmental influence.

Genetic and environmental influences on the continuous scales of the Myers-Briggs Type Indicator: an analysis based on twins reared apart.

The Myers-Briggs Type Indicator was administered to a sample of 61 monozygotic twins reared apart (MZA), 49 dizygotic twins reared apart (DZA), and 92 spouses, who participated in the Minnesota Study of Twins Reared Apart (MISTRA) from 1979 to 1995. Twins' scores on the continuous scales were subjected to behavior genetic model-fitting procedures. Extraversion-Introversion and Thinking-Feeling yielded heritabilities of about .60, consisting largely of nonadditive genetic variance. Sensing-Intuition and Judgment-Perception yielded heritabilities of about .40, consisting largely of additive genetic variance. Spouse correlations for three of the four scales were near zero and not statistically significant; one spouse correlation (Sensing-Intuition) was modestly positive and statistically significant.

The genetic correlation between impulsivity and sensation seeking traits.

A number of studies have demonstrated associations between sensation seeking traits and measures of impulsivity. This study examined contributions to the observed correlations between imupulsivity and sensation seeking traits. Fifty-seven pairs of identical and 49 pairs of fraternal twins who were reared apart and 90 individuals who also participated in the Minnesota Study of Twins Reared Apart completed the Control scale of the Multidimensional Personality Questionnaire (MPQ; Tellegen, 1982) and the four subscales of the Sensation Seeking Scale (SSS; Zuckerman, 1979). Consistent with previous studies, the Control scale was significantly correlated with the SSS. A Cholesky decomposition of the data indicated that the phenotypic correlations between the Control scale and the four subscales of the SSS were mediated almost entirely by genetic factors. In the final reduced model the proportion of the genetic variance of the Control scale in common with the SSS was estimated as 55%, and the rest of the genetic variance (45%) was attributed to the genetic variance specific to the Control scale. The results emphasize the importance of common biological mechanisms underlying associations between impulsivity and sensation seeking traits.

Experience Producing Drive Theory: how genes drive experience and shape personality.

There is now a large body of evidence from twin and adoption studies linking genetic variation to phenotypic variation on virtually all human individual differences. Individuals unquestionably influence the nature of their experiences, e.g. high-sensation seekers surround themselves with like-minded peers and seek out quite different experiences than low-sensation seekers. We propose a theory called Experience Producing Drive Theory-Revised, to account for the current findings in behavior genetics. This theory is based on the Darwinian view that complex organisms are active agents "designed" by natural selection to survive in the environments in which they evolved. The theory assumes that the behavior of complex organisms can best be understood in terms of motives, preferences and emotional responses that have evolved to drive specific behavioral patterns. We propose a number of ways in which this theory might be falsified.

Heritability of MMPI personality indicators of psychopathology in twins reared apart.

This report presents Minnesota Multiphasic Personality Inventory (MMPI) findings from the Minnesota Study of Twins Reared Apart. Data from 65 unique pairs of monozygotic twins reared apart (MZA) and 54 unique pairs of dizygotic twins reared apart (DZA) were analyzed. As in other results from this sample, MZA twins evidenced substantial similarity, highlighting the influence of shared genes. Biometric modeling yielded estimates of heritability for the MMPI's standard validity and clinical scales and for the Wiggins content scales ranging from .26 to .62 (M = .44), echoing previous findings from the twin and adoption literature on personality. The pattern of MZA and DZA correlations suggested nonadditive genetic effects for 3 MMPI scales. Multivariate profile analyses also suggested genetic influence on both profile elevation and shape.

Genetic and environmental contributions to the acquisition of a motor skill.

Practice, with feedback, is a fundamental variable that influences the aquisition of motor skills: with it, everyone improves, but some improve more than others. This simple fact has led to frequent debate over the relative importance of genetic and environmental influences on motor learning. In principle these factors could influence subjects' initial level of proficiency, their rate of improvement or their final level of attainment. The problem has been investigated using the rotary pursuit (RP) task, in which subjects learn to track a rotating target with a stylus; this is a factorially pure task which is relatively unaffected by cognitive or verbal factors. Earlier studies of twins reared together indicated that heredity was the primary factor responsible for individual differences in motor skill. Here we have studied learning in a sample of monozygotic (MZA) and dizygotic (DZA) twins who had been reared apart. Heritability of performance was high even in the initial phase, and increased with practice. The rate of learning was also significantly heritable. We propose that the effect of practice is to decrease the effect of environmental variation (previous learning) and increase the relative strength of genetic influences on motor performance.

Genetic influences on perceptions of childhood family environment: a reared apart twin study.

This study estimates the extent to which heredity influences perceptions of childhood family environment in a sample of 58 monozygotic and 46 dizygotic pairs of adult twins who were reared apart. The measures used to assess family environments were the Family Environment Scale (FES) and Block Environmental Questionnaire (BEQ). A principal component factor analysis with a VARIMAX rotation of the FES and BEQ yielded 2 major factors--Support, and Organization and Cultural Orientation. Single and multiple indicator model-fitting techniques were applied to the reared apart twin data on the 2 factors. Perceived support in childhood family environments was fitted best by a model incorporating additive genetic and unshared environmental factors. Perceived organization was fitted most adequately by a model which includes only unshared environmental factors. Maximum-likelihood estimates of heritability from model-fitting analyses suggest that genetic factors explain 44% of the variance of perceptions of support dimension in childhood family environments.

Dental caries and treatment characteristics in human twins reared apart.

The 'reared apart' model, eliminating the effect of common environmental factors, is used extensively in twins research. In this study, teeth present (TP), teeth present excluding third molars (TPX3), teeth restored (TR), teeth restored index (TRI), surfaces restored (SR), surfaces restored index (SRI) and surfaces restored or carious (SRCI), were compared in 46 monozygotic (MZ) twin pairs and 22 dizygotic (DZ) twin pairs separated during infancy and raised apart. The dental examination included charting, panoral and bitewing radiographs, study models and a questionnaire. Data were analysed using intraclass correlations (ICC) and analysis of variance (corrected for age and sex). For all characteristics, the MZ pairs showed greater within-pair similarity than DZ pairs. For MZ pairs, the ICC values for the dental characteristics were all statistically significant (p < 0.005): TP, 0.45; TPX3, 0.49; TR, 0.57; TRI, 0.61; SR, 0.46; SRI, 0.67; SRCI, 0.58. For DZ pairs, none of the ICC values reached statistical significance: TP, 0.04; TPX3, 0.11; TR, 0.30; TRI, 0.31; SR, 0.20; SRI, 0.17; SRCI, 0.26. Despite subjects being reared in different environments, receiving different diets and different dental professional care, the MZ findings demonstrated significant genetic variance (45-67%) for the number of teeth present, number of teeth and surfaces restored, and caries present; the DZ data supported this conclusion. These findings provide new evidence for a genetic contribution to dental caries.

Heritability of interests: a twin study.

The authors administered inventories of vocational and recreational interests and talents to 924 pairs of twins who had been reared together and to 92 pairs separated in infancy and reared apart. Factor analysis of all 291 items yielded 39 identifiable factors and 11 superfactors. The data indicated that about 50% of interests variance (about two thirds of the stable variance) was associated with genetic variation. The authors show that heritability can be conservatively estimated from the within-pair correlations of adult monozygotic twins reared together. Evidence for nonadditive genetic effects on interests may explain why heritability estimates based on family studies are so much lower. The authors propose a model in which precursor traits of aptitude and personality, in part genetically determined, guide the development of interests through the mechanisms of gene-environment correlation and interaction.