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.

Stoolmiller on restriction of range in adoption studies: a comment.

Stoolmiller has recently presented a model featuring an inferred dimension of family quality which, he suggests, is severely truncated in adoption studies such as the Texas Adoption Project, resulting in gross underestimation of the effects of shared family environment. We discuss some potential limitations of his approach in general and as he applies it to the Texas adoption data on IQ.

Dimensions of psychological masculinity-femininity in adult twins from opposite-sex and same-sex pairs.

Male and female twins with opposite-sex co-twins were compared to twins with same-sex co-twins on three independent dimensions of masculinity-femininity, in order to examine the hypothesis that the hormones of the co-twin might have an effect on prenatal masculinization. The analysis was originally carried out for an older cohort from the Australian Twin Registry (2647 pairs, mean age 41.2), and then repeated in a younger cohort (1503 pairs, mean age 23.2). For women, the results on two of the three scales support and extend that of an earlier large study in Finland by Rose et al. (1994), who found no effect of sex of co-twin on feminine interests. One of the two scales contrasted worried and calm individuals, the other, confiding and reserved ones. The third scale, willingness to break or bend rules, showed a small effect of shared environmental influence, and it lay in the expected direction for a prenatal hormonal effect--females with a male co-twin scored higher (more like males). Most previous studies have not looked at the effect of sex of co-twin on males. The present study detected several such effects, although all were small in magnitude. The pattern was complex: sometimes the effect was in the masculine direction, sometimes in the feminine direction; sometimes there was agreement between the older and younger cohorts, sometimes not. Overall, the results suggest that no simple masculinization hypothesis--prenatal or postnatal--will adequately account for all the evidence. Age, sex, and aspect of masculinity-femininity must be taken into account.

Environmental influences that may precede fertilization: a first examination of the prezygotic hypothesis from maternal age influences on twins.

The prezygotic hypothesis considers the possibility that development is subject to environmental influences on the oocyte prior to conception. Such influences may occur in the maternal grandmother's uterus where oogenesis is completed or in the mother before fertilization. According to this hypothesis, the separate eggs from which DZ twins are derived may be sensitive to microenvironmental variations within an ovary. As a first approach, we examined same-sex MZ and DZ twins for maternal age effects on differences between pairs in cognitive and behavioral traits. While no differences between MZ and DZ pairs were found that would indicate a major effect of the prezygotic environment, suggestions are made for further experimental studies of this unexplored question in human development.

A comparison of adult female twins from opposite-sex and same-sex pairs on variables related to reproduction.

In several litter-bearing species, prenatal exposure of a female fetus to hormones from adjacent male fetuses can lead to later effects on various anatomical and behavioral characteristics of the female, including a number related to reproduction. To see if such traits are also affected in humans, adult female twins from a large Australian sample who had male cotwins were compared to females with female cotwins on 90 questionnaire items related to reproductive functions. No substantial effects could be clearly demonstrated, although some weak effects remained a possibility. Some variables, such as age at first menstruation, age at first pregnancy, and height, were consistent in direction with results from the animal literature, although the effect sizes were small and not statistically significant.

Genetic and environmental influences on MMPI factor scales: joint model fitting to twin and adoption data.

In general, the shared family environment appears to play a negligible role in determining individual differences in personality and interests. Nevertheless, scattered reports of significant shared environmental influence on such variables appear in the literature. Using data from the Texas Adoption Project (TAP), the current study attempted to replicate twin study findings of significant shared environmental variance on four of nine Minnesota Multiphasic Personality Inventory (MMPI) factor scales (Rose, 1988). Conventional behavioral genetic analyses of the adoption data agreed in affirming a significant shared environmental influence on individual differences in Religious Orthodoxy only. Subsequent simultaneous modeling of Rose's twin data and TAP adoption data resulted in three scales (Extraversion, Inadequacy, and Religious Orthodoxy) showing significant shared environmental influence. Again, effects were most substantial for Religious Orthodoxy, where the shared environment accounted for nearly 50% of the variance. It is argued that assortative mating cannot explain this finding.

Behavioral genetics '97: ASHG statement. Recent developments in human behavioral genetics: past accomplishments and future directions.

The field of behavioral genetics has enormous potential to uncover both genetic and environmental influences on normal and deviant behavior. Behavioral-genetic methods are based on a solid foundation of theories and methods that successfully have delineated components of complex traits in plants and animals. New resources are now available to dissect the genetic component of these complex traits. As specific genes are identified, we can begin to explore how these interact with environmental factors in development. How we interpret such findings, how we ask new questions, how we celebrate the knowledge, and how we use or misuse this knowledge are all important considerations. These issues are pervasive in all areas of human research, and they are especially salient in human behavioral genetics.

Additional findings on heritability and prenatal masculinization of cochlear mechanisms: click-evoked otoacoustic emissions.

A previous demonstration of a substantial genetic contribution to the expression of spontaneous otoacoustic emissions (SOAEs) is here extended to an aspect of click-evoked otoacoustic emissions (CEOAEs). CEOAEs were measured in the same twins and non-twins used for the SOAE heritability study. The stimuli were 100-microsecond clicks presented a nominal rate of 2/s; the emitted waveforms from 50 clicks were summed, and a 20-ms sample of that averaged waveform (beginning 6 ms after click presentation) was subjected to spectral analysis. The total power in the spectrum from 1 to 5 kHz in this temporal segment of the CEOAE waveform was used as the primary dependent variable. This overall power was significantly greater in female and right ears than in male and left ears, but the difference between dark- and light-eyed subjects was not significant. The overall power in the two left, and two right, ears of monozygotic co-twins was more highly correlated than in dizygotic co-twins, and structural modeling indicated that about 65-85% of the individual variation in the expression of CEOAE power could be attributed to genes-essentially the same heritability estimate as obtained previously from the SOAE data. Within-subject correlations between CEOAE power and number of SOAEs ranged from about 0.3 to 0.7, suggesting that these two forms of otoacoustic emission may depend upon somewhat different aspects of the same underlying mechanism and, thus, that heritability estimates based on one measure are not completely redundant to those from the other. While the average spectral power of the CEOAEs in opposite-sex dizygotic (OSDZ) females was smaller than that in same-sex dizygotic (SSDZ) females- and thus approached the value for males-the difference did not achieve statistical significance. Thus, the evidence for a prenatal masculinizing effect was less definitive in these CEOAE data than in the SOAE data obtained from the same subjects. An interpretation that accounts for both the CEOAE and SOAE results is that the strength of the so-called cochlear amplifiers is under genetic control that is to some extent mediated and/or modified through prenatal exposure to androgens. The indicated direction of effect is that weak cochlear amplifiers result when prenatal androgen levels are high. Under this view, then, androgen level contribute both to the sex differences observed in otoacoustic emissions and the prenatal masculinizing effects observed in opposite-sex twins, and they may be a factor in individual differences in OAE expression as well. Additionally it is shown that, although the powers of the CEOAE waveforms were reasonably highly correlated for the two ears of subjects in all groups, and across MZ co-twins, cross-correlations on the fine structures of those same pairs of CEOAE waveforms were essentially zero-presumably owing largely to the synchronizing of (different) SOAE frequencies in the ears being compared.

On the heritability of spontaneous otoacoustic emissions: A twins study.

Spontaneous otoacoustic emissions (SOAEs) were measured in human monozygotic (MZ) and dizygotic (DZ) twins and in a sample of non-twins. The number of SOAEs exhibited was more highly correlated in MZ co-twins than in same-sex DZ co-twins. Model-fitting to the correlations suggested that about three-quarters of the individual variation in the expression of SOAEs is attributable to genes. There was no convincing evidence for the heritability of specific SOAE frequencies. In accord with past surveys, SOAEs were more numerous in right than left ears, and in female than male subjects. Also investigated were the numbers of SOAEs exhibited by dark-versus light-eyed people and by MZ versus DZ twins. Those differences in our data were small and not statistically significant, but they were in a direction consistent with other studies: more SOAEs in dark-eyed individuals and in MZ twins. The view presented here is that SOAEs themselves are unlikely objects for natural selection, and probably are epiphenomena resulting from selection for those cochlear mechanisms that contribute to good hearing sensitivity - which is related to SOAE expression. It is argued that, in addition to genetics, other factors have the potential to affect the specific numbers of SOAEs that are expressed. For example, some aspects of the complex prenatal process of producing a male fetus are presumed to be responsible for the smaller number of SOAEs seen in males than females.

Nature, nurture, and conservatism in the Australian Twin Study.

Church attendance, educational level, and six conservatism scales were the subject of a multivariate behavior-genetic analysis by Truett et al. (Behav. Genet. 22, 43-62, 1992), based on responses from a large sample of adult Australian twins. These data are here analyzed in a different way to elicit general conservatism factors in the genetic, shared environmental, and unshared environmental covariation. The general genetic factor appears mainly to reflect intellectual sophistication; the general environmental factors, religious affiliation. These factors are similar, although not identical, for men and women.

An adoption and a cross-fostering study of the Minnesota Multiphasic Personality Inventory (MMPI) Psychopathic Deviate Scale.

The first of two complementary studies compared biological and adoptive parents of teenage adoptees with either higher (n = 21) or low (n = 51) MMPI Psychopathic Deviate (Pd) scale scores. In comparison to biological mothers of the low-Pd adoptees, biological mothers of the high-Pd adoptees obtained significantly higher MMPI scores on six of eight clinical scales. Fewer differences existed between the corresponding groups of adoptive mothers, but adoptive mothers of the high Pd's did obtain significantly higher scores on the Pd and Hypomania scales. Substantial genetic correspondences also existed for Harris-Lingoes content subscales, with fewer correspondences between adoptees and their adoptive mothers. There were indications that adoptive mothers of the high-Pd children had personality traits which may have made them less effective in attenuating early signs of antisocial behavior. The second study employed a cross-fostering design dividing all biological and adoptive mothers (n = 138 each) by their respective median Pd raw scores to examine effects on offspring. Results confirmed the effect of biological mother Pd score, but only a trend suggested an adoptive mother effect, with no hint of an interaction.

Using EQS for a simple analysis of the Colorado Adoption Project data on height and intelligence.

Simple models in the general spirit of Coon et al. (1990) are fit to the Colorado Adoption Project height and intelligence data as the second step in a two-step process. In the first step, over-time data on height and IQ are reduced to level and slope parameters. In the second, these are incorporated in path models along with parental data and fitted using EQS in a multiple-group design. Some comparisons are made between EQS and LISREL for this kind of modeling.

Heredity, environment, and personality change: evidence from the Texas Adoption Project.

Personality changes over time can be analyzed by the same twin and adoption methods used to analyze the genetic and environmental influences on a trait at a given time. Composite parent rating measures of Extra-version, Socialization, and Stability made on two occasions approximately 10 years apart on 229 adopted and 83 nonadopted children from the Texas Adoption Project were used to illustrate this point in two ways. The first was based on correlations among family members, from which it appeared that by far the chief source of individual change was neither the genes nor shared family environment, but individual experience (and/or measurement error). The second was via a path-analytic approach to changes in the means of adopted and natural children, from which it appeared that, nonetheless, the children were tending to change on the average in the direction of their genetic parents' personalities.

Genetic and environmental variation on the California Psychological Inventory vector scales.

Three recently introduced vector scales for the California Psychological Inventory (CPI) were scored for the National Merit Twin Sample and subjected to a heredity-environment analysis. Confidence intervals for genetic and environmental parameters were obtained by a bootstrap method. Two of the scales, Internality (Vector 1) and Self-Realization (Vector 3), showed the substantial heritability and near-zero family environment effects typically found for personality scales, whereas Norm-Favoring (Vector 2) showed an appreciable effect of family environment and a nonsignificant heritability.