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What can we learn from thestudy of twins about geneticand environmental influenceson interpersonal affiliation,aggressiveness, and social anxiety?: ameta-analytic studyMichael J. Beatty a , Alan D. Heisel a , Alice E. Hall a , Timothy R.Levine b & Betty H. La France aa Department of Communication, University of Missouri-St. Louisb Department of Communication, Michigan State UniversityPublished online: 21 Oct 2010.

To cite this article: Michael J. Beatty , Alan D. Heisel , Alice E. Hall , Timothy R. Levine & BettyH. La France (2002) What can we learn from the study of twins about genetic and environmentalinfluences on interpersonal affiliation, aggressiveness, and social anxiety?: a meta-analytic study,Communication Monographs, 69:1, 1-18, DOI: 10.1080/03637750216534

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What Can We Learn from the Study of Twins aboutGenetic and Environmental Influences on

Interpersonal Affiliation, Aggressiveness, and SocialAnxiety?: A Meta-Analytic Study

Michael J. Beatty, Alan D. Heisel, Alice E. Hall, Timothy R. Levine,and Betty H. La France

Over the past ten years, studies comparing monozygotic (identical) and dizygotic (fraternal) twin pairs interms of intelligence, personality, and social behavior have increasingly appeared in major researchjournals. Importantly, many of the variables examined are central constructs in communication theory.Assertiveness, social anxiety, self-monitoring, and empathy are but a few of the variables studied in thetwins literature. In the present study, mean heritability estimates for three clusters (interpersonalaffiliation, aggressiveness, and social anxiety) were computed. The studies selected for analysis focused onvariables that have been frequently cited in mainstream reference texts and/or professional journals incommunication. Conservative data analytic procedures were adopted. Specifically, (1) unless evidence ofadditive gene effects was absolute, nonadditive gene effects were assumed; (2) unless reliability coefficientswere reported in a study, the highest reliability reported in the literature for the measure was employed incorrection procedures; and (3) observer ratings, although imperfect, were not corrected for attenuation dueto error in measurement. Results indicated that interpersonal affiliation was 70% heritable, aggressive-ness was 58% heritable, and social anxiety was 65% heritable. Notably, heritability estimates were notlarger for self-report measures than for behavioral observation. Criticisms of the twins design, possiblemoderators, and theoretical implications are discussed.

Over the past decade, studies comparing monozygotic (identical) twin pairs todizygotic (fraternal) twin pairs along the lines of intelligence, personality, and

social behavior have increasingly appeared in major research journals. The interestin twins research shown by personality theorists

hinges on the fact that monozygotic (MZ) twin pairs are genetically identical, but dizygotic (DZ)twin pairs share on average only 50% of their genes. Doubling the difference between MZ and DZwithin-pair correlations therefore provides an estimate of the proportion of trait variance attribut-able to genetic influences, the heritability for that trait. (Hughes & Cutting, 1999, p. 429)

In general, the research literature based on twins studies indicates that “the averageheritability of psychological traits seems to be about 50% based on single measure-ments, and perhaps 75% when based on estimates of the stable components of traits(e.g., on the means of repeated measures)” (Lykken, 1995, p. 108).

Importantly, many of the variables examined in the twins literature are centralconstructs in communication theory. Assertiveness, social anxiety, empathy, verbalaggressiveness, and self-monitoring are but a few of the variables studied in the twins

Michael J. Beatty (Ph.D., Ohio State University, 1976) is Professor and Chair, Alan D. Heisel (Ed.D., WestVirginia University, 2000), Alice E. Hall (Ph.D., University of Pennsylvania, 2001), and Betty H. La France(Ph.D., Michigan State University, 1998) are Assistant Professors in the Department of Communication,University of Missouri–St. Louis, and Timothy R. Levine (Ph.D., Michigan State University, 1992) isAssociate Professor of Communication, Michigan State University. An earlier version of this study waspresented by the first author at the Department of Communication Colloquium Series, Michigan StateUniversity.

Communication Monographs, Vol. 69, No. 1, March 2002, pp 1–18Copyright 2002, National Communication Association

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literature, which are frequently cited in the communication literature (e.g., Knapp &Miller, 1994). Despite the potential relevance of the twins research literature tocommunication theory and research, little more than a general and cursory acknowl-edgement of this body of research can be found in the communication literature (c.f.,Beatty & McCroskey, 1997, 2000; Beatty, McCroskey, & Heisel, 1998; Beatty &Valencic, 2000; Horvath, 1995). Moreover, there has been disagreement about thenature of the twins literature and the magnitude of the findings (c.f., Beatty &McCroskey, 2000; Condit, 2000).

As with any approach to research, the twins design has limitations. Challenges tothe validity of the assumptions underlying the twins design have appeared in theliterature (e.g., Hoffman, 1991, 1994; Rose, 1995; Wilson, 1998). However, satisfac-tory responses to critics (see for example, Bouchard, 1993, 1994; Goldsmith, 1983;Lykken, 1995; Martin, Boomsma, & Machin, 1997; Scarr & Carter-Saltzman, 1979;Segal, 1997, 1999) have led contemporary behavior geneticists to describe the twinsdesign as “the perfect natural experiment” (Martin et al., 1997. p. 387).

1

In this essay,we (1) present a quantitative summary and analysis of the twins studies related toconstructs frequently cited in the communication literature, and (2) explore theimplications of the twins literature for communication theory and research.

Method

Selection of Studies

Our literature search consisted of three steps. First, we conducted an electronicsearch of databases including PsychInfo, Biological Abstracts, Bioethics Online,EBSCOhost, Eric, HealthStar, and General Science Index, producing over 5000citations pertaining to the study of twins through the year 2000. Second, we reviewedthe journals that have published twins studies. Third, we scanned the referencesections of studies found in our search to locate studies undetected by the first twosteps of our initial search strategy. Although there was considerable overlap in theoutput of our search, we did locate a number of important studies that otherwisewould have been undiscovered.

After sorting the duplications, we narrowed our focus to studies that containedvariables considered important to communication theory. Although an argumentcould be made for many different personality variables (e.g., verbal intelligence), welimited our sample to variables that have been cited in essays appearing inmainstream reference texts (e.g., Knapp & Miller, 1994) and/or professional journalsin communication (e.g., NCA and ICA journals). Application of this requirementproduced 40 studies and included over 20 different variables. These variablesrepresented three dimensions of social behavior: interpersonal affiliation, aggressive-ness, and social anxiety. Each study was coded for sample size, age of subjects,variable, method of measurement, within-pair correlations, heritability estimates,and reliability coefficients.

Analytic Criteria

Measurement error. A typical summary of the twins literature depicts genetic sourcesas contributing between 30–60% of the variance in personality (e.g., Segal, 1999).However, this range of effects is based on attenuated correlations. Importantly, most

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of the reliability coefficients for the measures employed in the twins literature arebetween .65 and .80. Because environmental effects are calculated by subtracting theheritability estimate (h2) from Rmz (Plomin, 1986), use of attenuated correlationsdeflates heritability estimates and inflates environmental effects.

The inflation of environmental effects is particularly problematic given theevolution of the environmental construct. Across the twins research, results indicatevirtually no effect of shared environment (i.e., environmental factors such associoeconomic status shared by both twins). Instead, the variance unexplained bygenetic factors is attributed to “nonshared environment,” which can erroneously betaken to mean the unique experiences of one sibling. In an effort to explain theconcept of nonshared environment, Buss and Plomin (1984) suggested that “parentallove could easily be construed as a source of difference among children in the samefamily, for most parents love one child more than another” (p. 152). Although thenonshared environment construct makes intuitive sense and tends to rescue environ-mental factors, the term “nonshared environment” is to some extent misleading.Researchers involved in twins studies understand that nonshared environmentshould not be interpreted literally. In fact, nonshared environment is a “residualterm that is composed of many sources, including measurement error and varioussources of genetic and environmental interactions” (Rushton et al., 1986, p. 1195).Reducing measurement error associated with Rmz would therefore reduce thevariance attributable to nonshared environment. Indeed, this can be seen in theformula used to estimate nonshared environment (e2): e2 � 1 � Rmz. Accordingly,increases in Rmz achieved through correction for attenuation, for example, wouldreduce per force estimated nonshared environmental effects.

In our analysis, both monozygotic and dizygotic twin correlations were correctedfor attenuation prior to calculating heritability and environment effects. Whenreliability coefficients were not reported in the study, the estimate reported for thevariable in the original study was employed. In almost every case, the reliabilityreported in the original study is the largest depicted in the literature. Correlationsbased on observer ratings (although attenuated by imperfect observation) were notcorrected for attenuation because corrections based on reliability coefficients associ-ated with observer ratings sometimes lead to inflated disattenuated coefficients(Hoyt, 2000). The decision not to correct these types of correlations leads to slightlymore conservative heritability estimates and somewhat inflated environmentaleffects estimates depending on the degree of unreliability and the number ofobservational studies in the cluster.

Heritability (h2), shared environment (c2), and nonshared environment (e2). As men-tioned, when dizygotic twin pair correlations are at least one-half the magnitude ofmonozygotic twin correlations, the assumptions of additivity are met and heritabilitymay be calculated by multiplying by two the difference between Rmz and Rdz

(Falconer, 1989). However, in the face of evidence indicating nonadditive geneeffects, Rmz should be employed as the index of heritability.

2

Although it could beargued that Rdz correlations should deviate significantly from .5 Rmz before assumingnonadditivity, a more conservative approach was employed in our analysis. Specifi-cally, we used Rmz as the estimate of h2 whenever Rdz was less than .5 Rmz. FollowingPlomin’s (1986) formulae, shared environment was calculated as c2 � Rmz � h2 andnonshared environment was estimated as e2 � 1 � Rmz.

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Results

Attenuated correlations for the monozygotic and dizygotic twin pairs, heritabilityestimates, shared environmental effects, nonshared environmental effects, andreliability estimates by study for interpersonal affiliation, aggressiveness, and socialanxiety appear in Tables 1, 2, and 3, respectively. The associated correlationscorrected for attenuation and heritability and environment effects derived fromdisattenuated correlations are reported in Tables 4, 5, and 6. The average weightedcorrelation corrected for attenuation was .72 for MZ pairs and .28 for DZ pairs. Theaverage weighted heritability estimate (h2) based on corrected correlations for allstudies irrespective of cluster (total N � 51,637) was .68 (95% confidence interval �.64 to .73).

Effects of Correction for Attenuation

Because reliability estimates were not reported for many of the studies containedin the data set, estimates based on the highest coefficients reported in the literaturewere employed. Analysis of the effects of estimated reliabilities on correctedheritability estimates indicated that effects were not inflated by the procedure. First,the mean incremental increase in heritability effects based on estimated reliabilitycoefficients (Mchange � .10) was less than that observed for studies in which reliabilityestimates were reported (Mchange � .12). Second, the average of the estimatedreliability coefficients (Malpha � .82) was consistent with the average of the reliabilitycoefficients for studies that reported them (Malpha � .83).

Heterogeneity of Effects

An overall examination of the distribution of heritability estimates showed that itwas negatively skewed (�1.27) and leptokurtic (2.55), indicating that the effectstended to be high and generally clustered about the mean (Figure 1). As might beexpected given the high N, the small error term resulted in significant heterogeneityof the effects [�2 (53) � 4795.79, p � .001]. Examination of the data revealed twoextreme outliers (Plomin & Foch, 1980; Plomin, Foch, & Rowe, 1981), whichreported effects over 25 standard errors away from the mean (these studies arelocated on the far left of the distribution depicted in Figure 1). When the outlierswere removed, the average weighted effect was .69 (N � 51,219, K � 52, 95%confidence interval � .65 to .73). The remaining effects, however, were alsoheterogeneous [�2 (51) � 4271.13, p � .001]. Thus, the data were examined formoderator effects following the procedures outlined by Hunter and Schmidt (1990).Specifically, method (self-report versus behavior observation), cluster (social anxi-ety, aggressiveness, or interpersonal affiliation), and age (children versus adults)were probed. In light of the tiny error associated with our sample size, potentialmoderator effects were inspected for both magnitude and statistical significance.

Method effects. Critics of the twins research often argue that the heritabilityestimates reported in the literature are due to the exclusive use of self-reportmeasures. Because our data set contained 10 studies based on behavior observation(Ntotal � 3,749) it was possible to compare heritability estimates derived fromself-reports to estimates based on observation of behavior. Complicating our analy-ses of possible moderator variables is that heritability estimates for observationalstudies were not corrected for attenuation. As such, comparing observational studies

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TABLE 1CORRELATIONS, HERITABILITY ESTIMATES, ENVIRONMENTAL EFFECTS, AND RELIABILITY ESTIMATES FOR THE

INTERPERSONAL AFFILIATION CLUSTER

Study

Attenuated results

Method/Variable N Rmz Rdz h2 c2 e2 Rel.

Scarr (1969) Observer-rating 52 86 32 86 00 14 —(Friendliness)

Horn, Plomin, & Rosenman (1976) Self-report 1062 51 18 51 0 49 85b(Affiliation)

Buss, Plomin, & Willerman (1973) Other-report 254 58 22 58 0 42 81b(Sociability)

Cohen, Dibble, & Grawe (1977) Other-report(Sociability) 530 55 12 55 0 45 81b(Verbal expressiveness) 530 56 48 16 32 44 81b

Dworkin, Burke, Maher, &Gottesman (1977)

Self-report(Social Introversion) 84 45 26 38 12 55 85b

84 50 25 50 0 50 85bGoldsmith & Gottesman (1977) Self-report

(Sociability) 296 46 8 46 0 54 81b(Spontaneous conversation) 680 37 17 37 0 63 —

Horn et al. (1976) Self-report(Affiliation) 1062 51 18 51 0 49 85b

Plomin & Rowe (1977) Other-report(Affiliation) 182 56 5 56 0 44 85b

Dworkin (1979) Self-report(Self-Monitoring) 88 32 11 32 00 68 70b

Floderus-Myrhed, Pedersen, &Rasmuson (1980)

Self-report(Extraversion) 25,554 51 20 51 0 49 63a

Matheny & Dolan (1980) Other-report(Sociability) 210 56 6 56 0 44 81b

Matthews, Batson, Horn, &Rosenman (1981)

Self-report 230 41 05 41 00 59 70a(Empathic concern)

Rushton et al. (1986) Self-report(Empathy) 573 54 20 54 00 46 79a(Nurturance) 49 14 49 00 51 79a

Tellegen et al. (1988) Self-report(Social closeness) 331 57 24 57 00 43 85c

Heath, Neale, Kessler, Eaves, &Kendler (1992)

Self-report*(Extraversion) 811 39 �04 39 00 61 83a

Heath, Cloninger, & Martin (1994) Self-report(Extraversion) 758 59 24 59 00 41 83b

McGuire, Neiderhiser, Reiss,Hetherington, & Plomin (1994)

Self-report(Social competence) 190 49 15 49 00 51 75c

Saudino, McGuire, Reiss,Hetherington, & Plomin (1995)

Self-report(Sociability) 720 52 06 52 00 48 81b

Horvath (1995) Self-report(Sociability) 208 42 05 42 00 58 81b(Open-style) 48 09 48 00 52 75a

Jang, Livesley, & Vernon (1996) Self-report 83b(Extraversion) 500 53 23 53 00 47 83c(Gregariousness) 56 19 56 00 44 81b

Losoya, Callor, Rowe, & Goldsmith(1997)

Self-report(Pleasantness) 313 56 07 56 00 44 89c(Extraversion) 43 18 43 00 57 83c

Rieman, Angleitner, & Strelau(1997)

Self-report(Extraversion) 1928 56 28 56 00 44 80aPeer-report(Extraversion) 3856 38 22 38 00 62 79a

Jang et al. (1998) Self-report(Gregariousness) 998 44 22 44 00 54 87a

Loehlin, McCrae, Costa, & John(1998)

Self-report(Extraversion) 807 49 09 49 00 54 87a

Spinath & Angleitner (1998) Observer-report(Sociability) 354 48 00 48 00 52 59a

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to self-report studies would confound method of data collection with measurementerror. Therefore, to control for the effects of correction for attenuation, we probedmethod as a possible moderator variable by analyzing the attenuated effects. Theresults indicated that contrary to common criticisms, the average weighted effect forthe observational studies (h2 � .52, K � 10, N � 3,749, 95% confidence � .38 to.65) was larger than the average weighted effect for uncorrected self-report studies(h2 � .50, K � 37, N � 41,606, 95% confidence interval � .49 to .52). Althoughthe difference between the two effects was statistically significant (z � 2.17, p� .03), owing to the huge sample size, the magnitude of the differences wasexamined by computing Cohen’s (1988) q, which provides a relatively stableestimate of the difference between coefficients when r ranges from .20 to .80 (small �.10, medium � .30, large � .50). The difference between behavioral observationstudy effects and self-report-based effects was extremely small (q � .03).

When self-report studies were compared to a set consisting of both observationalstudies and other-reports (e.g., parents, peers), the heritability effects for self-reportstudies (.50) were significantly larger (z � 2.98, p � .003) than the observational/other-report set (h2 � .47, K � 15, N � 9,613, 95% confidence interval � .40 to.55), but the difference was also quite small (q � .04).

Cluster effects. Although we calculated an overall average effect for heritabilityacross the entire set of variables, we also coded the studies along three conceptual

TABLE 2CORRELATIONS, HERITABILITY ESTIMATES, ENVIRONMENTAL EFFECTS, AND RELIABILITY ESTIMATES FOR THE

(VERBAL) AGGRESSIVENESS CLUSTER

Study

Attenuated results

Method N Rmz Rdz h2 c2 e2 Rel.

Owen & Sines (1970) Self-report 42 45 �10 45 0 55 80bGoldsmith & Gottesman (1977) Self-report 296 51 26 51 0 49 87bPlomin & Foch (1980) Observer-report 216 44 42 4 40 56 —Plomin et al. (1981) Observer-report 202 42 42 0 42 58 —Rushton et al. (1986) Self-report 573 52 20 52 00 48 77aGhodsian-Carpey & Baker (1987) Observer-report 38 78 31 78 00 22 91aTellegen et al. (1988) Self-report 331 43 14 43 00 57 78bCates, Houston, Vavak, Crawford, & Uttley (1993) Self-report 109 41 06 41 00 59 67bJang et al. (1996) Self-report 500 42 10 42 00 58 87bCoccaro, Bergeman, Kavoussi, & Seroczynski (1997) Self-report 300 28 07 28 00 82 67aJang et al. (1998) Self-report 998 47 20 47 00 53 87aEley, Lichtenstein, & Stevenson (1999) Observer-rating 1551 75 43 64 11 25 —

Note: Subscripts a and b same as Table 1.

TABLE 1 continued

Study

Attenuated results

Method/Variable N Rmz Rdz h2 c2 e2 Rel.

Hughes & Cutting (1999) Self-report(Perspective-taking) 250 66 32 68 00 32 83a

Beatty et al. (2001) Self-report(Wit) 210 60 22 60 00 40 83a(Social confirmation) 20 06 20 00 80 78a

Note: Subscript a indicates reliability coefficient reported for the data in the study cited. Subscript b indicatesthat reliability was not reported in the study. The estimate represents the upper bound coefficient for themeasure reported in the literature. Subscript c indicates only the range of reliabilities was reported (the estimaterepresents the largest coefficient reported).

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clusters. The first cluster was composed of variables related to pro-social, interper-sonal affiliation orientation and tactics. Specifically, this cluster consisted of acombination of observer ratings, peer ratings, parents’ ratings, and self-reports offriendliness, empathy, nurturance, gregariousness, wit, social confirmation, perspec-tive-taking, sociability, extraversion, social closeness, openness, pleasantness, self-monitoring, verbal expressiveness, and social competence. The second clusterconsisted of aggressiveness constructs. This group of studies included self-reports,parents’ ratings, and observer ratings of aggressiveness. In virtually all of the studies,aggression measures included verbal hostility and verbal aggressiveness items. Thefinal cluster consisted of social anxiety constructs, including observer ratings of socialinhibition, parent’s ratings of children’s shyness, and self-reports of shyness andsocial composure. These three clusters parallel the BIG THREE personality factors(i.e., extraversion, psychoticism, and neuroticism) derived through higher-orderfactor analysis of personality traits (Eysenck & Eysenck, 1985). Eysenck (1986) notedthat these factors “embody the three ways in which individuals can interact” (p. 14).The three clusters employed in our analysis permitted a relatively parsimoniousorganization of the variables, which was consistent with personality research, andsorted variables into domains of interest for communication researchers.

The average, weighted heritability effect for the corrected data was largest for theinterpersonal affiliation cluster (h2 � .70, K � 35, N � 44,677, 95% confidenceinterval � .65 to .75), second largest for social anxiety (h2 � .65, K � 7, N � 1,804,95% confidence interval � .56 to .75), and the smallest for aggressiveness (with thetwo outliers deleted, h2 � .58, K � 10, N � 4,738, 95% confidence interval .54 to.63). Although the average, weighted heritability estimates were significantly differ-ent for all comparisons (affiliation versus social anxiety, z � 3.50, p � .01;interpersonal affiliation versus aggression, z � 12.99, p � .01; aggression versussocial anxiety, z � 4.13, p � .01), the magnitude of the differences was small for theinterpersonal affiliation-social anxiety (q � .09) and the social anxiety-aggressive-ness comparisons (q � .06). The difference between the affiliation effects and theaggressiveness effects, however, can be described as between small and medium(q � .20).

TABLE 3CORRELATIONS, HERITABILITY ESTIMATES, ENVIRONMENTAL EFFECTS, AND RELIABILITY ESTIMATES FOR THE

SOCIAL ANXIETY CLUSTER

Study

Attenuated results

Method N Rmz Rdz h2 c2 e2 Rel.

Scarr (1969) Observer-rating(Shyness) 52 88 28 88 00 12 —

Owen & Sines (1970) Self-report(Inhibition/withdrawal) 84 34 16 34 00 66 80b

Dworkin (1979) Self-report(Anxiety) 176 36 19 38 0 64 87b

Horvath (1995) Self-report(Relaxed style) 208 50 19 50 00 50 80a

Saudino et al. (1995) Self-report*(Shyness) 720 63 11 63 00 37 85b

Spinath & Angleitner (1998) Observer-rating*(Shyness) 354 57 �06 57 00 43 83a

Beatty et al. (2001) Self-report(Social composure) 210 69 28 69 00 31 87a

Note: *Average weighted effects for males and females. Subscripts a and b are same as Table 1 and 2.

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TABLE 4CORRECTED CORRELATIONS, HERITABILITY ESTIMATES, ENVIRONMENTAL EFFECTS, AND RELIABILITY

ESTIMATES FOR THE INTERPERSONAL AFFILIATION CLUSTER

Study

Disattenuated results

Method/Variable N Rmz Rdz h2 c2 e2

Scarr (1969) Observer-rating(Friendliness) 52 86 32 86 00 14

Buss et al. (1973) Other-report 254 72 27 72 00 28(Sociability)

Dworkin et al. (1977) Self-report(Introversion) 84 48 31 48 00 52(Introversiond) 84 59 29 59 00 41

Horn et al. (1976) Self-report 1062 60 21 60 00 40(Affiliation)

Cohen et al. (1977) Other-report(Sociability) 530 68 15 68 00 32(Expressiveness) 530 69 59 20 49 31

Plomin & Rowe (1977) Other-report 182 66 06 66 00 34(Sociability)

Goldsmith & Gottesman (1977) Self-report 296 57 10 57 00 43(Sociability)

Dworkin (1979) Self-report(Self-monitoring) 88 46 16 46 00 54

Floderus-Myrhed et al. (1980) Self-report 25554 81 32 81 00 19(Extroversion)

Matheny & Dolan (1980) Other-report 210 69 07 69 00 31(Sociability)

Matthews et al. (1981) Self-report(Empathic concern) 230 59 07 59 00 41

Goldsmith & Gottesman (1981) Observational 680 37 17 37 00 63(Spontaneous Conversation)

Rushton et al. (1986) Self-report(Empathy) 573 68 25 68 00 32(Nurturance) 68 19 68 00 32

Tellegen et al. (1988) Self-report(Social closeness) 331 67 28 67 00 33

Heath et al. (1992) Self-report(Extraversion) 811 47 �05 47 00 53

Heath et al. (1994) Self-report(Extraversion) 758 59 24 59 00 41

McGuire et al. (1994) Self-report(Social competence) 190 65 20 65 00 35

Saudino et al. (1995) Self-report(Sociability) 720 64 07 64 00 36

Horvath (1995) Self-report(Sociability) 208 52 06 52 00 48(Open-style) 64 10 64 00 36

Jang et al. (1996) Self-report(Extraversion) 500 64 28 64 00 36(Gregariousness) 64 22 64 00 36

Losoya et al. (1997) Self-report(Pleasantness) 313 63 08 63 00 37(Extraversion) 52 22 52 00 48

Rieman et al. (1997) Self-report(Extraversion) 1928 70 35 70 00 30Peer-report(Extraversion) 3856 48 28 40 00 60

Jang et al. (1998) Self-report(Gregariousness) 998 51 25 51 00 49

Loehlin et al. (1998) Self-report(Extraversion) 807 61 10 61 00 39

Spinath & Angleitner (1998) Observer-report(Sociability) 354 48 00 48 00 52

Hughes & Cutting (1999) Self-report(Perspective-taking) 250 80 32 80 00 20

Beatty et al. (2001) Self-report(Wit) 210 72 27 72 00 26(Social Confirmation) 26 07 26 00 74

Mean Effects Corrected for Attenuation 63 25 63 00 37

Note: Subscript d designates non-adult samples within the same study.

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Age effects. Analysis of heritability effects for the corrected data indicated the largestestimate was for the studies of adults (h2 � .73, K � 30, N � 38,700, 95%confidence interval � .69 to .78), the second largest for studies of grade schoolchildren (with the two outliers deleted, h2 � .59, K � 16, N � 5,883, 95%confidence interval .50 to .70), and smallest for studies of high school students (h2 �.50, K � 6, N � 6,636, 95% confidence interval .40 to .61). The difference in effectsfor high school and grade school effects was significant but small (z � 6.42, p �.001, q � .13), the difference between adults and grade school effects was significantand almost medium (z � 19.03, q � .25), and the difference in adult and highschool effects was significant and medium to large (z � 28.70, q � .38). Althoughage appears to contribute to the heterogeneity of the overall analysis, the direction ofthe effects is opposite than expected for environmental effects over a lifespan.

TABLE 5CORRECTED CORRELATIONS, HERITABILITY ESTIMATES, ENVIRONMENTAL EFFECTS, AND RELIABILITY

ESTIMATES FOR THE (VERBAL) AGGRESSIVENESS CLUSTER

Study

Disattenuated results

Method N Rmz Rdz h2 c2 e2

Owen & Sines (1970) Self-report 42 56 13 56 00 44(Aggressiveness)

Goldsmith & Gottesman (1977) Self-report 296 59 30 59 00 41(Hostility)

Plomin & Foch (1980) Observational 216 44 42 04 40 56(Aggressiveness)

Plomin et al. (1981) Observational 202 42 42 00 42 58(Aggressiveness)

Rushton et al. (1986) Self-report 573 68 30 68 00 32Ghodsian-Carpey & Baker (1987) Observer-report 38 78 31 78 00 22Tellegen et al. (1988) Self-report 331 55 18 55 00 45Cates et al. (1993) Self-report 109 61 09 61 00 59Jang et al. (1996) Self-report 500 48 11 48 00 58Coccaro et al. (1997) Self-report 300 42 10 42 00 82Jang et al. (1998) Self-report 998 54 23 54 00 48Eley et al. (1999) Observer-rating 1551 75 43 64 11 25

TABLE 6CORRECTED CORRELATIONS, HERITABILITY ESTIMATES, ENVIRONMENTAL EFFECTS, AND RELIABILITY

ESTIMATES FOR THE SOCIAL ANXIETY CLUSTER

Study

Disattenuated results

Method N Rmz Rdz h2 c2 e2

Scarr (1969) Observer-rating(Shyness) 52 88 28 88 00 12

Owen & Sines (1970) Self-report 84 43 20 43 00 57(Inhibition)

Dworkin (1979) Self-report 176 41 22 41 00 59(Anxiety)

Horvath (1995) Self-report(Relaxed style) 208 63 24 63 00 37

Saudino et al. (1995) Self-report(Shyness) 720 74 13 74 00 26

Spinath & Angleitner (1998) Observer-rating(Shyness) 354 57 �06 57 00 43

Beatty et al. (2001) Self-report(Social composure) 210 79 32 79 00 21

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Discussion

In the present study, we provided a quantitative summary of the twins literaturefocussing on frequently referenced variables in the mainstream communicationliterature. We adopted conservative procedures in conducting this analysis. Specifi-cally, we (1) assumed nonadditive gene effects whenever the within-pair correlationfor dizygotic twins was less than one-half that for the monozygotic pairs, (2)employed attenuated heritability estimates for all studies using observer ratings, and(3) included methodologically deficient studies (which tend to contain low heritabil-ity estimates) to avoid any appearance of selectivity bias. Given the formula forcalculating environmental effects, conservative estimates of heritability translate intoliberal estimates of environmental effects. Even so, the mean heritability estimatewas .70 for interpersonal affiliation, .58 for aggressiveness, .65 for social anxiety, and.69 across all of the studies. Relaxation of the analytic criteria yields larger heritabil-ity estimates. For instance, a reasonable case can be made for deriving the heritabil-ity estimate from the weighted average correlation for MZ twin pairs for the entiresample, rather than averaging heritability estimates. Such a procedure produces anoverall heritability estimate of .72. Likewise, although correcting effects for attenua-tion on the basis of intercoder reliability is a questionable practice (Hoyt, 2000),behavioral observations are imperfect. Within our sample, the average intercoderreliability for behavioral observation studies was .84. We included attenuated effectsfrom these studies in our own analysis that reduced the overall heritability estimate

FIGURE 1HISTOGRAM DEPICTING CORRECTED HERITABILITY ESTIMATES.

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by approximately .02. Deleting such studies and employing the weighted averageMZ correlation would yield a heritability estimate of .74. Finally, the case can bemade that the difference between the weighted mean of the observed DZ correla-tions (rdz � .28) and the criterion for additivity (i.e., 1⁄2 .72 or .36) is small enough(q � .08) to justify the application of Falconer’s formula. Under this assumption, theheritability for the entire sample equals .88. Incidently, Beatty, McCroskey, andHeisel’s (1998) projection that heritability could be as much as 80% under idealmethodological conditions, falls approximately midway between the .69 conserva-tive estimate and this .88 liberal estimate. Although a point estimate of heritabilitydepends on the analytic assumptions made, it is sufficient at this juncture torecognize that even under the most conservative guidelines, over two-thirds of thevariation in patterns of social behavior appear to be attributable to genetic sources.

The aforementioned heritability estimates reflect average contributions to pat-terns of behavior and as such should not be extrapolated to mean that geneticinfluences are evenly distributed across single, isolated acts. It should be pointed out,however, that (1) we are aware of no theory in the discipline with sufficientlydemonstrated predictive power to account for appreciable variance in single acts,and (2) patterns of behavior are every bit as important as single acts. Indeed, patternsof behavior are important in the evolution of human relationships, in general, andprovide the infrastructure supporting social rituals. Our results indicate that asubstantial percentage of variance in patterns of behavior as well as predispositionscan be attributed to genetic inheritance. As indicated in our examination ofdata-collection method as a possible moderator variable, the effect for self-reportstudies was not larger than for observations of behavior when variance due tocorrection for attenuation was controlled. In addition to addressing the assertion thatheritability estimates are inflated by self-report methodology (e.g., Condit, 2000),this analysis extends the influence of heritability beyond traits to behavior. Thisextension is especially emphasized by the observational work of Scarr (1969) and themeasurement of accuracy in Hughes and Cutting (1999) wherein sizable heritabilityestimates were reported.

The heterogeneous nature of the heritability effects requires attention. Althoughmethod was not a significant moderator when the effects of correction for attenua-tion were removed, both cluster and age appear to contribute to heterogeneity. Incontrast to the assumption that genetic influence might be greatest on emotion-basedconstructs such as social anxiety or aggression (Beatty, McCroskey, & Valencic,2001), the heritability effects were largest for the interpersonal affiliation cluster,which consisted of several variables. Although the interpersonal affiliation clusterwas far from exhaustive in terms of the variables that comprise affiliation, all but sixof the variables in our data set appear in Knapp and Miller’s (1994) index and theremaining variables are described within various chapters as subcomponents ofbroader constructs. For instance, although pleasantness and composure are notcontained in the index, Burgoon (1994) included them within the discussion of socialinfluence (pp. 269–270).

The probe into age as a potential moderator also produced interesting results.Although age of the participant was identified as contributing to the overallheterogeneity of effects, the larger effect was for adults. This finding is opposite theclaim that environmental effects cumulate across time. It is possible that methodologi-cal explanations, however, are as viable as theoretical ones regarding age to the

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extent that adult subjects more than children might be expected to take researchprotocols and/or experimental conditions more seriously. Interestingly, as indicatedin both tables, there was no effect attributable to common environment (c2).Virtually all the variance was attributable to either genetic inheritance and non-shared environmental factors (i.e., residual). Given these findings and our inability tocompletely account for the heterogeneity of genetic effects in our data, the results ofour quantitative analysis of twins studies have important implications for researchand theory.3

Directions for Future Research

The results of our analysis suggest that further research employing the classictwins design is indicated. Although a few studies have been published in thecommunication literature (Beatty, Marshall, & Rudd, 2001; Horvath, 1995), most ofthe twins studies have appeared outside the discipline. One of the problemsassociated with deriving heritability estimates from research in psychology, forexample, resides in the way variables are conceptualized and measured. Because theperspectives and interests are different across disciplines, many of the variablesimportant to communication scholars are inextricably bound up with other variablesin composite measures. With respect to aggressiveness, for instance, items pertainingto physical aggressiveness and verbal hostility are typically combined into a singleindex (e.g., Rushton, Fulker, Neal, Nias, & Eysenck, 1986). In other studies, it isdifficult to differentiate verbal aggressiveness from assertiveness (e.g., Jang, McCrae,Riemann, & Livesley, 1998). Indeed, Infante and Wigley (1986) made these sameobservations in their rationale for the development of a measure of verbal aggressive-ness. As it stands, however, it is difficult to determine the heritability of verbalaggressiveness relative to other forms of aggressiveness. Similarly, research existsthat cannot be included in our analysis although the composite scores containvariables of interest to communication scholars. Robinson, Kagan, Reznick, andCorley (1992), for example, conducted an excellent twins study of inhibited anduninhibited temperament in children. However, avoidance of strangers, whichwould interest communication apprehension researchers, was combined into asingle index with avoidance of new objects (e.g., toys) and experiences unrelated tosocial interaction. Clearly, twin studies using communication variables as we con-ceive and measure them would contribute to the research literature and probablysharpen the precision of our heritability estimates.

In addition to the problem of multiple variables combined into a single index,other measurement problems exist in the twins literature. For example, Dworkin(1979) treated the measure of self-monitoring as unidimensional although in laterstudies Briggs, Cheek, and Buss (1980) and Gabrenya and Arkin (1980) showed thatthe measure was multidimensional. It is possible that examining factors separatelywould produce larger heritability estimates (at least for some of the factors) thanreported by Dworkin.

Further research employing alternative research strategies to corroborate thetwins research is needed. However, it would be incorrect to assume that evidence forgenetic influence on communication variables depends solely on twins studies.Indeed, fairly comprehensive reviews of the biochemical signatures of inheritedneurobiological functioning across a robust array of personality variables, many ofwhich are communication related, have appeared in the personality literature (e.g.,Zuckerman, 1995). Furthermore, recent MRI studies have indicated correlations

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greater than .60 between self-reported extraversion and activation in 10 regions ofthe brain (Johnson et al., 1999). Over a decade ago, Kagan, Reznick, and Snidman(1988) correlated observer ratings of social avoidance with a physiological indexcomposed of variables such as salivatory cortisol and urinary norepinephrine, whichare considered indicants of inherited thresholds for arousal. Kagan et al. (1988)reported an attenuated correlation of .70 between the physiological index andobserver ratings for children at the age of 21 months and a correlation of .64 forchildren at 7.5 years. These correlations are comparable to the mean heritabilityestimate for social anxiety (i.e., .65) computed in our analysis. Recent research (Lal,Fisher, Hurst, Vargha-Khadem, & Monaco, 2001) has identified the gene (FOXP2)responsible for triggering the cascade of other genes necessary for speech andlanguage fluency. Although much corroborative evidence exists, studies specificallyfocused on variables of interest to communication scholars would greatly enhanceour understanding of the influence of genetics on communication.

Implications for Theory

The mean effects reported in our analysis, especially the overall estimate, empha-size the importance of potential genetic influences on human communication.Accordingly, these findings suggest that communication theorists should affordmore attention to biological sources of variation in both traits and behavior thancurrently is reflected in explanations of communication processes. Thus, whentypologies are developed, whether in interpersonal, family, relational, or organiza-tional contexts, biologically imposed constraints that render some behavioral alter-natives less likely than others need to be recognized. With respect to the specificclusters examined in our analysis, the findings for social anxiety are consistent withexpectations based on Beatty, McCroskey, and Heisel’s (1998) reanalysis of commu-nication apprehension. Reasoning from a neurobiological model of social anxiety,they concluded that approximately 80% of the variance in individual differences incommunication apprehension was attributable to genetic influence, assuming highlevels of methodological rigor. In light of the conservative analytic proceduresutilized throughout this study, the 65% heritability estimate calculated in our analysisis not far from Beatty et al.’s (1998) projection. Although comparatively lessemphasis has been placed on biological models of verbal aggressiveness in thecommunication literature, the heritability estimate reported for aggressiveness in ourstudy (h2 � .58) was similar to the .60 estimate Valencic, Beatty, Rudd, Dobos, andHeisel (1998) derived from their self-report data.

Results of our analysis indicate virtually no effects of environment. Situationaltheorists, however, need to acknowledge that genetic sources of variation are oftenimbedded in estimates of environmental impact, especially when retrospective andself-reports are employed to assess the environment. A growing stream of studies(e.g., Chipeur, Plomin, Pedersen, McClearn, & Nesselroade, 1993; Phillips &Matheny, 1997; Saudino, Pedersen, Lichtenstein, McClearn, & Plomin, 1997;Saudino & Plomin, 1997) indicates that perceptions of the environment are notindependent of genetically-based selectivity processes. Rather, apparent situationalor environmental factors are moderated by genetically-based perceptual filters.Personality theorists are recognizing that studies which seem to document environ-mental effects might be confounded by indirect genetic effects (e.g., Phillips &Matheny, 1997).

A discussion of the results reported in our analysis would not be complete without

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addressing the question, “what exactly is inherited?” Critics (e.g., Condit, 2000) ofgenetic models of communication sometimes argue that there exists no evidence of asingle genetobehavior link. However, as others (e.g., Beatty & McCroskey, 2000)have pointed out, this position amounts to a straw argument. Behavioral geneticistsdo not contend that specific behaviors are necessarily due to single gene properties.Strictly speaking, traits and behaviors are theorists’ constructions and, therefore, arenot inherited per se. The perspective articulated in the temperament and psychobiol-ogy literature is that the socially significant behavior patterns that attract scholars’interest are observable expressions of inborn differences in thresholds of neurobio-logical systems (e.g., Bates, 1989, 1994; Gray, 1991; Strelau, 1994; Zuckerman, 1994,1995), which are the product of combinations of genes–not merely a single gene.This perspective has been taken by Beatty and his colleagues (e.g., Beatty, McCros-key, & Valencic, 2001) in the discipline of communication. For example, Beatty,McCroskey, and Heisel (1998) have proposed that communication apprehension isa function of low thresholds for activation of avoidance systems. Thus, patterns ofsocial behavior may be the product of multiple genes, often hierarchically config-ured, mediated by neurobiological systems, but such are genetic patterns neverthe-less.

One implication of such an approach concerns the way in which constructs evolvewithin a discipline. Traditionally, communication theorists, like other behavioralscientists, worked inductively from observations of social behavior to the develop-ment of constructs. Zuckerman (1994) refers to the traditional approach as “bottom-up” theorizing (i.e., genetically inherited neurobiological activity is placed at the topof the model and observable behavior is placed at the bottom). According to thisperspective, less than perfect statistical associations between observable behaviorand neurobiological functions are the products of “imperfect conceptualizations” ofsocial behavior. Any attempt, therefore, to understand the possible genetic mecha-nisms contributing to the construct requires mapping the neurobiology underlyingthe behavior, affect, or cognition of interest. Gray (1991) proposed that alternatively,theorists could infer behavioral constructs from known facts about neurobiologicalfunctioning. Zuckerman (1994) calls this the “top-down” approach. Although present-ing a radical departure from conventional theorizing in the communication litera-ture, a top-down approach would likely result in stronger statistical relationshipsbetween constructs and genetics than observed for constructs produced via thetraditional bottom-up approach used in the social sciences.

Conclusion

In general, the results of our quantitative summary of the twins literature providesan empirical rationale for further exploration of the possible genetic influences oncommunication variables. However, the magnitude of the effects, while substantial,does not rule out possible environmental effects. Furthermore, the variance attribut-able to nonshared environment, even when heritability estimates were based oncorrected correlations, was sufficient to accommodate the environmental and reme-diation effects reported in the extant communication apprehension (for a review, seeBeatty, McCroskey, & Valencic, 2001, p. 31) and aggressiveness literatures (e.g.,Paik & Comstock, 1994). Although the studies included in our analysis represented abroad spectrum of variables, we hope our report stimulates more focused futuretwins research in addition to other biologically-oriented investigations. It does seem,

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however, that if genetic and environmental effects are additive, the variance due toheritability will be manifest as large error terms in studies in which genetic influenceis uncontrolled. If genetics and environment are seen as interacting in the productionof behavior, interpretation of situational effects are meaningless when genetic factorsare ignored, given that within the context of statistical interaction main effects aretypically not interpreted. Although resolution regarding which model accuratelydepicts the combined role of genetics and environmental effects remains an empiri-cal question, our findings underscore the importance of genetic influences oncommunicators.

Footnotes1One of the initial questions raised by critics concerns the degree to which the high level of personality

resemblance that identical twin pairs possess can be attributed to their having been raised in the sameenvironment (e.g., Hoffman, 1994). Although the formula typically used to compute heritability estimatessubtracts the correlation for dizygotic pairs that also share common environments from the monozygotic twinpairs’ correlation, critics claim that monozygotic twins are probably treated more alike than are dizygotic twins.However, ignored in this claim is Bouchard’s (1993) earlier observation that the effects of similar treatmentdocumented in previous research are far too small to account for the degree of similarity in personality ofidentical twins. Moreover, two related lines of research provide compelling evidence against the argument thatcommon environment is responsible for the resemblance in personality of monozygotic twins.

First, research on monozygotic twins raised together (MZT) and monozygotic twins raised apart (MZA) indicates thatMZT and MZA pairs are nearly equivalent in their within-pair resemblance across personality measures. In hisreview of the literature, Zuckerman (1994) remarked that “There is little difference between the correlations foridentical twins who were raised apart and those who were raised together” (p. 245). Lykken (1995), whoconducted many of the MZT-MZA comparisons, pointed out that the correlations for the two groups aresimilar enough that “it justifies in most cases, the use of MZT correlation as a direct effect of heritability” (p. 75).A second line of research has compared the within-pair correlations of monozygotic twins, dizygotic twins, anda special type of sibling pairs (referred to as pseudo twins) who are nearly identical in age, adopted into a familyat the same time, but are biologically unrelated (Segal, 1997). The results showed that pseudo twins are far lesssimilar in personality years after adoption than are fraternal twins raised together, who in turn show far lessresemblance than identical twins raised apart. Overall, most contemporary researchers interested in behaviorgenetics do not attribute the high degrees of similarity in intelligence and personality observed in identicaltwin-pairs to common environment.

2Critics have also challenged the formulae typically employed to calculate heritability estimates from twins’data because the difference between monozygotic and dizygotic twins’ within-pair correlations is doubled inthe formulae (e.g., Falconer, 1989). Underlying these types of formulae are the assumptions that (1)monozygotic twins share 100% of their genes in common whereas dizygotic twins share on averageapproximately 50% of their genes, and (2) the gene effects are additive. When gene effects are nonadditive,however, the number of shared genes between dizygotic twins does not translate into sibling resemblance.Although resemblance of monozygotic twins is unaffected because MZ twins are genetically identical, “whennonadditive genetic effects are present, similarity of fraternal twins will be less than one half of that of identicaltwins” (Saudino, 1997, p. 87). For example, if both fraternal twins inherit the genes to be extraverted but onetwin inherits the genes to be neurotic while the other inherits genes to be emotionally stable, the twins sharehalf of the genes in common (i.e., extraversion) but a multiplicative interaction between extraversion andneuroticism might make the extraverted, neurotic twin more than twice as socially annoying as the extraverted,stable twin. The challenge to the twins research, therefore, is that when nonadditive gene effects are present,multiplying the difference between MZ and DZ within pair correlations inflates heritability estimates (e.g.,Wilson, 1998). Evidence of nonadditive effects has been reported in the personality literature, thereby makingthe issue relevant to the interpretation of the studies considered in the present essay. Lykken (1995), forinstance, observed that “for some psychological traits, we now know that Rdzt tends to be less than half the Rmztvalue, and for some variables, Rmzt is substantial while Rdzt is essentially zero” (p. 76). Because dizygoticwithin-pair correlations indicate possible nonadditive gene effects, formulae such as Falconer’s (1989) “willoverestimate the actual heritability” (Lykken, 1995, p. 76).

Several points in response are worth considering with respect to the issue of nonadditive gene effects. First,possible nonadditive gene effects can be detected by comparing the monozygotic and dizygotic correlations.Whenever Rdz � .5 Rmz, nonadditive gene effects are possible with the magnitude of those effects indicated bythe deviation of Rdz from .5 Rmz. The degree to which Falconer’s (1989) formula inflates heritability estimatesdepends on the degree of nonadditivity and whether the deviation is a real nonadditive gene effect or simplydue to sampling error. Lykken (1995) recommended that when nonadditivity is a concern, a conservativeapproach is to employ the monozygotic twin within-pair correlation as the estimate of heritability. As Lykken

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put it, “Rmzt alone provides both a more efficient and more effective accurate estimate” (p. 76). In addition,“Rmzt has the added advantage of including the effects of both additive and nonadditive genetic variance”(Lykken, 1995, p. 75).

Second, the additive and nonadditive genetic effects on personality can be separated if the distinction istheoretically significant to researchers (e.g., Rushton et al., 1986). Additive effects define the portion of variancein a trait that is transmissable from parents to offspring. Nonadditive gene effects are genetic contributions to atrait that are not directly inherited from parents. The overall genetic effect consisting of both additive andnonadditive effects is often called broad heritability. In the past, many analytic techniques for estimating additiveand nonadditive gene effects have been developed (e.g., Rushton et al., 1986). More recently, some scholarshave begun to question whether the apparent nonadditive gene effects represent genetic phenomena. Saudinoet al. (1995), for example, noted that the effects of rater biases, which are manifest as intercoder or test-retestunreliability, “mimic nonadditive genetic variance” (p. 732). It turns out that the greatest evidence ofnonadditivity occurs in studies relying on parental ratings of their children’s personalities. When these ratingbiases are removed, the nonadditive gene effect is extremely small.

A second type of gene nonadditivity often mentioned in critiques of twins research, is a gene-environmentinteraction (Wilson, 1998). According to critics, behavior geneticists relying on twins data ignore possiblegene-by-environment interactions in the development of personality. However, studies of MZ and DZ twins,some of whom were raised apart, indicate no such effects (Tellegen et al., 1988). Furthermore, if true, thedetection of gene-by-environment interactions would not reduce genetic impact on personality alreadydocumented. The variance explained by such interactions would only reduce the residual term, not the geneticeffect. In light of the relatively small main effects due to environment in twins research, these interactionswould merely add to the role of genetics by documenting that environmental effects are contingent upongenetic predispositions. The independent effect of genetics on personality has already been shown.

3This is particularly true given that our sample included studies conducted in various cultures but culture didnot produce outliers.

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Received: 25 June 2001Accepted: 6 November 2001

18 COMMUNICATION MONOGRAPHS

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