Conduct Problems in Children and Adolescents: A Twin Study | Adolescent Medicine | JAMA Psychiatry | JAMA Network
Figure 1.

Independent pathway model. Dataare shown as components of variance (95% confidence interval). The fit ofan ADE model is as follows: χ224 goodness of fit= 14.52, P = .93, AIC = −33.48. The fit ofan ACE model is as follows: χ224 goodness of fit= 19.23, P = .74, AIC = −28.77. A indicatesadditive genetic effects; AIC, Akaike Information Criterion; C in ACE, sharedenvironmental effects; D, nonadditive or dominance genetic effects; E, uniqueenvironment (which includes measurement error); Pt, parent report; S, adolescentself-report; and T, teacher report.

Figure 2.

Common pathway model. Data areshown as components of variance (95% confidence interval). The variance ofthe latent variable, "P," was constrained to equal 1.00. The fit of an ADEmodel is as follows: χ228 goodness of fit = 16.81, P= .95, AIC = −39.19. The fit of an ACE model isas follows: χ228 goodness of fit = 19.52, P= .88, AIC = −36.48. A indicates additive geneticeffects; AIC, Akaike Information Criterion; C in ACE, shared environmentaleffects; D, nonadditive or dominance genetic effects; E, unique environment(which includes measurement error); P, conduct problems' phenotype commonto all raters; Pt, parent report; S, adolescent self-report; and T, teacherreport.

Figure 3.

Simple multiple rater model. Dataare shown as components of variance (95% confidence interval). The varianceof the latent variable, "P," was constrained to equal 1.00. The fit of anADE model is as follows: χ234 goodness of fit =58.48, P = .006, AIC = −9.52. The fit of anACE model is as follows: χ234 goodness of fit =58.62, P = .005, AIC = −9.38. A indicates additivegenetic effects; AIC, Akaike Information Criterion; C in ACE, shared environmentaleffects; D, nonadditive or dominance genetic effects; E, unique environment(which includes measurement error); EPt, ES, and ET, error for parent rating, self-rating, and teacher rating, respectively;P, conduct problems' phenotype common to all raters; Pt, parent report; S,adolescent self-report; and T, teacher report.

Table 1.
Data for Conduct Problem Scores in MZ and DZ Pairs
Table 2.
Results of Model Fitting Using SDQ Conduct Problem ScoresFrom Different Raters
1.
Melzer  HGatward  RGoodman  RFord  T The Mental Health of Children and Adolescents inGreat Britain.  London, England Office for National Statistics2000;
2.
Anderson  JCWilliams  SMcGee  RSilva  PA DSM-III disorders in preadolescent children:prevalence in a large sample from the general population.  Arch Gen Psychiatry. 1987;4469- 76PubMedGoogle ScholarCrossref
3.
Cohen  PCohen  JKasen  SVelez  CNHartmark  CJohnson  JRojas  MBrook  JStreuning  EL An epidemiological study of disorders in late childhood and adolescence,1: age- and gender-specific prevalence.  J Child Psychol Psychiatry. 1993;34851- 867PubMedGoogle ScholarCrossref
4.
Robins  L Sturdy childhood predictors of adult antisocial behavior: replicationsfrom longitudinal studies.  Psychol Med. 1978;8611- 622PubMedGoogle ScholarCrossref
5.
Rutter  M Relationships between mental disorders in childhood and adulthood.  Acta Psychiatr Scand. 1995;9173- 85PubMedGoogle ScholarCrossref
6.
Yeager  CLewis  D Mortality in a group of formerly incarcerated juvenile delinquents.  Am J Psychiatry. 1990;147612- 614PubMedGoogle Scholar
7.
Shaffer  DPiacentini  J Suicide and attempted suicide. Rutter  MTaylor  EHersov  Leds. Child andAdolescent Psychiatry: Modern Approaches. Oxford, England BlackwellScience Ltd1994;407- 424Google Scholar
8.
Zocollilo  MPickles  AQuinton  DRutter  M The outcome of childhood conduct disorder: implications for definingadult personality disorder and conduct disorder.  Psychol Med. 1992;22971- 986PubMedGoogle ScholarCrossref
9.
Rhee  SHWaldman  ID Genetic and environmental influences on antisocial behavior: a meta-analysisof twin and adoption studies.  Psychol Bull. 2002;128490- 529PubMedGoogle ScholarCrossref
10.
Eaves  LJSilberg  JLMeyer  JMMaes  HHSimonoff  EPickles  ARutter  MNeale  MCReynolds  CAErikson  MTHeath  ACLoeber  RTruett  KRHewitt  JK Genetics and developmental psychopathology, 2: the main effects ofgenes and environment on behavioral problems in the Virginia Twin Study ofAdolescent Behavioral Development.  J Child Psychol Psychiatry. 1997;38965- 980PubMedGoogle ScholarCrossref
11.
Slutske  WHeath  ACDinwiddie  SHMadden  PAFBucholz  KKDunne  MPStatham  DJMartin  NG Modelling genetic and environmental influences in the etiology of conductdisorder: a study of 2,682 adult twin pairs.  J Abnorm Psychol. 1997;106266- 279PubMedGoogle ScholarCrossref
12.
Rowe  DC Biometrical genetic models of self-reported delinquent behavior: atwin study.  Behav Genet. 1983;13473- 489PubMedGoogle ScholarCrossref
13.
Graham  PStevenson  J A twin study of genetic influences on behavioral deviance.  J Am Acad Child Psychiatry. 1985;2433- 41PubMedGoogle ScholarCrossref
14.
Rowe  D Genetic and environmental components of antisocial behavior: a studyof 265 twin pairs.  Criminology. 1986;24513- 532Google ScholarCrossref
15.
Stevenson  JGraham  P Behavioral deviance in 13-year-old-twins: an item analysis.  J Am Acad Child Adolesc Psychiatry. 1988;27791- 797PubMedGoogle ScholarCrossref
16.
Silberg  JErickson  MMeyer  JEaves  LJRutter  MLHewitt  JK The application of structural equation modelling to maternal ratingsof twins' behavioral and emotional problems.  J Consult Clin Psychol. 1994;62510- 521PubMedGoogle Scholar
17.
Edelbrock  CRende  RPlomin  RThompson  LA A twin study of competence and problem behavior in childhood and earlyadolescence.  J Child Psychol Psychiatry. 1995;36775- 785PubMedGoogle ScholarCrossref
18.
Schmitz  SFulker  DMrazekm  D Problem behavior in early and middle childhood: an initial behaviorgenetic analysis.  J Child Psychol Psychiatry. 1995;361443- 1458PubMedGoogle ScholarCrossref
19.
Simonoff  EPickles  AHewitt  JSilberg  JRutter  MLoeber  RMeyer  JNeale  MEaves  L Multiple raters of disruptive child behavior: using a genetic strategyto examine shared views and bias.  Behav Genet. 1995;25311- 326PubMedGoogle ScholarCrossref
20.
21.
Meyer  JRutter  MSilberg  JLMaes  HHSimonoff  EShillady  LLPickles  AHewitt  JKEaves  L Familial aggregation for conduct disorder symptomatology: the roleof genes, marital discord and family adaptability.  Psychol Med. 2000;30759- 774PubMedGoogle ScholarCrossref
22.
Jacobson  KPrescott  CKendler  K Genetic and environmental influences on juvenile antisocial behaviourassessed on two occasions.  Psychol Med. 2000;301315- 1325PubMedGoogle ScholarCrossref
23.
Miles  DRvan den Bree  MBPickens  RW Sex differences in shared genetic and environmental influences betweenconduct disorder symptoms and marijuana use in adolescents.  Am J Med Genet. 2002;114159- 168PubMedGoogle ScholarCrossref
24.
Plomin  RDeFries  JCMcClearn  GERutter  M Behavioral Genetics. 3rd New York, NY WH Freeman & Co1997;
25.
Rutter  MMaughan  BPickles  ASilberg  JSimonoff  ETaylor  E Heterogeneity of antisocial behavior: causes, continuities, and consequences.  The Nebraska Symposium on Motivation. 44 Lincoln University of Nebraska Press1997;45- 118Google Scholar
26.
Goodman  R The Strengths and Difficulties Questionnaire: a research note.  J Child Psychol Psychiatry. 1997;38581- 586PubMedGoogle ScholarCrossref
27.
Scourfield  JMartin  NCLewis  GMcGuffin  P The heritability of social cognitive skills in children and adolescents.  Br J Psychiatry. 1999;175559- 564PubMedGoogle ScholarCrossref
28.
Goodman  RScott  S Comparing the Strengths and Difficulties Questionnaire and the ChildBehaviour Checklist: is small beautiful?  J Abnorm Child Psychol. 1999;2717- 24PubMedGoogle ScholarCrossref
29.
Nichols  RCBilbro  WC  Jr The diagnosis of twin zygosity.  Acta Genet Stat Med. 1966;16275PubMedGoogle Scholar
30.
Cohen  DDibble  EGrawe  JMPollin  W Reliably separating identical from fraternal twins.  Arch Gen Psychiatry. 1975;321371- 1375PubMedGoogle ScholarCrossref
31.
SPSS.  Chicago, Ill SPSS Inc2001;
32.
Neale  MCBoker  SMXie  GMaes  HH Mx: Statistical Modelling. 6th Richmond Dept of Psychiatry, Virginia Commonwealth University2002;
33.
Neale  MCardon  L Methodology for Genetic Studies of Twins and Families.Dordrecht, the Netherlands: Kluwer Academic Publishers; 1992. NATO ASI Series.
34.
Townsend P, Philimore P, Beattie A., Health and Deprivation: Inequality and the North.  London, England Croon Helm1988;
35.
Muthén  BKaplan  D A comparison of some methodologies for the factor analysis of nonnormalLikert variables.  Br J Math Stat Psychol. 1985;38171- 189Google ScholarCrossref
36.
Arsenault  LAMoffitt  TECaspi  ATaylor  ARijsdijk  FVJaffee  SRAblow  JCMeaselle  JR Strong genetic effects on cross-situational antisocial behaviour among5-year-old children according to mothers, teachers, examiner-observers, andtwins' self-reports.  J Child Psychol Psychiatry. 2003;44832- 848PubMedGoogle ScholarCrossref
37.
Martin  NScourfield  JMcGuffin  P Observer effects and heritability of childhood attention-deficit hyperactivitydisorder symptoms.  Br J Psychiatry. 2002;180260- 265PubMedGoogle ScholarCrossref
38.
39.
Moffitt  TE "Life-course-persistent" and "adolescence-limited" antisocial behaviour:a developmental taxonomy.  Psychol Rev. 1993;100674- 701PubMedGoogle ScholarCrossref
40.
Cadoret  RYates  WRTroughton  EWoodworth  GStewart  MA Genetic-environmental interaction in the genesis of aggressivity andconduct disorders.  Arch Gen Psychiatry. 1995;52916- 924PubMedGoogle ScholarCrossref
41.
Ge  XRand  DCCadoret  RJNeidheiser  JMYatew  WTroughton  EStewart  MA The developmental interface between nature and nurture: a mutual influencemodel of child antisocial behavior and parent behaviors.  Dev Psychol. 1996;32574- 589Google ScholarCrossref
42.
Dunn  JPlomin  R Separate Lives: Why Siblings Are So Different.  New York, NY Basic Books Inc Publishers1990;
43.
Reiss  DPlomin  RHetherington  M Genetics and psychiatry: an unheralded window on the environment.  Am J Psychiatry. 1991;148283- 291PubMedGoogle Scholar
44.
Reiss  DHetherington  MPlomin  RHowe  GWSimmens  SJHenderson  SHO'Connor  TJBussell  DAAnderson  ERLaw  T Genetic questions for environmental studies: differentialparenting and psychopathology in adolescence.  Arch Gen Psychiatry. 1995;52925- 936PubMedGoogle ScholarCrossref
• Cite This

### Citation

Scourfield J, Van den Bree M, Martin N, McGuffin P. Conduct Problems in Children and Adolescents: A Twin Study. Arch Gen Psychiatry. 2004;61(5):489–496. doi:10.1001/archpsyc.61.5.489

Original Article
May 2004

# Conduct Problems in Children and Adolescents: A Twin Study

Author Affiliations

From the Department of Psychological Medicine, University of WalesCollege of Medicine, Cardiff. Dr McGuffin is now with the SGDP Research Centre,Institute of Psychiatry, London, England.

Arch Gen Psychiatry. 2004;61(5):489-496. doi:10.1001/archpsyc.61.5.489
Abstract

Background  Evidence supports a genetic influence on conduct problems as a continuous measure of behavior and as a diagnostic category. However, there is a lack of studies using a genetically informative design combined with several different informants and different settings.

Objectives  To examine genetic and environmental influences on conduct problems rated by parent and teacher reports and self-reports and to determine whether their ratings reflect a common underlying phenotype.

Design  A twin study design was used to examine conduct problem scores from ratings by teachers, parents, and twins themselves.

Setting  General community.

Participants  Twins aged 5 to 17 years participating in the Cardiff Study of All Wales and North England Twins (CaStANET) project.

Main Outcome Measures  Conduct problem scale from the Strengths and Difficulties Questionnaire.

Results  Conduct problem scores were significantly heritable based on parent and teacher reports and self-reports. Combining data from all 3 informants showed that they are rating a common underlying phenotype of pervasive conduct problems that is entirely genetic, while teacher ratings show separate genetic influences that are not shared with other raters.

Conclusions  Conduct problems are significantly heritable based on parent and teacher reports and self-reports, and are also influenced by environmental effects that impinge uniquely on children from the same family. There is a cross-situational conduct problems' phenotype, underlying the behavior measured by all informants, that is wholly genetic in origin. No significant influence of shared environmental effects was found.

In a recent government survey1 of themental health of children in the United Kingdom, conduct disorder emergedas the most common child psychiatric disorder and one of the most frequentreasons for referral to specialist services. Children with conduct disorderare more likely to have comorbid substance abuse, depression, and anxiety,2,3 and in adult life are much more likelyto develop antisocial personality disorder and other psychiatric disorders.4,5 Follow-up studies have found high ratesof premature death, often from violent causes,6 andup to 46% of those who attempt suicide have had conduct disorder.7 Even those without a frank psychiatric disorder inadulthood have pervasive social difficulties compared with their peers.8

There has been increasing awareness in recent years that genetic influenceshave a role in the development of conduct problems, whether measured as acontinuum of behavior or as a diagnostic category. A recent meta-analysis9 reported a heritability of 41% (additive and nonadditivegenetic effects) for antisocial behavior measured in various ways. It is unlikelythat there will turn out to be genes for these behaviors, in the sense thata specific genetic constellation would provide necessary and sufficient cause.Rather, the path from genes to behavior is likely to be a complex one involvingenvironmental stresses and individual differences in liability. There is evidencesupporting a genetic influence on conduct disorder as a diagnostic category10,11 and on continuous measures of conductproblems and antisocial behavior,12-23 althoughfindings vary among studies.

Discrepant findings are likely to be the result of different samplecharacteristics and measurement strategies and the fact that these behaviorsprobably represent a heterogeneous group.24,25

Because children's behavior can vary in different settings, the mostvalid measurements are those that include data from more than one informantand from more than one setting. Behavioral genetic approaches can indicatethe extent to which different informants are rating the same substrate ofbehavior and the degree of unique perspective that different informants contribute.The present investigation examines genetic and environmental influences ona dimensional measure of conduct problems using parent and teacher reportsand self-reports from the Strengths and Difficulties Questionnaire (SDQ),26 a measure widely used in the United Kingdom clinicallyand epidemiologically.

Methods
Sample

The Cardiff Study of All Wales and North England Twins (CaStANET) isa population-based twin register of all twin births in Wales and greater Manchester,England. This investigation is based on the subsample of families from southWales: 1109 twin pairs aged 5 to 17 years, consisting of all twin births betweenJanuary 1, 1980, and December 31, 1991, in the former counties of Mid andSouth Glamorgan. The methods used to identify and trace twins have been describedelsewhere.27

Measures

Data collection was by mail. Parents were asked to complete a packetof questionnaires about their twins, including the SDQ.26 Threereminders were sent, and telephone reminders were used when numbers couldbe traced. Parental consent was obtained to contact a teacher, who was thenasked to complete the teacher version of the SDQ. Self-report versions weresent to twins 11 years and older. The SDQ is a brief behavioral screeningquestionnaire that can be completed in 5 minutes. Its brevity and its inclusionof strengths as well as difficulties make it particularly suitable for usein general population samples. It includes a continuous measure of behaviorproblems, the Conduct Problems Scale. The following 5 items contribute tothe Conduct Problems Scale: "Often has temper tantrums or hot tempers; generallyobedient, usually does what adults request; often fights with other childrenor bullies them; often lies or cheats; steals from home, school or elsewhere."Parents were asked to report behavior observed in the past 6 months, and itemsare scored "not true" (0), "somewhat true" (1), or "certainly true" (2), withreverse scoring when appropriate. This conduct scale correlates highly withthe conduct/externalizing scales of the longer-established Rutter Scales andthe Achenbach Child Behavior Checklist.26,28 Zygositywas assigned using a twin similarity questionnaire that is more than 90% accuratein distinguishing monozygotic (MZ) from dizygotic (DZ) twins.29,30

Analyses
Univariate Analyses

The influence of latent genetic factors (the sum of effects of individualalleles at a locus), shared environmental factors, and unique environmentalfactors on conduct problem scores from parents, teachers, and adolescentswere first estimated in univariate models. Because MZ twins share 100% oftheir genes and DZ twins share, on average, 50%, additive genetic effectsare reflected in a pattern of correlations by which the MZ correlation istwice the DZ correlation. When DZ correlations are less than half the MZ correlations,nonadditive or dominance genetic effects can be examined. This refers to interactioneffects between alleles at the same locus. Dizygotic twins share only 25%of nonadditive or dominance genetic effects, so these will be reflected ina DZ correlation that is less than half the MZ correlation. Alternatively,when DZ correlations are less than half the MZ correlation, a sibling interactionmodel can be tested; this model examines genetic and environmental influencesin the presence of sibling interactions. Shared environmental influences arethose that tend to make twins more similar, and will inflate DZ correlationsrelative to MZ correlations, while unique environmental influences are thosethat make twins different from one another. They are seen in the extent towhich the MZ correlation is less than 1.0. The overall acceptability of amodel is evaluated using a χ2 test (assessing goodness of fit)and Akaike Information Criterion (assessing parsimony), which has a more negativevalue the better the fit. The significance of individual parameters withina model is assessed by constraining their effects to 0 and then calculatingthe change in χ2 for the model. A significant deteriorationin the goodness of fit implies a significant influence from the constrainedparameter. A commercially available software program (SPSS for Windows) wasused31 to obtain descriptive statistics andthe covariance matrices for genetic analysis. A software package (Mx)32 was used for genetic model fitting. The methods andprinciples of the model-fitting procedure are described in detail by Nealeand Cardon.33 Before model fitting, scoreswere transformed to approximate normality using log(score + 1).

Multivariate Analyses

Data from all informants were subsequently examined together in theadolescent sample, in which 3 sources of data were potentially available.For these analyses, scores were first standardized for age and sex so thatany influence of age or sex would not bias parameter estimates. To maximizethe available data, a raw data approach in the software package (Mx) was usedat this stage. This removes the need to summarize data in covariance matricesacross all informants, which would result in the exclusion of any twin pairsin which data from one informant were missing. This raw maximum likelihoodapproach generates a value (−2 × log likelihood), which is thencompared with a saturated model to generate a goodness-of-fit χ2.32

First, an independent pathway model was tested (Figure 1). This tests the extent to which genetic and environmentalinfluences are shared across informants, while also allowing for discretegenetic and environmental influences on ratings from each informant. Next,a more stringent model, described as a psychometric or common pathway model,33 was tested (Figure2). This type of model implies that parents, teachers, and adolescentsare all rating the same underlying conduct problems' phenotype (with contributinggenetic and environmental influences), but allows for rater-specific estimatesof genetic and environmental influences. The arrows from the latent phenotype,"P," represent the factor loadings of the latent phenotype on each of theobserved measures. This type of model was chosen because it best representsthe sources of data herein, with each rater basing his or her appraisal ondifferent circumstances (ie, teachers rating behavior in school, parents ratingbehavior primarily at home, and adolescents rating behavior across settings).Finally, a simple multiple rater model was tested (Figure 3). This again allows the 3 observed measures (parent, teacher,and adolescent ratings) to be influenced by a latent conduct problems' phenotype.It also allows for variation in the extent to which different raters contributeto the latent phenotype and for variation in the error associated with eachrater, but it does not allow for discrete genetic and shared environmentalinfluences on each observed measure.

Results

Replies were received from 682 families, a response rate of 61.5% froman initial sample of 1109. There were 12 families in which either the parentor one of the twins failed to return a questionnaire, leaving 670 familieswho returned parent- and twin-rated measures. Of these families, permissionto contact a teacher was given by 561 (83.7%), and 443 teachers replied (79.0%of those families giving consent). Adolescent self-report packs were sentto 570 twin pairs aged 11 to 17 years, and were returned by 286 twin pairs,a response rate of 50.2%.

The age of the twins ranged from 5 to 17 years, with a mean for bothsexes of 10.6 years. There were 14 pairs in which zygosity could not be determinedbecause of missing or inconsistent data.

A comparison of responders and nonresponders using Townsend Scores,34 which are based on National Census data for eachelectoral ward, revealed no significant (P = .10)sociodemographic differences between the 2 groups, suggesting that the sampleis representative of the local population in sociodemographic terms. Furthermore,a comparison with population norms for the SDQ showed that mean conduct problemscores for this sample are not significantly different from those in the generalpopulation. For twins, the mean (95% confidence interval) scores were as follows:parent report, 1.83 (1.39-2.27); teacher report, 0.98 (0.51-1.45); and self-report,2.41 (1.85-2.97). For singletons, the mean (95% confidence interval) scoreswere as follows: parent report, 1.60 (1.35-1.85); teacher report, 0.90 (0.64-1.16);and self-report, 2.20 (1.88-2.52).

Mean conduct problem scores, standard deviations, correlations, andfinal sample sizes of twin pairs with complete data are shown in Table 1. Conduct problem scores were higherin male than female twins, in keeping with the existing literature, for parentand teacher reports and self-reports (parent-rated t1300 = 3.3, P<.001; teacher-rated t831 = 3.4, P<.001;and adolescent-rated t530 = 2.6, P = .01). There were no significant differences in meanscores between MZ and DZ twins for any of the measures (parent-rated t1273 = 0.65, P = .50;teacher-rated t809 = 1.6, P = .10; and adolescent-rated t519 =−0.2, P = .90). The F ratio of variance inMZ/DZ pairs was not significant for any of the measures (parent-rated F =1.4, P = .20; teacher-rated F = 2.4, P = .20; and adolescent-rated F = .02, P =.90). The correlation between conduct problem scores and age was significantfor parent report (Spearman ρ = −0.09, P =.02), but not for other informants. The distribution of conduct scores waspositively skewed so scores were transformed to approximate normality beforemodel fitting using log(score + 1). After transformation, skewness and kurtosisindices were between −1.0 and 1.0, implying that not much distortionis to be expected.35

Model fitting: univariate analyses

Table 2 presents the resultsof univariate model fitting for the 3 informants. For results from differentinformants to be comparable (because self-reports are only available for 11-to 17-year-old subjects), results for children (those aged <11 years) andadolescents (those aged 11-17 years) are presented separately.

Parent-Rated Conduct Problems

Complete data were available from 621 twin pairs for these analyses.The pattern of correlations (0.63 MZ/0.16 DZ in male pairs and 0.53 MZ/0.33DZ in female pairs) suggested genetic influence on the variance of scores,and this was supported by model fitting. The best-fitting models show thatmore than half the variance was accounted for by additive genetic influences,while the remainder was influenced by unique environmental factors. Sharedenvironmental influences and nonadditive genetic influences were insignificant.

Teacher-Rated Conduct Problems

The same model-fitting procedure was applied to teacher-rated SDQ conductproblems. There were 39 twin pairs in which the twins were rated by differentteachers, and these were excluded from the present analyses. Complete datawere available from 387 twin pairs. The pattern of correlations (0.69 MZ/0.13DZ in male pairs and0.75 MZ/0.26 DZ in female pairs) suggested genetic influences.The results of model fitting are summarized in Table 2, and show that, in adolescents, almost three quarters ofthe variance in scores was attributable to genetic influences, with the remainderaccounted for by unique environmental factors. In children, the same proportionof variance was due to genetic influence, but in this case the genetic influencewas nonadditive. A sibling interaction model was also tested, but fitted thedata less well than the nonadditive or dominance genetic effects and uniqueenvironmental effects model. Again, the influence of shared environment wasinsignificant.

Complete data were available from 254 twin pairs for these analyses.The pattern of correlations (0.39 MZ/0.05 DZ in male pairs and 0.53 MZ/0.17DZ in female pairs) suggested genetic influence. The same model-fitting procedurewas applied, and results are summarized in Table 2. Of the variance in scores, 35% was accounted for by additivegenetic influences, with the remainder due to influences of the twins' uniqueenvironment. Again, the influences of the shared environment and the nonadditivegenetic effects were insignificant.

Multivariate models combining parent-, teacher-, and adolescent-ratedscores

Results for the independent pathway model are shown in Figure 1. A model including dominance genetic effects is shown,but a model including additive genetic effects, shared environmental effects,and unique environmental effects was also tested; fit statistics for bothmodels are given in the legend to Figure 1. The model including additive genetic effects, nonadditive or dominancegenetic effects, and unique environmental effects provides a slightly betterfit.

Results for the simple multiple rater model are shown in Figure 3. This model provides a poor fit with the data. The multiplerater model is more stringent than the other 2 models because it offers fewerparameters and, in particular, does not allow for any separate influences,other than error, on each measure. This restriction results in a significantdeterioration of fit compared with the common pathway model (change in χ26 = 41.67, P<.001), indicatingthat the presence of discrete rater influences provides a better explanationof the data.

Comment

The results of this investigation lend further support to existing evidencesuggesting that conduct problems in children and adolescents have a heritablecomponent. In this community sample of twins, a substantial genetic influencehas been shown using a dimensional measure that is widely used in clinicalpractice and epidemiology.26 Parent, teacher,and adolescent reports all provided significant heritability estimates. Therewas no significant effect of the shared environment in any of these models,and only in the teacher-report scores for adolescents were the confidenceintervals fairly wide around this estimate (0%-32%), suggesting that a largersample might have detected a significant effect.

When conduct scores from all informants were combined in a single model,the best results were achieved for a model that included an underlying phenotypethat was common to all informants and separate genetic and environmental influencesacting on each measure. This suggested that all informants were, to a considerableextent, rating a common underlying phenotype that was wholly influenced bygenetic factors, additive and nonadditive. This highly genetic phenotype refersonly to aspects of behavior that are common to all informants and, therefore,pervasive across settings. However, ratings by teachers also showed a significantspecific genetic influence, suggesting that different genes may be influencingthe behavior that teachers were rating. Conduct problems occurring in schoolseem to be different from those that are pervasive across settings. This isa phenomenon that has also been observed in ratings of antisocial behaviorin younger children36 and in childhood hyperactivity.37,38 It supports the value of SDQ datafrom teachers because they are clearly contributing a measure of behaviorthat is not tapped by parent reports or self-reports. It also has implicationsfor phenotype definition in molecular genetic research, implying that differentgenes are influencing measures from different informants.

Another approach to data from multiple informants is to examine theextent of rater bias, but this approach is better suited to multiple ratersof behavior in one setting (eg, mothers and fathers rating behavior at home)and does not easily generalize to multiple raters in different settings. Giventhat children may behave differently at home and at school, it is possiblethat parents and teachers provide valid ratings of behavior in their respectivedomains, and to what extent this represents rater bias or true phenotypicdifferences is difficult to disentangle.

The genetic literature on conduct/externalizing problems in childrenand adolescents is unusual insofar as more than any other behavioral trait,it has shown a significant effect of shared environment.24 However,the lack of shared environmental influence shown herein is a far from uniquefinding. These results are in agreement with those of Rowe,14 whoused a self-report delinquency scale in adolescents; Schmitz et al,18 who used the delinquency subscale of the Child BehaviorChecklist in 4- to 18-year-old subjects; McGuffin and Thapar,20 whoused the Olweus adolescent self-report scale; Slutske et al,11 whomade retrospective DSM-III-R conduct disorder diagnosesin adults; Eaves et al,10 who used interview-baseddiagnoses of conduct disorder and oppositional defiant disorder in childrenand adolescents; and Miles et al,23 who usedinterview-derived symptom counts. The only other twin study36 weare aware of that has used multiple informants in different settings alsofound a highly heritable shared latent phenotype with no significant influencefrom the shared environment. Arsenault et al36 useddata from mothers, teachers, observers, and children themselves, and showedthat heritability was higher (82%) for a shared latent phenotype than forcorresponding single measures of antisocial behavior, as found in this investigation.In their sample of 5-year-old twins, a common pathway model was the best explanationof the data, as found herein. Their results and ours support the notion thatconduct problems that are pervasive across settings are a highly genetic phenotypein young children and adolescents. This is in keeping with a theory of antisocialdevelopment39 that suggests that pervasiveantisocial behavior that develops in childhood is a result of heritable neurodevelopmentalabnormalities and is likely to persist into adolescence and adulthood, althoughlongitudinal cross-situational data are required to examine this questionfurther.

The lack of significant shared environmental influences in this investigationwarrants some consideration. When shared environmental influences depend ongenotype (as in gene × environment interactions), then their effectsare subsumed in the estimate of genetic influences in twin analyses. Thereis some evidence40,41 suggestingboth gene × environment interaction and correlation in conduct problemsand related behaviors, and this may have contributed to the lack of any significantshared environmental influence in the present investigation.

The estimates for unique environmental effects represent those environmentalinfluences that are not shared and tend to make individuals different fromone another, while also including measurement error. Unique environmentalinfluences emerged for each informant, but not for the shared underlying phenotype.This is in keeping with another twin study19 thatused data from mothers, fathers, and twins, and found that the influence ofthe nonshared environment reduced and disappeared when comparing single withmultiple-rater models. This observation is likely to reflect a reduction inerror variance when data from multiple informants are combined. For observedphenotypes, nonshared environment estimates include all measurement error,but with a latent phenotype, as used in the multiple-rater models herein,much of this error is taken up elsewhere. Although the present investigationdoes not measure which specific environmental influences are at work for eachinformant, a significant role for unique environmental influences is in keepingwith other literature. There is evidence42,43 suggestingthat children, while growing up in the same family, can nevertheless experiencequite different environments, and Reiss et al44 haveshown a strong association between child-specific parental conflict withinfamilies and children's behavior problems.

The finding that conduct problems are highly heritable is not in anyway a suggestion that these behaviors are wholly genetically determined and,therefore, not amenable to intervention. Environmental influences of the kindthat tend to make twins different from one another had a significant influencein this investigation, and appreciating genetic influence is only a smallstep in understanding the complex pathway from genes to behavior. Complexgene-environment correlation and interaction suggests that individuals witha genetic propensity to certain behaviors will seek out environments thatexacerbate the behavior. Nature and nurture are inextricably entwined.

Three limitations of the present study warrant discussion. First, thefindings presented are based on questionnaire data; diagnostic interviewsare obviously preferable, but are expensive and were not a feasible optionfor this investigation. Second, another limitation is the response rate: 61.5%of a population-based sample of 1109 twin pairs. However, comparison of responderswith nonresponders revealed no significant differences in sociodemographicmeasures reflecting neighborhood social deprivation. Because these are thetype of environmental stresses associated with conduct problems, it was assumedthat the sample adequately represents the general population and there wasno systematic bias with regard to conduct problems. In addition, a comparisonof mean conduct problem scores with population norms revealed no significantdifference. However, we cannot completely discount the possibility of a responsebias. Third, the main measure of conduct problems26,28 usedis a relatively new and brief measure, with 5 items that generate the ConductProblems Scale. Despite this, its brevity and its inclusion of strengths aswell as difficulties were seen as an advantage in a general population postalsurvey of this sort.

Corresponding author: Jane Scourfield, PhD, MRCPsych, Departmentof Psychological Medicine, University of Wales College of Medicine, FourthFloor, Heath Park, Cardiff CF14 4XN, Wales (e-mail: scourfieldj@cardiff.ac.uk).

Submitted for publication August 11, 2003; final revision received December30, 2003; accepted January 18, 2004.

This study was supported by the Medical Research Council, London, England.

References
1.
Melzer  HGatward  RGoodman  RFord  T The Mental Health of Children and Adolescents inGreat Britain.  London, England Office for National Statistics2000;
2.
Anderson  JCWilliams  SMcGee  RSilva  PA DSM-III disorders in preadolescent children:prevalence in a large sample from the general population.  Arch Gen Psychiatry. 1987;4469- 76PubMedGoogle ScholarCrossref
3.
Cohen  PCohen  JKasen  SVelez  CNHartmark  CJohnson  JRojas  MBrook  JStreuning  EL An epidemiological study of disorders in late childhood and adolescence,1: age- and gender-specific prevalence.  J Child Psychol Psychiatry. 1993;34851- 867PubMedGoogle ScholarCrossref
4.
Robins  L Sturdy childhood predictors of adult antisocial behavior: replicationsfrom longitudinal studies.  Psychol Med. 1978;8611- 622PubMedGoogle ScholarCrossref
5.
Rutter  M Relationships between mental disorders in childhood and adulthood.  Acta Psychiatr Scand. 1995;9173- 85PubMedGoogle ScholarCrossref
6.
Yeager  CLewis  D Mortality in a group of formerly incarcerated juvenile delinquents.  Am J Psychiatry. 1990;147612- 614PubMedGoogle Scholar
7.
Shaffer  DPiacentini  J Suicide and attempted suicide. Rutter  MTaylor  EHersov  Leds. Child andAdolescent Psychiatry: Modern Approaches. Oxford, England BlackwellScience Ltd1994;407- 424Google Scholar
8.
Zocollilo  MPickles  AQuinton  DRutter  M The outcome of childhood conduct disorder: implications for definingadult personality disorder and conduct disorder.  Psychol Med. 1992;22971- 986PubMedGoogle ScholarCrossref
9.
Rhee  SHWaldman  ID Genetic and environmental influences on antisocial behavior: a meta-analysisof twin and adoption studies.  Psychol Bull. 2002;128490- 529PubMedGoogle ScholarCrossref
10.
Eaves  LJSilberg  JLMeyer  JMMaes  HHSimonoff  EPickles  ARutter  MNeale  MCReynolds  CAErikson  MTHeath  ACLoeber  RTruett  KRHewitt  JK Genetics and developmental psychopathology, 2: the main effects ofgenes and environment on behavioral problems in the Virginia Twin Study ofAdolescent Behavioral Development.  J Child Psychol Psychiatry. 1997;38965- 980PubMedGoogle ScholarCrossref
11.
Slutske  WHeath  ACDinwiddie  SHMadden  PAFBucholz  KKDunne  MPStatham  DJMartin  NG Modelling genetic and environmental influences in the etiology of conductdisorder: a study of 2,682 adult twin pairs.  J Abnorm Psychol. 1997;106266- 279PubMedGoogle ScholarCrossref
12.
Rowe  DC Biometrical genetic models of self-reported delinquent behavior: atwin study.  Behav Genet. 1983;13473- 489PubMedGoogle ScholarCrossref
13.
Graham  PStevenson  J A twin study of genetic influences on behavioral deviance.  J Am Acad Child Psychiatry. 1985;2433- 41PubMedGoogle ScholarCrossref
14.
Rowe  D Genetic and environmental components of antisocial behavior: a studyof 265 twin pairs.  Criminology. 1986;24513- 532Google ScholarCrossref
15.
Stevenson  JGraham  P Behavioral deviance in 13-year-old-twins: an item analysis.  J Am Acad Child Adolesc Psychiatry. 1988;27791- 797PubMedGoogle ScholarCrossref
16.
Silberg  JErickson  MMeyer  JEaves  LJRutter  MLHewitt  JK The application of structural equation modelling to maternal ratingsof twins' behavioral and emotional problems.  J Consult Clin Psychol. 1994;62510- 521PubMedGoogle Scholar
17.
Edelbrock  CRende  RPlomin  RThompson  LA A twin study of competence and problem behavior in childhood and earlyadolescence.  J Child Psychol Psychiatry. 1995;36775- 785PubMedGoogle ScholarCrossref
18.
Schmitz  SFulker  DMrazekm  D Problem behavior in early and middle childhood: an initial behaviorgenetic analysis.  J Child Psychol Psychiatry. 1995;361443- 1458PubMedGoogle ScholarCrossref
19.
Simonoff  EPickles  AHewitt  JSilberg  JRutter  MLoeber  RMeyer  JNeale  MEaves  L Multiple raters of disruptive child behavior: using a genetic strategyto examine shared views and bias.  Behav Genet. 1995;25311- 326PubMedGoogle ScholarCrossref
20.
21.
Meyer  JRutter  MSilberg  JLMaes  HHSimonoff  EShillady  LLPickles  AHewitt  JKEaves  L Familial aggregation for conduct disorder symptomatology: the roleof genes, marital discord and family adaptability.  Psychol Med. 2000;30759- 774PubMedGoogle ScholarCrossref
22.
Jacobson  KPrescott  CKendler  K Genetic and environmental influences on juvenile antisocial behaviourassessed on two occasions.  Psychol Med. 2000;301315- 1325PubMedGoogle ScholarCrossref
23.
Miles  DRvan den Bree  MBPickens  RW Sex differences in shared genetic and environmental influences betweenconduct disorder symptoms and marijuana use in adolescents.  Am J Med Genet. 2002;114159- 168PubMedGoogle ScholarCrossref
24.
Plomin  RDeFries  JCMcClearn  GERutter  M Behavioral Genetics. 3rd New York, NY WH Freeman & Co1997;
25.
Rutter  MMaughan  BPickles  ASilberg  JSimonoff  ETaylor  E Heterogeneity of antisocial behavior: causes, continuities, and consequences.  The Nebraska Symposium on Motivation. 44 Lincoln University of Nebraska Press1997;45- 118Google Scholar
26.
Goodman  R The Strengths and Difficulties Questionnaire: a research note.  J Child Psychol Psychiatry. 1997;38581- 586PubMedGoogle ScholarCrossref
27.
Scourfield  JMartin  NCLewis  GMcGuffin  P The heritability of social cognitive skills in children and adolescents.  Br J Psychiatry. 1999;175559- 564PubMedGoogle ScholarCrossref
28.
Goodman  RScott  S Comparing the Strengths and Difficulties Questionnaire and the ChildBehaviour Checklist: is small beautiful?  J Abnorm Child Psychol. 1999;2717- 24PubMedGoogle ScholarCrossref
29.
Nichols  RCBilbro  WC  Jr The diagnosis of twin zygosity.  Acta Genet Stat Med. 1966;16275PubMedGoogle Scholar
30.
Cohen  DDibble  EGrawe  JMPollin  W Reliably separating identical from fraternal twins.  Arch Gen Psychiatry. 1975;321371- 1375PubMedGoogle ScholarCrossref
31.
SPSS.  Chicago, Ill SPSS Inc2001;
32.
Neale  MCBoker  SMXie  GMaes  HH Mx: Statistical Modelling. 6th Richmond Dept of Psychiatry, Virginia Commonwealth University2002;
33.
Neale  MCardon  L Methodology for Genetic Studies of Twins and Families.Dordrecht, the Netherlands: Kluwer Academic Publishers; 1992. NATO ASI Series.
34.
Townsend P, Philimore P, Beattie A., Health and Deprivation: Inequality and the North.  London, England Croon Helm1988;
35.
Muthén  BKaplan  D A comparison of some methodologies for the factor analysis of nonnormalLikert variables.  Br J Math Stat Psychol. 1985;38171- 189Google ScholarCrossref
36.
Arsenault  LAMoffitt  TECaspi  ATaylor  ARijsdijk  FVJaffee  SRAblow  JCMeaselle  JR Strong genetic effects on cross-situational antisocial behaviour among5-year-old children according to mothers, teachers, examiner-observers, andtwins' self-reports.  J Child Psychol Psychiatry. 2003;44832- 848PubMedGoogle ScholarCrossref
37.
Martin  NScourfield  JMcGuffin  P Observer effects and heritability of childhood attention-deficit hyperactivitydisorder symptoms.  Br J Psychiatry. 2002;180260- 265PubMedGoogle ScholarCrossref
38.