A test of the equal-environment assumption in twin studies of psychiatric illness

Marriage and Genetic Variation across the Lifespan: Not a Steady Relationship?

The prevalence of marriage varies across the lifespan, as does its importance to reproduction and the nurturance of children. We examined genetic and environmental influences on self-reported marriage at each decade from 20 through 70 years of age, using data collected for the Duke Dementia Study, a followed-up subset of the World War II Veteran Twin Registry. Genetic influences best fit a common factor model, supplemented by another, age-specific, genetic factor at age 30. Broad heritability increased from age 20 through 40, and then decreased to zero by ages 60 and 70. A longitudinal Cholesky model best described environmental influences on marriage across the lifespan. Shared environmental factors showed their greatest influence at age 20, no influence at 30 or 40 years, and then, reappeared with influence at 60 and 70. Variance due to error and unique environmental influences increased steadily to age 50 years and then declined slightly.

The equal environments assumption of classical twin studies may not hold

The classical twin method - comprising comparisons of monozygotic (MZ) and dizygotic (DZ) twins - in the domain of cognitive abilities and attainments has led to wide acceptance of results suggesting a large amount of additive genetic variance, with far-reaching implications both for the nature of future studies on the causes of cognitive variance and for intervention policies, as in education. However, this interpretation is only valid if the method observes a number of conditions, which have to hold. Here, we show that the most crucial of these, namely, the equal environments assumption (EEA), may not hold. Consequently, differences in twin correlations might be at least partly explained by treatment effects from parents, teachers, peers, and so on. In addition, well-known interactions at various levels confound the model of simple additive effects on which the classical twin method is predicated and results are interpreted. For example, at a socio-cognitive level, DZ twins may respond to treatments differently from MZ twins. This interaction may further explain MZ-DZ correlation differences. There is abundant evidence for such interactive effects in published twin data. We suggest that there is a need for a more thorough examination of these problems.

Age at first sexual intercourse, genes, and social context: Evidence from twins and the dopamine D4 receptor gene

We carried out two distinct types of genetic analysis with data from the National Longitudinal Study of Adolescent Health. The first was a non-DNA twin analysis using monozygotic (identical) and same-sex dizygotic (fraternal) twins. The second analysis investigates the association between age at first sexual intercourse and the 48-bp repeat polymorphism in the dopamine receptor D4 gene (DRD4). The twin analysis shows that MZ twins correlate their timing of first sex to a much greater extent than do the same-sex DZ twins. Our analysis of the polymorphisms in DRD4 indicates that those with an any-3R genotype experienced a risk of first sexual intercourse 23% (p=.016), 233% (p=.0001), 28% (p=.012), and 69% (p=.006) higher than those with an other/other (or any-4R) genotype in the all-ethnicities (n=2,552), Asian, white, and Hispanic samples, respectively. The risk of first sex does not differ between the two genotypes in the African American sample. These results were obtained after adjusting the standard socioeconomic covariates, including gender, parental education, family structure, and community poverty in the regression model. Evidence from both twin and genetic-variant analyses points to a role of genes in the timing of first sexual intercourse.

Inherited auditory-cortical dysfunction in twin pairs discordant for schizophrenia

BACKGROUND

Information on the inheritance of neurophysiological abnormalities might help elucidate the molecular genetic basis of schizophrenia. We used magnetoencephalography (MEG) and electroencephalography (EEG) to investigate the inheritance of auditory-cortical deficiencies in twin pairs discordant for schizophrenia.

METHODS

Auditory EEG/MEG responses to frequent standard and occasional deviant tones were measured in mono- and dizygotic (MZ and DZ) twin pairs discordant for schizophrenia and demographically matched healthy twin pairs, recruited from a total population cohort. The MEG/EEG results were regressed against the genetic resemblance to patients with schizophrenia across the patients' unaffected MZ/DZ co-twins and control subjects (with genetic correlations of 1, .5, and 0 to schizophrenia patients, respectively).

RESULTS

The EEG responses P50, N100, and mismatch negativity (MMN), as well as the MEG response P50m, were reduced in the schizophrenic patients. P50 and N100 were significantly decreased also in their unaffected co-twins, as compared with the control subjects. Importantly, the P50 and N100 decrease correlated with the unaffected subjects' genetic resemblance to schizophrenia patients.

CONCLUSIONS

Our results suggest inherited abnormalities in cortical auditory processing in schizophrenia, reflected by the decreased P50/P50m and N100 amplitudes, whereas the MMN abnormalities might reflect predominantly state-dependent neurodegeneration.

Heritability of cigarette smoking and alcohol use in Chinese male twins: the Qingdao twin registry

BACKGROUND

China has the world's largest concentration of smokers (350 million) and rising alcohol consumption, yet little is known about tobacco and alcohol use aetiology. In 2000, the Chinese National Twin Registry was established to provide a genetically informative resource for investigation of health behaviour including tobacco and alcohol use.

METHODS

Using standard twin methodology, this study aimed to examine the relative contribution of genetic and environmental influences on cigarette smoking and alcohol drinking in a sample of adult Chinese twins (n = 1010 individual twins). More than half of the male twins were smokers (58%), and 32.5% reported alcohol consumption. Among male smokers, 46.4% smoked 20 or more cigarettes per day (heavy smokers) and among drinkers, 32.8% consumed one or more drinks per day. Nearly all female twins were non-smokers (99.2%) and non-drinkers (98.7%); therefore, genetic analysis was limited to male data.

RESULTS

In men, current smoking was significantly heritable [75.1%, 95% confidence interval (CI) 56.7-87.5] with no evidence for a significant contribution of shared environmental effects. Heavy smoking was more strongly influenced by genes (66.2%, 95% CI 0-88.4) than shared environment (8.7%, 95% CI 0-71.0). Similarly, current drinking was more strongly influenced by genetic effects (59.5%, 95% CI 0-87.8) than by shared environmental effects (15.3%, 95% CI 0-72.1). Amount of alcohol consumed was influenced to a similar degree by genetic (42.4%, 95% CI 0-91.8) and shared environmental factors (39.2%, 95% CI 0-82.7).

CONCLUSIONS

These results support findings from twins of Western origin on the aetiology of tobacco and alcohol use and encourage further work in Chinese twins.

Assumption-free estimation of heritability from genome-wide identity-by-descent sharing between full siblings

The study of continuously varying, quantitative traits is important in evolutionary biology, agriculture, and medicine. Variation in such traits is attributable to many, possibly interacting, genes whose expression may be sensitive to the environment, which makes their dissection into underlying causative factors difficult. An important population parameter for quantitative traits is heritability, the proportion of total variance that is due to genetic factors. Response to artificial and natural selection and the degree of resemblance between relatives are all a function of this parameter. Following the classic paper by R. A. Fisher in 1918, the estimation of additive and dominance genetic variance and heritability in populations is based upon the expected proportion of genes shared between different types of relatives, and explicit, often controversial and untestable models of genetic and non-genetic causes of family resemblance. With genome-wide coverage of genetic markers it is now possible to estimate such parameters solely within families using the actual degree of identity-by-descent sharing between relatives. Using genome scans on 4,401 quasi-independent sib pairs of which 3,375 pairs had phenotypes, we estimated the heritability of height from empirical genome-wide identity-by-descent sharing, which varied from 0.374 to 0.617 (mean 0.498, standard deviation 0.036). The variance in identity-by-descent sharing per chromosome and per genome was consistent with theory. The maximum likelihood estimate of the heritability for height was 0.80 with no evidence for non-genetic causes of sib resemblance, consistent with results from independent twin and family studies but using an entirely separate source of information. Our application shows that it is feasible to estimate genetic variance solely from within-family segregation and provides an independent validation of previously untestable assumptions. Given sufficient data, our new paradigm will allow the estimation of genetic variation for disease susceptibility and quantitative traits that is free from confounding with non-genetic factors and will allow partitioning of genetic variation into additive and non-additive components.

Quantitative geneticists attempt to understand variation between individuals within a population for traits such as height in humans and the number of bristles in fruit flies. This has been traditionally done by partitioning the variation in underlying sources due to genetic and environmental factors, using the observed amount of variation between and within families. A problem with this approach is that one can never be sure that the estimates are correct, because nature and nurture can be confounded without one knowing it. The authors got around this problem by comparing the similarity between relatives as a function of the exact proportion of genes that they have in common, looking only within families. Using this approach, the authors estimated the amount of total variation for height in humans that is due to genetic factors from 3,375 sibling pairs. For each pair, the authors estimated the proportion of genes that they share from DNA markers. It was found that about 80% of the total variation can be explained by genetic factors, close to results that are obtained from classical studies. This study provides the first validation of an estimate of genetic variation by using a source of information that is free from nature–nurture assumptions.

Etiological contributions to heavy drinking from late adolescence to young adulthood

The authors examined genetic and environmental contributions to stability and change in heavy drinking from late adolescence to young adulthood in a sample of 1,152 twin pairs. In men, heavy drinking was similarly heritable at ages 17 (h2=.57) and 20 (h2=.39), and its stability owed primarily to common genetic factors. In women, heavy drinking was less heritable than in men at ages 17 (h2=.18) and 20 (h2=.30) and its stability was primarily due to enduring shared environmental influences. P3 amplitude, an event-related brain potential marker of alcoholism risk, was less predictive of heavy drinking in women than in men, providing further support for the proposition that biological factors have less impact on heavy drinking in young adult women than in young adult men.

Genetic and environmental influences on skin pattern deterioration

Sun exposure has been known to cause histological changes in the dermal layer of the skin. Using deterioration in the fine reticular patterning of the epidermal stratum corneum (skin pattern, as measured on the Beagley-Gibson scale) as a proxy measure of histological changes in the dermal layer, previous studies have typically assumed that degradation of skin pattern is largely caused by sun exposure. A twin study comprising 332 monozygotic twin pairs and 488 dizygotic twin pairs at ages 12, 14, and 16 was used to investigate the etiology of variation in skin pattern, particularly in relation to measured sun exposure and skin color. Our results indicate that although self-reported sun exposure is a significant contributor to variation in skin pattern, its effect is small, explaining only 3.4% of variation in skin pattern at age 14. Additive genetic effects explain 86% of variation in skin pattern at age 12 but these effects reduce with age so that 75% of variation is due to additive genetic effects at age 14 and 72% at age 16. This trend of diminishing genetic influences continues into adulthood, with 62% of variation due to non-additive genetic factors in a smaller adult sample (aged 32-86). Skin color explains 10.4% of variation in skin pattern at age 12, which is due to additive genetic influences common to both. Melanin content appears to provide a protective effect against skin pattern deterioration, perhaps because of the structural differences in melanosomes between different skin types or the free radical scavenging properties of melanin.

A flexible model for multivariate interval-censored survival times with complex correlation structure

We focus on the analysis of multivariate survival times with highly structured interdependency and subject to interval censoring. Such data are common in developmental genetics and genetic epidemiology. We propose a flexible mixed probit model that deals naturally with complex but uninformative censoring. The recorded ages of onset are treated as possibly censored ordinal outcomes with the interval censoring mechanism seen as arising from a coarsened measurement of a continuous variable observed as falling between subject-specific thresholds. This bypasses the requirement for the failure times to be observed as falling into non-overlapping intervals. The assumption of a normal age-of-onset distribution of the standard probit model is relaxed by embedding within it a multivariate Box-Cox transformation whose parameters are jointly estimated with the other parameters of the model. Complex decompositions of the underlying multivariate normal covariance matrix of the transformed ages of onset become possible. The new methodology is here applied to a multivariate study of the ages of first use of tobacco and first consumption of alcohol without parental permission in twins. The proposed model allows estimation of the genetic and environmental effects that are shared by both of these risk behaviours as well as those that are specific.

Modifier genetics: cystic fibrosis

Cystic fibrosis (CF) is the most common lethal autosomal recessive disorder in the Caucasian population, affecting about 30,000 individuals in the United States. The gene responsible for CF, the CF transmembrane conductance regulator (CFTR), was identified 15 years ago. Substantial variation in the many aspects of the CF phenotype among individuals with the same CFTR genotype demonstrates that factors independent of CFTR exert considerable influence on outcome in CF. To date, the majority of published studies investigating the cause of disease variability in CF report associations between candidate genes and some aspect of the CF phenotype. However, a definitive modifier gene for CF remains to be identified. Despite the challenges posed by searches for modifier effects, studies of affected twins and siblings indicate that genetic factors play a substantial role in intestinal manifestations. Identifying the factors contributing to variation in pulmonary disease, the primary cause of mortality, remains a challenge for CF research.

Environmental and heritable factors in the etiology of oral diseases--a population-based study of Swedish twins

A population-based twin study is a useful design for quantification of the effects of genes and environmental factors in disease etiology. We used data from 10,000 Swedish twin pairs to quantify genetic and environmental contributions to tooth loss and periodontal health. Oral health information was obtained from telephone interviews. Structural equation models measured the relative importance of genetic and environmental factors. Genetic factors contributed to 14% of variation in tooth loss among women, and 39% among men. Non-shared environmental factors accounted for one-quarter of risk; environmental factors shared by twins comprised the remainder. Heritability estimates of periodontal disease were 39% and 33% for women and men, respectively, while non-shared environmental factors accounted for the remaining variation. Heritability for both conditions varied as a function of age and smoking status. Analysis of data from this large, population-based study demonstrates a moderate role of genetic factors in oral diseases, and suggests potential gene-environment interactions.

How are parent-child conflict and childhood externalizing symptoms related over time? Results from a genetically informative cross-lagged study

The present study attempted to determine the direction and etiology of the robust relationship between childhood externalizing (EXT) symptoms and parent-child conflict using a genetically informative longitudinal model and data from the ongoing Minnesota Twin Family Study. Participants consisted of 1,506 same-sex twins assessed at ages 11 and 14, and their parents. The relationship between EXT and parent-child conflict from ages 11 to 14 was examined within a biometrical cross-lagged design. The results revealed three primary findings: first, the stability of conflict and externalizing over time is largely, although not solely, a result of genetic factors. Second, there appears to be a bidirectional relationship between conflict and EXT over time, such that both conflict and EXT at 11 independently predict the other 3 years later. Finally, the results are consistent with the notion that parent-child conflict partially results from parental responses to their child's heritable externalizing behavior, while simultaneously contributing to child externalizing via environmental mechanisms. These results suggest a "downward spiral" of interplay between parent-child conflict and EXT, and offer confirmation of a (partially) environmentally mediated effect of parenting on child behavior.

Genetic and environmental influences on antisocial behavior and alcohol dependence from adolescence to early adulthood

Genetic and environmental influences on symptoms of adult antisocial behavior (AAB) and alcohol dependence at ages 17, 20, and 24 were examined cross-sectionally and longitudinally in 188 monozygotic and 101 dizygotic male twin pairs. A moderate genetic influence on both AAB and alcohol dependence was found at each age, with a substantial proportion of this influence common to the two disorders, suggesting they share susceptibility genes. Biometrical models showed that continuity effects accounted for most of the stable variance in symptoms of both AAB and alcohol dependence, indicating that genetic and environmental effects associated with each of these disorders were similar at each age. Significant cross-lag effects (effects of alcohol dependence contributing to variance in AAB and vice versa) were observed at ages 20 and 24 for both disorders. The largest and theoretically most interesting of these effects indicated that one sixth of the genetic influence on AAB at age 20 was due to genetic effects associated with alcohol dependence at age 17. Thus, alcohol dependence symptoms at age 17 in particular had an effect on antisocial behavior symptoms at age 20, suggesting that alcohol involvement in adolescence may ensnare otherwise desisting youth in persistent antisocial behavior.

Externalizing problem behaviors and headache: a follow-up study of adolescent Finnish twins

OBJECTIVE

To examine the association of teacher- and parent-rated behavior with headache in a prospective follow-up study of adolescent Finnish twins.

METHODS

Questionnaire data were collected during 1995-2001 from a nationwide sample of Finnish families of 11-year-old twins who were born 1983-1987 (n = 5393) and again at age of 14. Psychological factors were measured by using parents' and teachers' ratings of a 37-item multidimensional rating instrument at the ages of 11 and 14.

RESULTS

At age 11, headache frequency (5 categories) was associated with total scales of externalizing and internalizing problem behaviors and adaptive behaviors, assessed by parents, but only with externalizing problem behaviors assessed by teachers. Results were similar at age 14. The incidence of at least monthly headache between the ages of 11 and 14 years was predicted by externalizing problem behaviors and 2 subscales of adaptive behaviors: constructiveness and poor compliance. In twin pairs discordant for headache, externalizing and internalizing problem behaviors were more common among headache sufferers than among headache-nonsufferers. Headache-discordant monozygotic co-twins confirmed the association of externalizing problem behaviors with headache.

CONCLUSIONS

The frequency of adolescents' headache is predicted by psychological factors, especially by externalizing problem behaviors. This seems to be independent of genetic or familial influences on behavior and headache. Behavioral problems may be a sign of worsening of headache or vice versa.

Genetic epidemiological approaches in the study of risk factors for cardiovascular disease

The importance of genetic factors for the susceptibility to disease has been widely recognized in the last years. Genes have been identified for monogenic diseases and the challenge lying ahead is the identification of genetic components of importance and the environments in which they are expressed for complex diseases, that is, multiple genetic factors act and interact with each other or environmental factors to add to the complexity. This paper gives a brief overview of some genetic epidemiological approaches, concepts and recent methodological developments related to the study of risk factors for cardiovascular disease in twin and family studies.

Genetic aspects and genetic epidemiology of parasomnias

Parasomnias are undesirable phenomena associated with sleep. Many of them run in families, and genetic factors have been long suggested to be involved in their occurrence. This article reviews the present knowledge of the genetics of the major classical behavioral parasomnias as well as present results from genetic epidemiological studies. The level and type of evidence for genetic effects varies much from parasomnia to parasomnia. The genetic factors are best established in enuresis, with several linkages to chromosomal loci, but their functions are not so far known. Environmental causes and gene-environment interactions are most probably also of great importance in the origin of complex traits or disorders such as parasomnias.

Twin studies in schizophrenia with special emphasis on concordance figures

Twin studies in schizophrenia have been reviewed with special emphasis on concordance rates in population-based investigations. Sources of error have been discussed with particular focus on sampling. The pair-wise concordance rates in schizophrenia are 30-40% in MZ and 5-10% in DZ, with somewhat higher rates for proband concordance. The findings from twin studies support the diathesis stress model in schizophrenia, and it is argued that the polygenic model gives the best explanation for the empirical findings.

The heritability of alcohol abuse and dependence: a meta-analysis of behavior genetic research

A meta-analysis was performed on 50 family, twin, and adoption studies in which problem drinking and alcohol dependence served as the primary criterion measure. The results showed that far from being an established "fact," the genetic foundations of alcohol misuse are modest and heterogeneous. A weighted mean phi effect size of 0.12 (95% Confidence Interval = 0.11-0.12) was obtained for the total sample of 72 effect sizes. Four potential moderator variables (proband gender, sample nationality, pattern severity, year of publication) were examined with outcomes confirming that the heritability of alcohol misuse is stronger in males and in studies employing more severe definitions of abuse (alcoholism, alcohol dependence). When the effect size measure was restricted to studies using male subjects with more severe diagnoses of alcohol misuse, the unweighted mean phi effect size was only 0.18 (95% Confidence Interval = 0.15-0.21), with an even smaller weighted mean phi effect size of 0.15 (95% Confidence Interval = 0.12-0.18); results which indicate an upper limit of 30-36% for the heritability of alcohol misuse.

Shared rearing environment in migraine: results from twins reared apart and twins reared together

BACKGROUND

Studies of twins who are separated from each other early in life and are reared in different environments offer the opportunity to resolve variation in liability to disease.

OBJECTIVE

To evaluate the importance of genetic and environmental influences in migraine; in particular, addressing the role of the shared rearing environment.

METHODS

A population-based cohort of twins, including a subsample of 314 pairs reared apart and 364 matched control pairs reared together, was drawn from the Swedish Twin Registry. Data on lifetime migraine was collected via self-administered questionnaires mailed to twins aged 42 to 81 years. Quantitative genetic models and regression models were used to analyze sources of twin similarity.

RESULTS

We found nonsignificant shared rearing environmental influences on migraine for men (17%) and no rearing effects at all for women. The heritability of migraine was estimated at 38% (95% confidence interval, 0 to 73) for men and 48% (95% confidence interval, 27 to 65) for women. Among monozygotic twins reared apart, those separated at 3 years of age or earlier were more similar for lifetime migraine than those separated later, and this was especially true for women.

CONCLUSION

In agreement with previous twin data, family resistance in migraine is mainly due to genetic factors, whereas environmental influences make family members different, not similar.

Life events and depressiveness - the effect of adjustment for psychosocial factors, somatic health and genetic liability

OBJECTIVE

To study the nature of the relationship of life events with depressiveness.

METHOD

In 1990, 9947 Finnish adult twins aged 33-60 years participated in a questionnaire study, with no missing data on the analysed variables. The relationship between stressful life-events and depressiveness, based on Beck Depression Inventory (BDI) classified as normal, mild or moderate, was analysed using multinomial regression for all subjects with adjustment for the effect of age, sex, health status, social support, marital status, social class, and personality variables. These same factors were analyzed among 643 twin pairs discordant for depressiveness.

RESULTS

High BDI score categories were strongly associated with stressful life events in all individuals and equally strongly within discordant twin pairs, both monozygotic and dizygotic. Poor somatic health and lack of social support also increased depressiveness.

CONCLUSION

The effect of life events predicting depressiveness was independent of social support, somatic disease, sex and genetic liability.

Genetic and environmental influences on human psychological differences

Psychological researchers typically distinguish five major domains of individual differences in human behavior: cognitive abilities, personality, social attitudes, psychological interests, and psychopathology (Lubinski, 2000). In this article we: discuss a number of methodological errors commonly found in research on human individual differences; introduce a broad framework for interpreting findings from contemporary behavioral genetic studies; briefly outline the basic quantitative methods used in human behavioral genetic research; review the major criticisms of behavior genetic designs, with particular emphasis on the twin and adoption methods; describe the major or dominant theoretical scheme in each domain; and review behavioral genetic findings in all five domains. We conclude that there is now strong evidence that virtually all individual psychological differences, when reliably measured, are moderately to substantially heritable.

Biometrical genetics

Biometrical genetics is the science concerned with the inheritance of quantitative traits. In this review we discuss how the analytical methods of biometrical genetics are based upon simple Mendelian principles. We demonstrate how the phenotypic covariance between related individuals provides information on the relative importance of genetic and environmental factors influencing that trait, and how factors such as assortative mating, gene-environment correlation and genotype-environment interaction complicate such interpretations. Twin and adoption studies are discussed as well as their assumptions and limitations. Structural equation modeling (SEM) is introduced and we illustrate how this approach may be applied to genetic problems. In particular, we show how SEM can be used to address complicated issues such as analyzing the causes of correlation between traits or determining the direction of causation (DOC) between variables.

Emotional and behavioral problems among female twins: an evaluation of the equal environments assumption

OBJECTIVE

To evaluate the equal environments assumption (EEA) of the twin method for mother-reported symptoms of child and adolescent emotional and behavioral problems.

METHOD

Four emotional and behavioral problem scales (symptoms of separation anxiety disorder, attention-deficit/hyperactivity disorder, oppositional defiant disorder, and conduct disorder) and four environmental similarity measures (sharing friends, sharing classes, dressing alike, and perceived zygosity) were assessed by telephone interviews with the biological mothers of 1,948 female adolescent and young adult twin pairs. The effect of environmental similarity on the magnitude of the monozygotic and dizygotic twin correlations and on the parameter estimates from genetic model-fitting was examined for each symptom scale.

RESULTS

The measures of environmental similarity were not strongly or consistently related to the similarity of twins for mother-reported emotional and behavioral problems. On average, controlling for environmental similarity did not substantially affect estimates of genetic and environmental influences.

CONCLUSIONS

These results lend support for the validity of the EEA and suggest that estimates of genetic and environmental influences obtained from twin studies of mother-reported child and adolescent emotional and behavioral problems are not unduly biased by the greater environmental similarity of monozygotic than dizygotic twins.

The genetics of tobacco use: methods, findings and policy implications

Research on the genetics of smoking has increased our understanding of nicotine dependence, and it is likely to illuminate the mechanisms by which cigarette smoking adversely effects the health of smokers. Given recent advances in molecular biology, including the completion of the draft sequence of the human genome, interest has now turned to identifying gene markers that predict a heightened risk of using tobacco and developing nicotine dependence

Genetic relationships between personality and eating attitudes and behaviors

Genetic and environmental factors underlying relationships between personality traits and disordered eating were examined in 256 female adolescent twin pairs (166 monozygotic, 90 dizygotic). Eating behaviors were assessed with the Total Score, Body Dissatisfaction, Weight Preoccupation, Binge Eating, and Compensatory Behavior subscales from the Minnesota Eating Disorders Inventory (M-EDI; K. L. Klump, M. McGue, & W. G. Iacono, 2000). Personality characteristics were assessed with the Negative Emotionality, Positive Emotionality, and Constraint scales from the Multidimensional Personality Questionnaire (MPQ; A. Tellegen, 1982). Model-fitting analyses indicated that although genetic factors were more likely to contribute to MPQ and M-EDI phenotypic associations than environmental factors, shared genetic variance between the 2 phenotypes was limited. MPQ personality characteristics may represent only some of several genetic risk factors for eating pathology.

Sources of covariation among attention-deficit/hyperactivity disorder, oppositional defiant disorder, and conduct disorder: the importance of shared environment

Research has documented high levels of covariation among childhood externalizing disorders, but the etiology of this covariation is unclear. To unravel the sources of covariation among attention-deficit/hyperactivity disorder (ADHD), oppositional defiant disorder (ODD), and conduct disorder (CD), the authors studied 11-year-old twins (N = 1,506) from the Minnesota Twin Family Study. Symptom counts for each of these disorders were obtained from interviews administered to the twins and their mothers. A model was fit that allowed the parsing of genetic, shared environmental (factors that make family members similar to each other), and nonshared environmental (factors that make family members different from each other) contributions to covariation. The results revealed that although each disorder was influenced by genetic and environmental factors, a single shared environmental factor made the largest contribution to the covariation among ADHD, ODD, and CD.

Genetic and environemental factors in conduct problems and adult antisocial behavior among adult female twins

Most twin and adoption studies of conduct problems have demonstrated modest genetic effects but substantial contributions of shared family environment. Conversely, most investigations have shown marked genetic influences but modest contributions of shared family environment in adult antisocial behavior. However, most previous work has focused on male subjects. We obtained retrospective reports of DSM-III-R-defined conduct disorder (CD) and adult antisocial behaviors from a population-based sample of female-female twin pairs. Genetic and environmental contributions to conduct problems and adult antisocial behaviors were examined using polychoric correlation coefficients and univariate structural equation modeling. Statistically significant but modest heritability was observed for conduct problems. A small, statistically nonsignificant contribution of shared family environment to CD behavior was also noted. Adult antisocial symptoms showed modest contributions of both additive genetic and shared family environmental factors. In both childhood and adulthood, the largest influence on antisocial behavior was individual-specific environment. Our findings support the importance of both genetic and environmental factors in antisocial behavior among women as well as the possibility that the relative importance of each set of influences differs by sex in both childhood and adulthood.

Health locus of control in late life: a study of genetic and environmental influences in twins aged 80 years and older

The factor structure of health locus of control (Form A; K. A. Wallston, B. S. Wallston, & R. DeVellis, 1978) was examined in 420 octogenarians (M age = 83.2 years), and the contributions of genetic and environmental factors to health-control beliefs in 141 octogenarian twin pairs (71 identical, 70 same-sex fraternal) were estimated. Factor analyses reproduced previously proposed factors (Internal, Chance, and Powerful Others). Associations between health-control beliefs and life satisfaction, depression, and other health-related measures (e.g., self-rated health, outpatient contacts, and hospitalization), were modest. Quantitative genetic analyses revealed significant shared environmental influence on the Chance subscale, and significant familiality (attributable to a combination of genetic and shared environmental influences) on the Powerful Others subscale; there was no evidence of familiality on the Internal subscale.

Obsessive and compulsive symptoms in a general population sample of female twins

Obsessive-compulsive disorder (OCD) and obsessive-compulsive symptoms (OCS) exhibit a familial pattern of transmission. The different components of these conditions and the extent to which these components are inherited have not been studied well. A sample of 1,054 female twins, including both members of 527 pairs, from the Virginia Twin Registry returned questionnaires that included 20 items from the Padua Inventory of obsessive-compulsiveness. Their responses were used to estimate the heritability of the different factors of OCS in this population. Principal components analysis suggested two meaningful factors corresponding roughly to obsessions and compulsions. The best-fit model suggested heritabilities of 33 and 26%, respectively. The correlation between additive genetic effects on compulsiveness and obsessiveness was found to be +0.53. Self-report symptoms of obsessions and compulsions in women from the general population are moderately heritable and due, in part, to the same genetic risk factors. An understanding of the etiology of these symptoms is relevant to the study of OCD. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:791-796, 2000.

Genetic studies of substance abuse

Genetic studies of substance abuse indicate that variation in the risk for the disorder in the population is contributed by differences in both individual genotypes and environment. Recent developments in genetics raise the possibility of disentangling the complex system of genotype-environment interaction that determines the development of the individual behavioral phenotype. This paper reviews the concepts, methods and results pertaining to genetic investigation of substance abuse.

Twin studies of eating disorders: a review

OBJECTIVE

Twin methodology has been used to delineate etiological factors in many medical disorders and behavioral traits including eating disorders. Although twin studies are powerful tools, their methodology can be arcane and their implications easily misinterpreted.

METHOD

The goals of this study are to (a) review the theoretical rationale for twin studies; (b) provide a framework for their interpretation and evaluation; (c) review extant twin studies on eating disorders; and (d) explore the implications for understanding etiological issues in eating disorders.

DISCUSSION

On the basis of this review, it is not possible to draw firm conclusions regarding the precise contribution of genetic and environmental factors to anorexia nervosa. Twin studies confirm that bulimia nervosa is familial and reveal significant contributions of additive genetic effects and of unique environmental factors in liability to bulimia nervosa. The magnitude of the contribution of shared environment is less clear, but in the studies with the greatest statistical power, it appears to be less prominent than additive genetic factors.

Genetic and environmental influences on eating patterns of twins aged >/=50 y

BACKGROUND

Clinicians and researchers could benefit from a greater understanding of the role of genetic and environmental factors in human eating behavior.

OBJECTIVE

Our aim was to estimate the relative influence of genetic and environmental factors on habitual eating patterns in middle-aged and elderly men and women.

DESIGN

Male and female twins (n = 4640) aged >/=50 y completed a mailed version of the National Cancer Institute food-frequency questionnaire. Factor analysis was performed to identify eating patterns among respondents. Estimates of genetic, common environmental (shared by family members), and specific environmental (unique to an individual) influences were obtained for food use, serving size, and consumption frequency by comparing monozygotic and dizygotic twin-pair groups with structural equation analysis.

RESULTS

Two independent eating patterns were identified: the first consisted of items high in fat, salt, and sugar, and the second reflected healthful eating habits. Although the influence of environmental factors was larger, between 15% and 38% of the total variation in pattern 1 and between 33% and 40% in pattern 2 were explained by genetic influences. Models accounting for sex differences in genetic and environmental estimates fit the data significantly better for food use and serving size of foods in eating pattern 1 and for food use in eating pattern 2.

CONCLUSION

Although 60-85% of the variability in eating patterns was associated with environmental factors, genetic influences were also apparent and there was some evidence of sex specificity. These findings may be important in crafting dietary interventions and predicting adherence to these interventions.

Behavioral genetics as a tool for developmental psychology: anxiety and depression in children and adolescents

Over the past decade there has been a huge increase in the number of behavioral genetic studies looking into anxiety and depression in children and adolescents. There are now enough data in this area to make a review of the results useful. This paper begins with an outline of the methods used in such research and moves on to review the results in extant studies. Overall, these studies indicate modest to moderate genetic influence on both anxiety and depression. However, behavioral genetic methods are also paramount for exploring environmental influences in addition to genetic influences. Shared environment (that which makes family members resemble one another) is rarely identified in adult studies of personality or psychopathology and does not appear to be a significant influence for depression but it is for anxiety. Nonshared environment, which makes family members differ from one another, is found to be a significant influence for both anxiety and depression. Patterns within these results due to rater effects, age effects, sex effects, the precise phenotype measured, and the study design are explored.

Heritability of binge-eating and broadly defined bulimia nervosa

BACKGROUND

Using diagnostic information obtained at two different times, we incorporated error of measurement into structural equation twin models to evaluate the contribution of additive genetic, common environmental, and individual-specific environmental factors to the liability to binge-eating and broadly defined bulimia nervosa (BN). We also evaluated the validity of the equal environment assumption (EEA) with reference to these two phenotypes.

METHODS

We interviewed 1897 female twins (including both members of 854 twin pairs) from a population-based register about their lifetime history of binge-eating and of broadly defined BN twice, approximately 5 years apart.

RESULTS

The reliabilities of a lifetime history of binge-eating (kappa = .34) and of broadly defined BN (kappa = .28) were low. Based on single interviews, the heritability of binge-eating was estimated to be 50% and broad BN 60%, with the remaining variance attributable to individual-specific environment. Common environmental influences had no effect on liability to either trait. By combining information from two interview waves and thereby incorporating error of measurement into a structural equation model, the estimated heritability of the latent vulnerability to binge-eating (82%) and broadly defined BN (83%) increased substantially. Although there were no violations of the EEA detected for binge-eating, cosocialization influenced twin concordance for broadly defined BN.

CONCLUSIONS

Lifetime histories of binge-eating and broadly defined BN appear to be highly heritable conditions of low reliability.

Longitudinal study of interpersonal dependency in female twins

BACKGROUND

Interpersonal dependence is thought to be important in a number of physical and psychological disorders. There are several developmental theories that suggest environmental influences in childhood are important.

METHOD

A twin study methodology was used to look at the genetic and environmental influences on interpersonal dependence as measured by a sub-scale of the Interpersonal Dependency Inventory with a population-based sample of 2230 twins.

RESULTS

Psychometric analysis revealed that this was a stable measure and that there was a substantial degree of construct validity. Both univariate and longitudinal twin analysis suggested that there was a modest genetic influence and a large, specific environment influence on interpersonal dependency as measured by this scale. The longitudinal analysis revealed that the genetic influence was stable over the time-scale sampled and the environmental influence was moderately stable.

CONCLUSIONS

This finding is at odds with theories that suggest shared environment is important in the aetiology of interpersonal dependency.

A twin-pronged attack on complex traits

Before one starts the hunt for quantitative trait loci (QTLs) for a complex trait, it is necessary to show that the trait is genetically influenced. This evidence is most likely to come from the classical twin study--the demonstration that monozygotic twins are more similar for the trait than dizygotic twins. The strengths and weaknesses of twin studies are discussed, and it is suggested that, far from becoming irrelevant with advances in molecular biology, they can improve the efficiency of QTL detection and play an important role in unravelling developmental genetic mechanisms.

Twin closeness and co-twin risk for substance use disorders: assessing the impact of the equal environment assumption

Various environmental variables are hypothesized to operate differentially within identical and fraternal twin pairs. To the extent that these factors are correlated with behavioral outcomes, such as alcohol or drug abuse, traditional twin studies of concordance may be biased. Self-ratings of within-pair emotional closeness, assessed in 169 same-sex twin pairs ascertained through alcohol and drug treatment centers, were used to determine the impact of the twin relationship on concordance for alcohol dependence (N = 130 twin pairs) and other drug abuse and/or dependence (N = 85 twin pairs). In general, identical twin pairs reported significantly closer relationships than fraternal twin pairs, and female twin pairs reported significantly closer relationships than male twin pairs. The data did not indicate an overall effect of closeness on co-twin risk for alcohol dependence. In contrast, closeness was significantly related to co-twin risk for other drug abuse and/or dependence. However, the MZ/DZ concordance difference for other drug abuse and/or dependence remained significant when the effects of within-pair closeness were controlled. Thus, the initial zygosity and sex differences in concordance for substance use disorders cannot be explained solely by differences in twin relationship due to closeness as assessed in this study.

Heritability of respiratory sinus arrhythmia: dependency on task and respiration rate

In this study, we investigated the genetic and environmental origin of individual differences in respiratory sinus arrhythmia (RSA) during rest and during four stress tasks. We used a multivariate model including age, RSA, and respiration rate. Participants were 208 male and female pairs of middle-aged twins. A model without sex differences, specifying additive genetic and unique environmental factors, showed the best fit across all conditions. Heritability of RSA ranged from 28% to 43%. Correction for respiration rate yielded RSA heritabilities of similar size. The covariance between respiration rate and RSA was best explained by a combination of correlated unique environmental and correlated additive genetic factors. Combined with data from an earlier project, RSA from 317 adolescent and 712 middle-aged individuals of both sexes was available. This large data set showed that (a) sex differences in mean RSA are absent and (b) RSA decreases considerably from adolescence (111.5 ms) to middle age (60.0 ms).

The genetic epidemiology of psychiatric disorders: a current perspective

Psychiatric genetic epidemiology has, as a field, undergone considerable change in the last two decades. My goal here is to provide a brief, selective and inevitably personal overview of where the field has come from, where it is now and where it might be going. Methodologic issues will be emphasized, particularly those of an "epidemiologic" nature. I will organize this review around three of the major methods used in psychiatric genetics: family studies, twin studies and linkage studies.

A twin study of self-reported criminal behaviour

Twin studies can be used to investigate the contributions of genetic factors, the common or shared environment, and the unique or non-shared environment to individual differences in a measurable characteristic. This paper reports the results of preliminary analyses of self-reported data on arrests and criminal behaviour from the Vietnam Era Veteran Twin Registry. The subjects for the study were 3226 male twin pairs in which both members served in the military during the Vietnam era. There were significant influences from both genetic factors and the common environment on early arrests. Genetic factors, but not the common environment, significantly influenced whether subjects were ever arrested after age 15, whether subjects were arrested more than once after age 15, and later criminal behaviour. The common environment, but not genetic factors, significantly influenced early criminal behaviour. The environment shared by the twins has an important influence on criminality while the twins are in that environment, but the shared environmental influence does not persist after the individual has left that environment. Genes are likely to influence the occurrence of criminal behaviour in a probabalistic manner by contributing to individual dispositions that make a given individual more or less likely to behave in a criminal manner.

The "equal environments assumption" in MZ-DZ twin comparisons: an untenable premise of psychiatric genetics?

The comparison of MZ-DZ twins in behavioral genetics has produced what seems like irrefutable evidence for the heritability of many psychiatric disorders. But such research depends on the validity of the EEA--the "equal environments assumption"--as an underlying premise. In this paper, several empirical studies which support the EEA are critically reviewed in terms of methodology and the way data has been processed in a mathematical model called "path analysis". It turns out that studies investigating the EEA appear to be largely inadequate in terms of technique, as well as biased in the inferences drawn. Further, the "heritability" estimate--often taken to mean the influence of trait-specific genes--is merely a statistical abstraction derived from a matrix of correlations; this estimate encompasses many buried environmental effects so that "heritability" does not correspond to any underlying DNA structure. In conclusion, many MZ-DZ pedigree studies have dubious scientific value, given the non-viable premise of the EEA, as well as the misleading operational definition of what has been called "heritability".