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

The traditional twin method is predicated on the equal-environment assumption (EEA)--that monozygotic (MZ) and dizygotic (DZ) twins are equally correlated in their exposure to environmental events of etiologic importance for the trait under study. In 1968, Scarr proposed a test of the EEA which examines the impact of phenotypic similarity in twins of perceived versus true zygosity. We apply this test for the EEA to five common psychiatric disorders (major depression, generalized anxiety disorder, phobia, bulimia, and alcoholism), as assessed by personal interview, in 1030 female-female twin pairs from the Virginia Twin Registry with known zygosity. We use a newly developed model-fitting approach which treats perceived zygosity as a form of specified familial environment. In 158 of the 1030 pairs (15.3%), one or both twins disagreed with the project-assigned zygosity. Model fitting provided no evidence for a significant influence of perceived zygosity on twin resemblance for any of the five disorders. Although limited in power, these results support the validity of the EEA in twin studies of psychiatric disorders.

Genetic contribution to risk of smoking initiation: comparisons across birth cohorts and across cultures

Self-report data on smoking initiation (whether the respondent admitted ever having smoked) were obtained from three large adult twin samples (Australia, N = 3,808 pairs; Virginia, N = 2,145 pairs; AARP, N = 3,620 pairs). Data were broken down into birth cohorts, and genetic models were fitted to test whether the decline, in more recent birth cohorts, in the percentage of individuals becoming smokers has led to a change in the relative contributions of genes and environment to risk of becoming a smoker. Despite a marked change in the proportion of male respondents who reported ever having smoked, we found no evidence for cohort differences in genetic and environmental effects (no Genotype x Cohort interaction). Significant differences in genetic and environmental parameters were found between sexes, and between the Australian and the two U.S. samples. In the U.S. samples, estimates of the genetic contribution to risk of becoming a smoker were 60% in men, 51% in women. In the Australian sample, heritability estimates were 33% in men, but 67% in women. Significant shared environmental effects on smoking initiation also were found, accounting for 23% of the variance in U.S. men, 28% of the variance in U.S. women, 39% of the variance in Australian men, and 15% of the variance in Australian women. In models that allowed for the environmental impact of cotwin smoking on a twin's risk of smoking initiation, estimates of the direct genetic contribution to risk of smoking initiation were comparable or higher (49-58% in U.S. women and 71% in Australian women; 58-61% in U.S. men, and 37% in Australian men).

Familial influences on the clinical characteristics of major depression: a twin study

We sought in this study to clarify the role that familial factors play in influencing the clinical presentation of major depression (MD). We examined the similarity of the historical and symptomatic features of MD in 176 pairs of female-female monozygotic (MZ) and dizygotic (DZ) twins from a population-based registry, where both members reported a history of MD defined by DSM-III-R criteria. The age at onset and treatment-seeking were significantly correlated in all twin pairs and the correlation in concordant DZ pairs was actually somewhat higher than in concordant MZ twins. The degree of impairment was modestly correlated in all twin pairs with substantially higher correlations in MZ vs DZ twins. No twin resemblance was observed for number of episodes or longest duration of an episode. Twin resemblance for the clinical features of MD was modest, but so was their consistency for the same individual over successive 1-year periods. However, in 5 of the 6 neurovegetative symptoms involving changes in appetite, weight and sleep, MZ twins were significantly correlated and correlations were significantly greater in concordant MZ vs DZ twins. Although the familial factors that cause twin resemblance for the age at onset and treatment seeking appear to be largely environmental, twin resemblance for the degree of impairment and neurovegetative symptoms are probably due largely to genetic factors. Our results suggest that familial factors influence the predisposition to some clinical features of MD.

A population-based twin study of alcoholism in women

OBJECTIVE

To clarify the role of genetic factors in the etiology of alcoholism in women.

DESIGN AND SETTING

Personal structured psychiatric interviews conducted by researchers "blinded" to the status of the co-twin in an epidemiologic sample of 1030 female-female twin pairs of known zygosity from the population-based Virginia Twin Registry.

MEASURES

Three definitions of lifetime prevalence of alcoholism based on Diagnostic and Statistical Manual of Mental Disorders, Third Edition, Revised criteria: (1) alcoholism with tolerance or dependence; (2) alcoholism with or without tolerance-dependence; and (3) alcoholism with or without tolerance-dependence or problem drinking.

RESULTS

Using narrow, intermediate, or broad definitions, the probandwise concordance for alcoholism was consistently higher in monozygotic than in dizygotic twin pairs. Multifactorial threshold models suggested that the heritability of liability to alcoholism in women is in the range of 50% to 60%.

CONCLUSIONS

The results support the hypothesis that genetic factors play a major role in the etiology of alcoholism in women. Women should be well represented in the efforts currently under way to elucidate the molecular basis of the genetic susceptibility to alcoholism.

Major depression and generalized anxiety disorder. Same genes, (partly) different environments?

Bivariate twin analysis can determine the extent to which two disorders share common genetic, familial environmental, or individual-specific environmental risk factors. We applied this method to lifetime diagnoses of major depression and generalized anxiety disorder as assessed at personal interview in a population-based sample of 1033 pairs of female same-sex twins. Three definitions of generalized anxiety disorder were used that varied in minimum duration (1 vs 6 months) and in the presence or absence of a diagnostic hierarchy. For all definitions of generalized anxiety disorder, the best-fitting twin model was the same. Familial environment played no role in the etiology of either condition. Genetic factors were important for both major depression and generalized anxiety disorder and were completely shared between the two disorders. A modest proportion of the nonfamilial environmental risk factors were shared between major depression and generalized anxiety disorder. Within the limits of our statistical power, our findings suggest that in women, the liability to major depression and generalized anxiety disorder is influenced by the same genetic factors, so that whether a vulnerable woman develops major depression or generalized anxiety disorder is a result of her environmental experiences.

Bulimia nervosa and major depression: a study of common genetic and environmental factors

A genetic analysis of the co-occurrence of bulimia and major depression (MD) was performed on 1033 female twin pairs obtained from a population based register. Personal interviews were conducted and clinical diagnoses made according to DSM-III-R criteria. Additive genes, but not family environment, are found to play an important aetiological role in both bulimia and MD. The genetic liabilities of the two disorders are correlated 0.456. While unique environmental factors account for around half of the variation in liability to both bulimia and MD, these risk factors appear to be unrelated, i.e., each disorder has its own set of unique environmental risk factors. Thus, the genetic liability of bulimia and MD is neither highly specific nor entirely non-specific. There is some genetic correlation between the two disorders as well as some genetic and environmental risk factors unique to each disorder. Limitations and directions for future research are discussed.

Evidence for genetic influences on personality from self-reports and informant ratings

Self-report data on Extraversion (E) and Neuroticism (N), together with ratings by the co-twin, were obtained from a sample of 826 adult female twin pairs ascertained through a population-based twin register. Data were analyzed using a model that allowed for the contributions to personality ratings of the rater's personality (rater bias) as well as of the personality of the person being rated. For E, but not for N, significant rater bias was found, with extraverted respondents tending to underestimate, and introverted respondents tending to overestimate, the Extraversion of their co-twins. Good agreement between self-reports and ratings by the respondent's co-twin was found for both E and N. Substantial genetic influences were found for both personality traits, confirming findings from genetic studies of personality that have relief only on self-reports of respondents.

Genetic and environmental contributions to the variance of body height in a sample of first and second degree relatives

Height was measured in a health screening of the population in Nord-Trøndelag, Norway. Correlations were computed for 24,281 pairs of spouses, 43,613 pairs of parents and offspring, 19,168 pairs of siblings, 1,318 pairs of grandparents and grandchildren, 1,218 cognate avuncular pairs, 849 noncognate avuncular pairs, 175 pairs of same-sexed twins, and smaller groups of other types of relatives. Fitting of structural equation models showed proportions of additive genetic variance of approximately 0.8 for both sexes and small sex-specific effects that probably reflect genetic dominance or environmental sibling effects. The correlations between parents and offspring were significantly lower in old than young cohorts, seeming to imply some kind of interaction effect between genes and environment.

The analysis of parental ratings of children's behavior using LISREL

A common procedure for assessing children's behavior is to obtain parental ratings of the child. Since the ratings obtained are a function of both parent and child, disentangling the child's phenotype from that of the rater becomes an important methodological problem. For the analysis of genetic and environmental contributions to children's behavior, solutions to this are available when multiple raters, e.g., two parents, rate multiple children, e.g., twins. This paper describes and illustrates simple LISREL models for the analysis of parental ratings of children's behavior. We show how the assumption that mothers and fathers are rating the same behavior in children can be contrasted with the weaker alternative that parents are rating correlated behaviors. Given the stronger assumption, which appears adequate for ratings of children's internalizing behavior problems, the contribution of rater bias and unreliability may be separated from the shared and nonshared environmental components of variation in a behavior genetic analysis.

Generalized anxiety disorder in women. A population-based twin study

Little is known about the role of familial and genetic factors in the etiology of generalized anxiety disorder (GAD), a new disorder first proposed in DSM-III. We examine this question in 1033 female-female twin pairs from a population-based registry. Both members in each twin pair were "blindly" assessed by structured psychiatric interview. Our results suggest the following: (1) GAD is a moderately familial disorder; (2) the tendency for GAD to run in families seems to be due largely or entirely to genetic factors shared between relatives rather than to the effects of the familial environment; (3) the heritability of GAD, estimated at around 30%, is modest, with the remainder of the variance in liability resulting from environmental factors not shared by adult twins; (4) the heritability of GAD cannot be explained solely by the occurrence of GAD only during episodes of major depression or panic disorder; and (5) the etiologic role of genetic factors is probably similar in GAD with a 1- vs a 6-month minimum duration of illness.

A population-based twin study of major depression in women. The impact of varying definitions of illness

Although depression aggregates in families, the degree to which this aggregation results from genetic vs environmental factors remains uncertain. We examined this question in 1033 female-female twin pairs from a population-based registry. Both members of each twin pair were "blindly" assessed by structured psychiatric interview. Nine commonly used definitions of major depression, which produced life-time prevalence rates ranging from 12% to 33%, were examined. For all definitions, the results of model fitting to twin correlations suggested that the liability to depression results from genetic factors and environmental experiences unique to the individual. For seven of the definitions, the estimated heritability of liability was similar, ranging from 33% to 45%. For the two definitions that included only primary cases of depression, the heritability was lower (21% to 24%). The results document that in women (1) genetic factors play a substantial, but not overwhelming, role in the cause of depression; (2) the tendency for depression to aggregate in families results largely from shared genetic and not from shared environmental factors; (3) except for definitions that exclude secondary cases, the magnitude of genetic influence is similar in broadly and narrowly defined forms of major depression; and (4) most environmental experiences of causative importance for depression are those not shared by members of an adult twin pair.

The genetic epidemiology of phobias in women. The interrelationship of agoraphobia, social phobia, situational phobia, and simple phobia

In 2163 personally interviewed female twins from a population-based registry, the pattern of age at onset and comorbidity of the simple phobias (animal and situational)--early onset and low rates of comorbidity--differed significantly from that of agoraphobia--later onset and high rates of comorbidity. Consistent with an inherited "phobia proneness" but not a "social learning" model of phobias, the familial aggregation of any phobia, agoraphobia, social phobia, and animal phobia appeared to result from genetic and not from familial-environmental factors, with estimates of heritability of liability ranging from 30% to 40%. The best-fitting multivariate genetic model indicated the existence of genetic and individual-specific environmental etiologic factors common to all four phobia subtypes and others specific for each of the individual subtypes. This model suggested that (1) environmental experiences that predisposed to all phobias were most important for agoraphobia and social phobia and relatively unimportant for the simple phobias, (2) environmental experiences that uniquely predisposed to only one phobia subtype had a major impact on simple phobias, had a modest impact on social phobia, and were unimportant for agoraphobia, and (3) genetic factors that predisposed to all phobias were most important for animal phobia and least important for agoraphobia. Simple phobias appear to arise from the joint effect of a modest genetic vulnerability and phobia-specific traumatic events in childhood, while agoraphobia and, to a somewhat lesser extent, social phobia result from the combined effect of a slightly stronger genetic influence and nonspecific environmental experiences.

The relationship between age at first drug use and teenage drug use liability

Our analyses of Carey's (1992) simulated data set of substance abuse in a cohort of adolescent twins were aimed at answering the question What is the relationship between age at first drug use and EVER having used drugs (i.e., teenage drug use liability)? Three analytic methods were used to determine whether age at first drug use was (1) a "perfect" index of drug use liability, (2) correlated in relatives but conditionally independent of drug use liability, or (3) causally influenced by drug use liability and by factors independent of liability. The analytic methods included nonmetric multidimensional scaling, multifactorial threshold model-fitting to contingency tables, and pedigree-based likelihood formulations for the raw data. All approaches indicated that age at first drug use was a perfect index of drug use liability. Further, model-fitting results indicated that only shared environmental factors accounted for twin similarity in the onset and timing of drug use. We discuss the limitations of each of the analytic methods and integrate our findings with the true model used in Carey's simulation.

Social support, depressed mood, and adjustment to stress: a genetic epidemiologic investigation

A survey of 821 same-sex female twin pairs from a population-based registry assessed 8 dimensions of social support and social integration. Twin analyses documented significant common environmental influences on 5 of these 8 measures and significant genetic influences on 5 of the 8. A decomposition of the multiplicative association between support and a measure of stressful life experiences in predicting depressed mood--an association typically interpreted as providing evidence for a stress-buffering effect of social support--shows clearly that it is the environmental and genetic factors that cause support, rather than support itself, that buffer the effects of stress on mood in most cases. We discuss the implications of this result for future research on the relationship between social support and psychopathology.

Genetic and environmental factors in the aetiology of menstrual, premenstrual and neurotic symptoms: a population-based twin study

Symptoms during the premenstrual and menstrual phases of the female reproductive cycle were assessed in 827 pairs of female same-sex twins from a population-based registry. By conventional factor analysis, premenstrual and menstrual symptoms were relatively independent of one another and of baseline 'neurotic' symptoms (i.e. anxiety, depression and somatization). Familial resemblance for menstrual and premenstrual symptoms was due solely to genetic factors with heritability estimates of 39.2% and 35.1%, respectively. Multivariate genetic analysis revealed distinct genetic and environmental factors for menstrual, premenstrual and neurotic symptoms. The genes and individual-specific experiences that predispose to premenstrual symptoms appear to be largely distinct from those which predispose either to menstrual or to neurotic symptoms. The generalizability of these results may be limited because only a modest number of premenstrual and menstrual symptoms were assessed, all by retrospective self-report.

Childhood parental loss and adult psychopathology in women. A twin study perspective

We examine the relationship between parental loss prior to age 17 years and adult psychopathology in 1018 pairs of female twins from a population-based registry. The relationship between loss and adult psychopathology varied as a function of the kind of loss (death vs separation), the parent involved, and the form of psychopathology. Increased risk for major depression and generalized anxiety disorder was associated with parental separation but not parental death and with separation from either mother or father. Panic disorder was associated with parental death and maternal, but not paternal, separation. Increased risk for phobia was associated with parental death and not parental separation. Risk for eating disorder was unrelated to the experience of parental loss. A model that includes parental loss as a form of "specified" family environment shows that, if it is truly an environmental risk factor for adult psychopathologic conditions, it can account for between 1.5% and 5.1% of the total variance in liability to these disorders and is responsible for between 7.0% and 20.5% of the tendency for these disorders to aggregate in siblings.

Evidence for genetic influences on personality from self-reports and informant ratings

Self-report data on Extraversion (E) and Neuroticism (N), together with ratings by the co-twin, were obtained from a sample of 826 adult female twin pairs ascertained through a population-based twin register. Data were analyzed using a model that allowed for the contributions to personality ratings of the rater's personality (rater bias) as well as of the personality of the person being rated. For E, but not for N, significant rater bias was found, with extraverted respondents tending to underestimate, and introverted respondents tending to overestimate, the Extraversion of their co-twins. Good agreement between self-reports and ratings by the respondent's co-twin was found for both E and N. Substantial genetic influences were found for both personality traits, confirming findings from genetic studies of personality that have relief only on self-reports of respondents.

Aphidicolin-inducible common fragile-site expression: results from a population survey of twins

Common chromosomal fragile sites appear to be ubiquitous in humans and other mammals, and, although the molecular basis and function of these sites remain an enigma, it has been speculated that they may be a cytogenetic expression of gene activity. A population survey of 28 twin pairs was conducted to assess the heritability of common fragile-site expression. Our data yielded a heritability estimate of .88 for total site expression, suggesting that these sites may result from some common process that is under relatively stringent genetic control. An analysis of the expression of individual autosomal sites revealed that expression on both homologues in the same cell occurred more frequently than expected.

Genetic and environmental effects on blood pressure in a Norwegian sample

Systolic (SBP) and diastolic (DBP) blood pressures were measured in a health screening of the adult population in Nord-Trøndelag, Norway. Correlations were computed for 23,936 pairs of spouses, 43,586 pairs of parent and offspring, 19,151 pairs of siblings, 1,251 pairs of grandparents-grandchildren, 1,146 pairs of biological uncles/aunts-nephews/nieces (avuncular), 801 non-biological avuncular pairs, 169 pairs of same-sex twins, and smaller groups of other types of relationships. Spouse correlations of 0.08 and 0.09 were approximately constant or slightly decreasing with marital duration. The correlation values for SBP and DBP were approximately 0.16 for parents-offspring, 0.19 to 0.23 for same-sex siblings with similar values for DZ twins, 0.19 and 0.16 for opposite-sex siblings, 0.52 and 0.43 for MZ twins, and close to zero for most of the second-order relationships. Genetic additive variance was estimated at 0.29 and genetic dominance variance at 0.18 with the best model for SBP. The corresponding estimates from the best models for DBP were 0.29 or lower and 0.22 or lower, the sum not exceeding 0.35. There was evidence of a moderate effect of environmental factors shared by same-sex siblings and twins (for DBP), but no cultural transmission, and whether or not adult relatives live together does not affect familial resemblance for BP. The data did not permit a very precise resolution of the relative magnitude of genetic dominance and sibling effects. The correlation structure did not show sex-specific genetic effects.

Social support, depressed mood, and adjustment to stress: A genetic epidemiologic investigation

A survey of 821 same-sex female twin pairs from a population-based registry assessed 8 dimensions of social support and social integration. Twin analyses documented significant common environmental influences on 5 of these 8 measures and significant genetic influences on 5 of the 8. A decomposition of the multiplicative association between support and a measure of stressful life experiences in predicting depressed mood--an association typically interpreted as providing evidence for a stress-buffering effect of social support--shows clearly that it is the environmental and genetic factors that cause support, rather than support itself, that buffer the effects of stress on mood in most cases. We discuss the implications of this result for future research on the relationship between social support and psychopathology.

A model for comparative ratings in studies of within-family differences

Comparative ratings between pairs of siblings or other relatives are commonly used to refine measures of intrafamily variation. A simple model, based on signal detection theory, is proposed which shows how comparative ratings can be used to estimate within-pair variances of true scores, which can, in turn, be modeled with any of the conventional approaches to partitioning genetic and environmental variance within families.

The family history method: whose psychiatric history is measured?

OBJECTIVE

The family history method, in which an informant is asked about the history of psychiatric illness in relatives, is widely used in psychiatric research. Previous research has examined the influence on family history information of characteristics of the relative. In this report, the authors seek to clarify the impact on family history reporting of the psychiatric history of the informant.

METHOD

Both members of female twin pairs from a population-based twin registry were asked about the history of major depression, generalized anxiety disorder, and alcoholism in their mother and father. The authors examined twin pairs discordant for each of the three diagnoses and predicted that the affected twin would report higher rates of the same disorder in her parent than would the unaffected twin.

RESULTS

Twins with a history of major depression or generalized anxiety disorder but not twins with alcoholism were significantly more likely to report the same disorder in their parents than were their unaffected co-twins.

CONCLUSIONS

For major depression and generalized anxiety disorder, a family history diagnosis appears to reflect the psychiatric history of both the relative and the informant. Caution may be needed in the interpretation of results based on the family history method, although the magnitude of this problem may be attenuated by the use of multiple informants.

Coping: a genetic epidemiological investigation

This study examines data on self-report coping behaviour, life events and symptoms of anxiety and depression in a general population sample of 827 female twin pairs. Factor analysis identified three almost uncorrelated coping factors: turning to others; problem solving; and denial. Turning to others and problem solving were negatively and denial was positively related to levels of anxiety and depression. Turning to others and problem solving buffered the depressogenic and anxiogenic effects of stressful life events, while denial exacerbated the anxiogenic effects of life events. Structural equation model-fitting indicated that twin resemblance in turning to others and problem solving could be explained entirely by genetic factors with an estimated heritability of 30 and 31%, respectively. For denial, twin resemblance could be best explained by familial-environmental factors accounting for 19% of the total variation. Genes may affect the vulnerability to psychiatric disorders in part by influencing coping behaviour.

Personality and reproductive fitness

The relationship between reproductive success (number of biological children) and personality was explored in 1101 postmenopausal females from the Australian twin registry. The quadratic response surface relating fitness to extraversion (E) and neuroticism (N) showed a saddle point at intermediate levels of E and N. Selection was shown to be stabilizing, i.e., having an intermediate optimum, along the axis low E, low N-high E, high N and more mildly disruptive, having greater fitness in the extremes, along the axis low N, high E-high N, low E. Neither dimension of personality considered by itself showed a significant linear or quadratic relationship to reproductive success. Sections through the fitness surface, however, show selection tends to favor high neuroticism levels in introverts and low neuroticism levels in extroverts.

The analysis of assortative mating: a LISREL model

The use of linear models to discriminate between primary and secondary assortative mating has allowed a significant advance in our understanding of the mate selection process. We describe how these methods may be implemented in the LISREL and COSAN packages and illustrate the method with data on cognitive ability, education, and personality reported by Phillips et al. (Behav. Genet. 18:347-356, 1988). Issues discussed include the interface between path diagrams and computer program specification, the near-independence of parameters for within-person correlations from parameters for marital correlations, and the fact that almost all of marital resemblance for IQ seems to be due to assortative mating for educational level.

Estimating familial effects on age at onset and liability to schizophrenia. II. Adjustment for censored data

Genetic studies of disorders with adult onset often contain individuals who have not completed their age at risk when last observed. Without correction for such censoring, correlation in ages at onset among relatives is substantially underestimated. Moreover, without correction for the effect of correlated ages at onset, the relationship between age at onset in the proband and liability in relatives is substantially overestimated. The present paper describes methods for correcting the effects of censoring on these estimates. In a companion paper [Kendler and MacLean, Genet Epidemiol 7:409-417, 1990] these methods are applied to a large family study of schizophrenia.

Genetic and environmental effects on self-reported depressive symptoms in a general population twin sample

To determine the etiology of self-reported depressive symptoms and their co-occurrence in the general population, multivariate genetic models were fitted to the responses of 771 female twin pairs (463 MZ, 308 DZ) to a 20-item epidemiological depression inventory (CES-D scale). A model which contained one common genetic factor, one shared environmental factor, and four unique environmental factors provided a useful account of symptom covariation. Under this model, the four non-shared environmental factors explained the largest proportion of variance in response to the CES-D scale, whereas a single common genetic factor explained substantially less of the variation in symptomatology. Consistent with previous findings (Kendler, Heath, Martin, & Eaves, Archives of General Psychiatry 43, 213-221, 1986) shared environmental influences were found to play a relatively minor role in the report of depressive symptoms. These results suggest that while genetic factors do contribute to the covariation among symptoms of depression, it is the largely non-shared environmental factors that account for the co-occurrence of symptoms in the general population.

Analyzing the relationship between age at onset and risk to relatives

Correlations in age at onset between relatives affect risk to relatives of a given age. Either an increase or a decrease in risk may be observed for a relative of a proband, according to whether there is a causal relationship between liability to disease and age at onset. Likelihood formulas are given for pairs of relatives under a number of different sampling schemes, and it is shown how data collected from relatives enable maximum-likelihood estimation of parameters of a linear model relating disease liability and age at onset. A genotype-environment extension of this model was fitted to data on age at onset for schizophrenia that were obtained from the National Academy of Sciences-National Research Council Twin Registry. Age at onset is correlated between twins, but this correlation appears to be associated with factors that are separate from those which affect liability to disease. However, even this relatively large sample of twins is too small to draw firm conclusions about any causal relationship between disease liability and onset.

Bias in correlations from selected samples of relatives: the effects of soft selection

Martin and Wilson (1982) describe two forms of sampling bias in twin studies. One is "hard selection," where individuals above a threshold participate, and those below do not. The second is "soft selection," where the probability of including a pair of relatives varies over the range of the character. We present an alternative model of soft selection which has strikingly different consequences for the resemblance between relatives. In general, the softer the threshold, the more the correlation resembles that in the underlying population. Results are presented where the probability of selection equals the cumulative distribution function of a normal distribution with 10% of the variance of the selected variable. In these circumstances, soft selection usually leads to less severely attenuated correlations than truncate selection.

Rater bias in the EASI temperament scales: a twin study

Under trait theory, ratings may be modeled as a function of the temperament of the child and the bias of the rater. Two linear structural equation models are described, one for mutual self- and partner ratings, and one for multiple ratings of related individuals. Application of the first model to EASI temperament data collected from spouses rating each other shows moderate agreement between raters and little rating bias. Spouse pairs agree moderately when rating their twin children, but there is significantly rater bias, with greater bias for monozygotic than for dizygotic twins. MLE's of heritability are approximately .5 for all temperament scales with no common environmental variance. Results are discussed with reference to trait validity, the person-situation debate, halo effects, and stereotyping. Questionnaire development using ratings on family members permits increased rater agreement and reduced rater bias.

The effects of age, sex, and genotype on self-report drunkenness following a challenge dose of alcohol

Age is a potential source of variation that contributes to differences between, but not within, twin pairs. In most genetic analyses of twin data, linear and other functions of age are usually removed prior to model fitting. This correction is typically applied only within twin groups of the same sex and zygosity, and no heterogeneity test of age regressions is performed. Here we include age as a variable in the model-fitting procedure and allow for tests of heterogeneity of age regressions across sex and zygosity groups. The LISREL formulation of the approach is illustrated with data collected from Australian twins on subjective impressions of drunkenness following alcohol consumption. The results indicate significant negative covariation of impressions of drunkenness with age. The data support a simple model of additive genetic and unique environmental variation. No evidence was found for sex differences in genetic or environmental components of variation.

Partitioned twin analysis: a power study

Individual differences in the human genome may now be measured with molecular genetic techniques. Therefore, dizygotic (DZ) twins may be classified as sharing two, one, or zero "genes" identical by descent for any measured polymorphism. As a result, we may partition genetic variation into two sources: (i) genotypes at and closely linked to particular marker loci identified with restriction fragment length polymorphisms (RFLPs) and (ii) other genetic variation. The power of the classical twin study to reject false models lacking either a marker effect or a residual genetic effect is explored. Additivity of genetic effects at or near the locus and of the residual genetic variation as well as random environmental variation are assumed. Results indicate that statistical rejection of models could be achieved with sample sizes which are within the range of several current twin registers. A design including monozygotic (MZ) twins is compared with one consisting of only DZ twins. MZ twins add considerable power for the detection of residual genetic variation but provide no information to resolve genetic marker effects.

Genetic and environmental variation in the birth weight of twins

Two novel approaches to the analysis of twin data are illustrated with data from birth weight in twins. First, two possible covariates of birth weight are fitted to the data simultaneously, allowing for linear effects of these variables, and their correlation. Second, information on chorionicity is used to estimate the effects of chorion type on birth weight. The data were collected from a large sample of twins born in East Flanders, Belgium. Variation and covariation in twins were considered as a function of sex, chorionicity, maternal age, gestational age, and genotype. No evidence for sex differences in causes of variation was found. As expected, the largest source of variation in bith weight was associated with gestational age. Other common environmental influences were non-significant. Heritability was significant, constituting approximately 40% of variation not associated with maternal and gestational age. A small but significant effect of chorionicity was found, such that dichorionic twins show a greater similarity than monochorionic.

Fitting genetic models with LISREL: hypothesis testing

A brief introduction to the mathematical theory involved in model fitting is provided. The properties of maximum-likelihood estimates are described, and their advantages in fitting structural models are given. Identification of models is considered. Standard errors of parameter estimates are compared with the use of likelihood-ratio (L-R) statistics. For structural modeling, L-R tests are invariant to parameter transformation and give robust tests of significance. Some guidelines for fitting models to data collected from twins are given, with discussion of the relative merits of parsimony and data description.

Testing structural equation models for twin data using LISREL

Simple genetic models can be fitted to twin data using software packages such as LISREL (Jöreskog and Sörbom, 1986a). After discussion of data preparation and routine checks on possible violation of assumptions of the twin method, we illustrate univariate, bivariate, and multivariate genetic models which can be tested in cross-sectional twin data using LISREL. These include models for cohort or cohabitation effects, genotype x sex interaction, and certain types of genotype x environment interaction and genotype-environment correlation.

Rater bias in the EASI temperament scales: A twin study

Under trait theory, ratings may be modeled as a function of the temperament of the child and the bias of the rater. Two linear structural equation models are described, one for mutual self- and partner ratings, and one for multiple ratings of related individuals. Application of the first model to EASI temperament data collected from spouses rating each other shows moderate agreement between raters and little rating bias. Spouse pairs agree moderately when rating their twin children, but there is significantly rater bias, with greater bias for monozygotic than for dizygotic twins. MLE's of heritability are approximately .5 for all temperament scales with no common environmental variance. Results are discussed with reference to trait validity, the person-situation debate, halo effects, and stereotyping. Questionnaire development using ratings on family members permits increased rater agreement and reduced rater bias.

Chronic antidepressant drug regimes and food and water intake in rats

Food and water consumption were measured in rats prior to and during a course of antidepressant drug administration. Desmethylimipramine (DMI, 10 mg/kg/day), clorgyline (1.0 mg/kg/day) or saline were injected IP for 30 days. Food and water intake in the DMI- and clorgyline-treated rats was initially and significantly decreased but progressively returned towards pretreatment levels over the course of the drug administration. The effects of these antidepressant drug treatments on food and water intake appeared to consist of two components: (a) a rapid suppressive effect, possibly associated with an acute central action of these drugs (and perhaps a slight initial stress effect related to the drug administration) and (b) an adaptive effect over the course of the treatment which may involve changes in monoaminergic neurotransmitters or receptor status in those brain regions associated with feeding behavior. The similarities of the results of these treatments and those seen with chronic stress are discussed.

Genetic and environmental variation in menstrual cycle: histories of two British twin samples

Information about menstrual cycle variables was obtained by questionnaire using 462 female twin pairs. The twins were either members of the Institute of Psychiatry Volunteer Twin Register, or of the Birmingham Population-based Register. The two samples were analysed separately using univariate and multivariate methods so that an independent replication was obtained. Maximum likelihood estimation was used to fit simple models of genetic and environmental variation to these data. The results suggest that age of menarche, menstrual cycle regularity and premenstrual symptom reporting may be heritable, whereas menstrual cycle length is not. The result should be interpreted with caution as not all variables were replicated in the smaller sample, and the method of retrospective menstrual cycle data collection has been questioned.

Altruism and aggression: the heritability of individual differences

Five questionnaires measuring altruistic and aggressive tendencies were completed by 573 adult twin pairs of both sexes from the University of London Institute of Psychiatry Volunteer Twin Register. The questionnaires measured altruism, empathy, nurturance, aggressiveness, and assertiveness. The intraclass correlations for the five scales, respectively, were .53, .54, .49, .40, and .52 for 296 monozygotic pairs, and .25, .20, .14, .04, and .20 for 179 same-sex dizygotic pairs, resulting in broad heritability estimates of 56%, 68%, 70%, 72%, and 64%. Additional analyses, using maximum-likelihood model-fitting, revealed approximately 50% of the variance on each scale to be associated with genetic effects, virtually 0% with the twins' common environment, and the remaining 50% with each twins' specific environment and/or error associated with the test. Correcting for the unreliability in the tests raised the maximum-likelihood heritabilities to approximately 60%. Age and sex differences were also found: altruism increased over the age span from 19 to 60, whereas aggressiveness decreased, and, at each age, women had higher scores than men on altruism and lower scores on aggressiveness.

Altruism and aggression: The heritability of individual differences

Five questionnaires measuring altruistic and aggressive tendencies were completed by 573 adult twin pairs of both sexes from the University of London Institute of Psychiatry Volunteer Twin Register. The questionnaires measured altruism, empathy, nurturance, aggressiveness, and assertiveness. The intraclass correlations for the five scales, respectively, were .53, .54, .49, .40, and .52 for 296 monozygotic pairs, and .25, .20, .14, .04, and .20 for 179 same-sex dizygotic pairs, resulting in broad heritability estimates of 56%, 68%, 70%, 72%, and 64%. Additional analyses, using maximum-likelihood model-fitting, revealed approximately 50% of the variance on each scale to be associated with genetic effects, virtually 0% with the twins' common environment, and the remaining 50% with each twins' specific environment and/or error associated with the test. Correcting for the unreliability in the tests raised the maximum-likelihood heritabilities to approximately 60%. Age and sex differences were also found: altruism increased over the age span from 19 to 60, whereas aggressiveness decreased, and, at each age, women had higher scores than men on altruism and lower scores on aggressiveness.