Toward a comprehensive developmental model for major depression in men

OBJECTIVE

The multiple risk factors for major depression are interrelated through poorly understood developmental pathways. In 2002, the authors presented a developmental model for major depression in women. Based on similar methods, they here present an analogous model for men.

METHOD

Using data from 2,935 adult male twins, interviewed twice over a 2-4-year period, the authors constructed, by means of structural equation modeling, an integrated etiologic model for major depression that predicts depressive episodes over 1 year from 18 risk factors conceptualized as five developmental "tiers" reflecting childhood, early adolescence, late adolescence, adulthood, and the last year.

RESULTS

The best-fitting model, including six correlations and 76 paths, provided a good fit to the data, explaining 49% of the variance in the liability to depressive episodes. The overall results, similar to those seen in women, suggest that the development of major depression results from the action and interaction of three broad pathways of internalizing symptoms, externalizing symptoms, and adversity. Childhood parental loss and low self-esteem were more potent variables in the model in men than in women. Genetic risks for major depression had a broader spectrum of action in men than in women. The pathway to major depression through externalizing symptoms was not more prominent in men than in women.

CONCLUSIONS

Major depression in men, as in women, is an etiologically complex disorder influenced by risk factors from multiple domains that act in developmental time. The similarities in etiologic pathways to major depression for men and women outweigh the modest differences.

The differential heritability of regular tobacco use based on method of administration

Several large studies have demonstrated that the liability to smoke cigarettes is strongly genetically influenced. However, the role of genetic and environmental risk factors in the use of other common forms of tobacco use has yet to be studied. Data on the regular use of cigarettes, cigars, pipes, dip (moist snuff), and chewing tobacco from 2634 male twins were analyzed with ACE structural equation models. Twin similarity for regular cigarette and dip use was largely genetic in origin. However, twin resemblance for chewing tobacco was just about equally the result of genes and shared environment, and twin similarity for use of pipes and cigars was entirely the result of shared environmental factors. Thus, the genetic influences on the liability for regular tobacco use appear to vary based on tobacco type. The causes for the use of different forms of tobacco are complex and worthy of further study.

Genetic and environmental influences on illicit drug use and tobacco use across birth cohorts

BACKGROUND

The prevalence of use of many psychoactive substances has changed considerably in recent years. While genetic factors impact on overall risk for substance use, we know little about whether the etiological importance of these factors differs across birth cohorts. One theory, which postulates that heritability of deviant traits increases in permissive environments, predicts a positive relationship across cohorts between prevalence and heritability of substance use.

METHOD

The lifetime history of use of tobacco, cannabis, cocaine, sedatives and stimulants were assessed in 4826 twins from male-male and female-female pairs born in Virginia from 1934 to 1974. Using empirical methods based on prevalence by birth year, these twins were divided into three cohorts for each substance (e.g. for cannabis 1934-1953, 1954-1968 and 1969-1974). Structural equation modeling was performed using the Mx software package.

RESULTS

Prevalence rates for psychoactive substance use differed substantially across cohorts, most markedly for cocaine, sedatives and stimulants, which were highest in the 1958-1963 cohort. However, for all substances, the best-fit model constrained estimates of the etiological role of genetic and environmental risk factors to be equal across both sex and cohort.

CONCLUSIONS

We found no evidence in this sample for any systematic relationship between heritability and prevalence of psychoactive substance use--which should be a rough index of drug availability and/or acceptability. This sample had reasonable power to detect large changes in heritability across cohorts and at least moderate power to detect relatively small changes.

The Irish Affected Sib Pair Study of Alcohol Dependence: study methodology and validation of diagnosis by interview and family history

BACKGROUND

This article is the first report of the Irish Affected Sib Pair Study of Alcohol Dependence, whose goal is to detect the genomic location of susceptibility loci for alcohol dependence (AD). This article describes phenotypic characteristics of the probands, siblings, and parents included in the sample and examines agreement among different sources of diagnostic information, including the validity of family history (FH) assessment.

METHODS

Structured diagnostic interviews were conducted with 1414 individuals from 591 families ascertained in Ireland. AD was assessed among 1201 probands and affected siblings with use of the Semi-Structured Assessment for the Genetics of Alcoholism and among 213 parents with use of a modified version of the Structured Clinical Interview for DSM. Probands and siblings were also assessed for drinking history, comorbid disorders, and other clinical characteristics. FH reports based on FH-Research Diagnostic Criteria were obtained for 1113 of these individuals as well as for 3652 first-degree relatives who were not interviewed.

RESULTS

Sample characteristics confirm the severity of AD among the affected individuals. Agreement between FH ratings and diagnoses based on direct interviews was high for both parent-offspring and sibling-sibling comparisons (e.g., positive and negative predictive values > 80% for a range of cutoffs). Agreement among individuals about their family members was also high for a single item (1 month or more of drinking problems, tetrachoric r = 0.86-0.98), the total number of DSM-IV AD symptoms (polychoric r = 0.86-0.96), and classifications based on a range of cutoffs (kappa = 0.75-0.80). Use of multiple informants improved classification accuracy only slightly (6-10%).

CONCLUSIONS

The authors successfully collected data for a large sample of affected sibling pairs for molecular genetic analysis of AD. Individuals with AD were able to provide accurate evaluations of alcoholism symptoms in their parents and adult siblings. A single screening item performed nearly as well as the full scale. Collecting information from multiple informants may not be cost effective for the gain in predictive accuracy. FH information collected from affected informants can be a valuable source of diagnostic information for family studies of alcoholism.

Illicit drug use and abuse/dependence: modeling of two-stage variables using the CCC approach

Drug use and abuse/dependence are stages of a complex drug habit. Most genetically informative models that are fit to twin data examine drug use and abuse/dependence independent of each other. This poses an interesting question: for a multistage process, how can we partition the factors influencing each stage specifically from the factors that are common to both stages? We used a causal-common-contingent (CCC) model to partition the common and specific influences on drug use and abuse/dependence. Data on use and abuse/dependence of cannabis, cocaine, sedatives, stimulants and any illicit drug was obtained from male and female twin pairs. CCC models were tested individually for each sex and in a sex-equal model. Our results suggest that there is evidence for additive genetic, shared environmental and unique environmental influences that are common to illicit drug use and abuse/dependence. Furthermore, we found substantial evidence for factors that were specific to abuse/dependence. Finally, sexes could be equated for all illicit drugs. The findings of this study emphasize the need for models that can partition the sources of individual differences into common and stage-specific influences.

The differential impact of risk factors on illicit drug involvement in females

BACKGROUND

Initiation of drug use and progression to abuse/dependence involve complex pathways. Potential risk factors may correlate with initiation or progression or both. Are there risk factors that associate with illicit drug use or illicit drug abuse/dependence? Is the magnitude of the association the same for use and abuse/dependence? Does this pattern of association differ across categories of drugs?

METHODS

We used data from female-female adult twins to assess the association of 26 putative risk factors with use and abuse/dependence of six illicit psychoactive drugs. Drug involvement was represented by independent dichotomous outcomes and by a single ordinal variable. Odds ratios were obtained by logistic regression and a continuation ratio was used to test the magnitude of association.

RESULTS

Factors associate in similar patterns with different drug categories. Some associated factors interact only with initiation while others relate with both stages. There is a stronger association of significant socio-demographic factors with drug use while the psychiatric diagnoses are more strongly associated with progression to abuse/dependence.

CONCLUSIONS

Risk factors may be use-specific, abuse/dependence-specific or common to use and abuse/dependence. The trend of associations is similar across different illicit drugs. This suggests complex, interacting pathways that determine drug habits in individuals. These results are hypothesis-generating and future studies of causal relationships may draw from the outcomes presented in these analyses.

The types of regular cigarette smokers: A latent class analysis

The present study sought to refine cigarette smoking exposure by examining whether individuals with a history of regular cigarette smoking could be classified into meaningful subgroups based on their smoking habits, nicotine dependence, and quit attempts. Data were obtained from 3,025 male and female twins aged 22-59 years who participated in two longitudinal studies based on the Virginia Twin Registry. Latent class analysis was used to identify the number and nature of subgroups of smokers reporting similar smoking habits based on 18 characteristics. Estimates of prevalence for different classes of smoking were obtained along with expected frequencies of endorsements for each smoking behavior by class. Distributions of demographic characteristics, psychopathology, and personality were examined as external validators. The results demonstrated considerable heterogeneity and revealed eight classes of smokers who differed in their degree of nicotine dependence, cessation attempts, and ability to quit smoking. Smoking classes ranged from individuals who were all former smokers with low nicotine dependence who attempted to quit once and were successful, to individuals who were all current smokers with little desire to quit, to individuals with high nicotine dependence with multiple quit attempts, only a subset of which were successful. Estimates of prevalence for each class were 9%-20%. Our findings support the idea that regular cigarette smokers are highly heterogeneous and highlight the need for smoking exposure refinement in future studies. Acknowledging the complexity of cigarette smoking and classifying smokers into more specific subgroups based on their smoking behavior in future studies will enable more accurate evaluation of disease etiology and risk and could lead to more appropriate smoking cessation interventions.

The interaction of stressful life events and a serotonin transporter polymorphism in the prediction of episodes of major depression: a replication

CONTEXT

Prior evidence from twin studies suggested genetic moderation of the depressogenic effects of stressful life events (SLEs). Can the specific genes involved in this effect be identified?

OBJECTIVE

To replicate and extend a recent study that a functional variant in the serotonin transporter (5-HTT) might in part explain these findings.

DESIGN

Characterizing risk for major depression and generalized anxiety syndrome in the last year as a function of 5-HTT genotype, sex, and the occurrence of SLEs and ratings of the SLE-associated level of threat.

SETTING

A population-based sample of adult twins.

PARTICIPANTS

Five hundred forty-nine male and female twins with a mean age at participation of 34.9 years (SD 9.1).

MAIN OUTCOME MEASURE

Episodes of major depression and generalized anxiety syndrome in the last year with onset measured to the nearest month.

RESULTS

Individuals with 2 short (S) alleles at the 5-HTT locus were more sensitive to the depressogenic effects of all SLEs than were those with 1 or 2 long (L) alleles. When level of SLE-associated threat was examined, the interaction between genotype and SLE resulted from an increased sensitivity of SS individuals to the depressogenic effects of common low-threat events. These events had little impact on risk for those possessing the SL and LL genotypes. The 5-HTT genotype did not modify the effects of SLEs on risk for generalized anxiety syndrome.

CONCLUSION

Variation at the 5-HTT moderates the sensitivity of individuals to the depressogenic effects of SLEs largely by producing, in SS individuals, an increased sensitivity to the impact of mild stressors. Replication of these intriguing results is needed.

The Washington University Twin Study of alcoholism

Genetic contributions to the liability to develop alcoholism in males of Northern and Western European ancestry are well-established. However, questions remain concerning the role of genetic variation in the etiology of alcoholism among non-white populations, among women, and the possibility of etiological heterogeneity in subtypes of alcoholism. The answers to these questions are needed to help define phenotypes for molecular genetic studies searching for QTLs for alcoholism. Twins from 295 pairs were consecutively ascertained at inpatient and outpatient psychiatric and alcohol treatment facilities in St. Louis, MO in 1981-1986. Probands and willing cotwins were evaluated by structured psychiatric interviews, psychometric assessment, and lifetime treatment records. One hundred fifty-four probands met criteria for alcohol abuse/dependence (AAD), including twins from 45 MZ, 50 same-sex DZ, and 59 opposite-sex pairs. Twin-pair resemblance was evaluated for AAD and alcohol dependence (AD), as well as for subsets defined by gender, patterns of comorbidity, ethnic background, and clinical features. Among males, heritability of AAD and AD was substantial, with little evidence for common environmental contributions to family resemblance. Pair resemblance among females was also substantial, but similar for MZ and DZ pairs, yielding near-zero heritability estimates. However, based on these sample sizes, the sex differences were not statistically significant. The results confirm prior studies of strong genetic influences on alcoholism in males, but suggest lower genetic influence in females. Power to test other sources of heterogeneity was limited, but the results suggest no evidence for higher heritability for male early onset alcoholism or for alcoholism with comorbid antisocial personality.

Personality and comorbidity of common psychiatric disorders

BACKGROUND

We know little about the degree to which comorbidity, socommonly seen among psychiatric disorders, arises from variation in normal personality.

AIMS

To study the degree to which variation in normal personality accounts for the comorbidity of eight common psychiatric and substance use disorders.

METHOD

Internalising disorders (major depression, generalised anxiety and panic disorders, phobias), externalising disorders (alcohol and drug dependence, antisocial personality and conduct disorders) and personality dimensions of neuroticism, extraversion and novelty seeking were assessed in 7588 participants from a population-based twin registry. The proportion of comorbidity explained by each personality dimension was calculated using structural equation modelling.

RESULTS

Neuroticism accounted for the highest proportion of comorbidity within internalising disorders (20-45%) and between internalising and externalising disorders (19-88%). Variation in neuroticism and novelty seeking each accounted for a modest proportion (10-12% and 7-14%, respectively) of the comorbidity within externalising disorders. Extraversion contributed negligibly.

CONCLUSIONS

High neuroticism appears to be a broad vulnerability factor for comorbid psychiatric disorders. Novelty seeking is modestly important for comorbid externalising disorders.

Sex differences in the relationship between social support and risk for major depression: a longitudinal study of opposite-sex twin pairs

OBJECTIVE

Compared to men, women have larger and more intimate social networks and higher rates of major depression. Prior studies have suggested that women are more sensitive to the depressogenic effects of low social support, but most of these studies had substantial methodologic limitations.

METHOD

In two interview waves at least 1 year apart, 1,057 pairs of opposite-sex dizygotic twin pairs ascertained from a population-based register were assessed. The authors predicted risk of major depression in the year before the wave 2 interview from levels of social support assessed at wave 1.

RESULTS

Women reported higher levels of global social support than their twin brothers. Global social support at wave 1 predicted risk for major depression at wave 2 significantly more strongly in female than in male members of these pairs, and the same effect was seen when the analysis controlled for the history of major depression in the year prior to wave 1. Women were more sensitive than men to the depressogenic effects of low levels of social support, particularly from the co-twin, other relatives, parents, and spouses. Levels of social support did not explain the sex difference in risk for major depression.

CONCLUSIONS

Emotionally supportive social relationships are substantially more protective against major depression for women than for men. While these effects cannot explain sex effects on the prevalence of major depression, they do suggest important sex differences in pathways of risk. Clarification of the nature of the causal links between low social support and depression in women is needed.

Childhood sexual abuse, stressful life events and risk for major depression in women

BACKGROUND

In animals, early trauma can produce long-lasting changes in sensitivity to the pathogenic effects of stress. To explore whether similar processes occur in humans, we examine whether childhood sexual abuse (CSA) in women alters sensitivity in adulthood to the depressogenic effects of stressful life events (SLEs).

METHOD

A history of CSA was obtained from a population-based sample of 1404 female adult twins. Cox Proportional hazard models were used to predict onsets of episodes of DSM-III-R major depression (MD) in the past year from previously assessed levels of neuroticism (N), CSA and past-year SLEs scored on long-term contextual threat.

RESULTS

In the best-fit model, onset of MD was predicted by CSA, SLEs and N. Individuals with CSA (and especially with severe CSA) had both an overall increased risk for MD and a substantially increased sensitivity to the depressogenic effects of SLEs. A 'dose-response' relationship between severity of CSA and sensitivity to SLEs was clearer in those with low to average levels of N than in those with high levels of N.

CONCLUSION

As documented with physiological responses to a standardized laboratory stressor, CSA increases stress sensitivity in women in a more naturalistic setting. Both genetic and early environmental risk factors can produce long-term increase in the sensitivity of individuals to depressogenic life experiences.

A twin study of personality and illicit drug use and abuse/dependence

Although personality measures such as neuroticism (N), extraversion (E) and novelty-seeking (NS) are associated with the use and abuse/dependence of illicit drugs, little is known about the degree to which these associations are due to genetic or environmental factors. The goal of this analysis was to estimate the extent of genetic and environmental overlap between three dimensions of personality (N, E and NS) and illicit psychoactive substance use and abuse/dependence. Using data from adult male and female twins from the Mid-Atlantic Twin Registry, we used the structural equation modeling package Mx to perform bivariate Cholesky decompositions for personality measures of N, E and NS, individually with cannabis, cocaine, sedatives, stimulants and hallucinogens. This was done separately for use and for a polychotomous diagnosis of abuse and/or dependence. Sex differences were tested. The phenotypic relationship between personality and use and abuse/dependence of illicit drugs were moderate and most of the covariance was explained by genetic factors. Sexes could be equated for N and E but not for NS. For NS, use and abuse/dependence of illicit drugs showed greater phenotypic and genetic overlap in males than females. Of the personality measures, NS and illicit drug use and abuse/dependence were most closely related. NS was most closely related to cannabis use while N showed significant genetic overlap with sedative use. NS in males appears to be a good indicator of risk for cannabis use. This result may be useful for candidate gene studies.

Sources of individual differences in stressful life event exposure in male and female twins

The roles of genetic and environmental influences on stressful life events were examined in 3938 twin pairs (MZ, same-sex DZ, and opposite-sex DZ) using a sex-limitation model. Life events were assessed by personal interview, and were categorized as being either personal (i.e., events that occur directly to the individual) or network (i.e., events that occur to someone within the individual's social network, thus affecting the individual indirectly). Consistent with previous reports, genetic factors were found to exert more influence on personal events than network events. Genetic correlations between males and females suggest that many of the same genetic factors are acting within both genders.

Level of family dysfunction and genetic influences on smoking in women

BACKGROUND

An adoption study of alcoholism suggests that in women, the impact of genetic risk factors become greater in the presence of conflict in the family of origin. Is the same true for cigarette smoking (CS)?

METHOD

We obtained, in a sample of 1676 twins from female female twin pairs from a population-based register, a measure of maximum lifetime CS (divided into six ordinal categories) and family dysfunction (FD) assessed as the mean report of up to four informants (twin, co-twin, mother, father). Statistical analysis was conducted by traditional regression analysis and a moderator structural equation twin model using the computer program Mx.

RESULTS

With increasing levels of FD, maximum CS increased substantially while correlations for CS in monozygotic (MZ) and dizygotic (DZ) twins decreased modestly. Regression analyses demonstrated reduced twin-pair resemblance for CS with increasing levels of FD. The best-fit structural equation model found high levels of heritability for CS and no evidence for a role of shared environment. With increasing levels of FD, the proportion of variance in CS due to genetic factors (i.e. heritability) decreased while that due to unique environmental effects increased.

CONCLUSIONS

Several different statistical methods suggested that, contrary to prediction, heritability of CS decreased rather than increased with higher levels of dysfunction in the family of origin. The hypothesis that genetic effects for psychiatric and drug-use disorders become stronger in more adverse environments is not universally true.

A twin study of genetic and environmental influences on tobacco initiation, regular tobacco use and nicotine dependence

BACKGROUND

Numerous twin studies have reported significant genetic contributions to the variability of tobacco initiation (TI), while fewer studies have shown similar results for the persistence of smoking behavior, or nicotine dependence (ND). As the development of ND requires regular tobacco use (RTU) which in turn requires TI, a conditional approach is necessary.

METHOD

We used structural equation modeling of multi-step conditional processes to examine the relationship between genetic and environmental risk factors for TI, RTU and ND. The tobacco variables were assessed by personal interview in female, male and opposite-sex twin pairs from the population-based Virginia Twin Registry.

RESULTS

The results suggested that the liabilities to TI, RTU and ND were correlated. Over 80 % of the variance in liability to TI and RTU were shared, and a smaller proportion was shared between RTU and ND. The heritabilities were estimated at 75 %, 80 % and 60 % respectively for TI, RTU and ND. The variance specific to liability to RTU was entirely accounted for by additive genetic factors. Only a modest part of the heritability in liability of ND was due to genetic factors specific to ND. Shared environmental factors were not significant. No sex differences were found for the sources of variation or causal paths, but prevalences were significantly greater in males versus females.

CONCLUSIONS

This study showed significant overlap in the contribution of genetic factors to individual differences in TI, RTU and ND. Furthermore, there was evidence for significant additional genetic factors specific to RTU and ND.

A twin study of early cannabis use and subsequent use and abuse/dependence of other illicit drugs

INTRODUCTION

Cannabis use is strongly associated with the use and abuse/dependence of other illicit drugs. Gateway and common liabilities models have been employed to explain this relationship. We sought to examine this association using a combination of the discordant twin design and modeling methods.

METHOD

We assess the relationship between early cannabis use and the subsequent use and abuse/ dependence of other illicit drugs in a population-based sample of male and female twin pairs using four analyses: (i) analysis of the association between early cannabis use and other illicit drug use and abuse/dependence in the entire sample of twins, (ii) assessment of the influence of early cannabis use in twin 1 on twin 2's use or abuse/dependence of other illicit drugs, (iii) use of twin pairs discordant for early cannabis use in a discordant twin design and (iv) a model-fitting procedure.

RESULTS

We found: (i) a strong association between early cannabis use and use and abuse/dependence of other illicit drugs in the sample, (ii) twin 1's early cannabis use is significantly associated with the twin 2's other illicit drug use, (iii) the role of correlated genetic factors with some evidence for a causal influence, and (iv) the correlated liabilities model fits the data well.

CONCLUSIONS

Early cannabis use is strongly associated with other illicit drug use and abuse/dependence. The relationship arises largely due to correlated genetic and environmental influences with persisting evidence for some causal influences.

Genetic and environmental sources of covariation between generalized anxiety disorder and neuroticism

OBJECTIVE

The authors examined the sources of covariation between generalized anxiety disorder and the personality trait of neuroticism. Because women have higher levels of neuroticism and twice the risk of lifetime generalized anxiety disorder of men, gender-specific effects were also explored.

METHOD

Lifetime generalized anxiety disorder and neuroticism were assessed in more than 8,000 twins from male-male, female-female, and opposite-sex pairs through structured diagnostic interviews. Sex-limited Cholesky structural equation models were used to decompose the correlations between generalized anxiety disorder and neuroticism into genetic and environmental components, including sex-specific factors.

RESULTS

Genetic correlations between generalized anxiety disorder and neuroticism were high and differed (nonsignificantly) between men and women (1.00 and 0.58, respectively). When nonsignificant gender differences were removed from the models, correlations between generalized anxiety disorder and neuroticism were estimated at 0.80 (95% confidence interval=0.52-1.00). The individual-specific environmental correlation between generalized anxiety disorder and neuroticism was estimated at 0.20 for both genders.

CONCLUSIONS

There is substantial overlap between the genetic factors that influence individual variation in neuroticism and those that increase liability for generalized anxiety disorder, irrespective of gender. The life experiences that increase vulnerability to generalized anxiety disorder, however, have only modest overlap with those that contribute to an individual's level of neuroticism.

Forms of cannabis and cocaine: a twin study

Cannabis and cocaine are illicit psychoactive substances that have fallen under intense scrutiny by epidemiologists and behavioral geneticists. However, most analyses have used a composite variable to represent the use of these two drugs. For example, the composite variable of cannabis use often includes use of marijuana or hashish. Similarly, cocaine use involves different preparations (crack vs. cocaine hydrochloride) and varying routes of administration (intranasal insufflation vs. smoking). While there is some epidemiological evidence for the difference in addictive potentials between crack and intranasal cocaine, genetically informative studies have not examined the relationship between the forms of cannabis or cocaine. We used data from male and female same-sex twin pairs to examine the extent of genetic, shared environmental, and unique environmental overlap between (i) marijuana and hashish for cannabis use and (ii) intranasal and crack cocaine for cocaine use. Bivariate Cholesky models were fit using the structural equation modeling software Mx. Our results indicate that for both drugs, the individual drug forms show a complete overlap of genetic factors and a substantial overlap of shared environmental influences. While marijuana and hashish share a moderate proportion of their unique environment, crack and intranasal cocaine only show a modest overlap of unique environmental factors, adding some evidence for form-specific environmental factors. In conclusion, there is substantial overlap of familial factors between forms of a single drug and preference is primarily determined by unique environmental influences. These findings also reinforce the validity of composite variables in epidemiological and genetic research.

Cannabis and Other Illicit Drugs: Comorbid Use and Abuse/Dependence in Males and Females

Cannabis and other illicit drugs are often used or abused comorbidly. Two competing theories to explain this comorbidity are (i) the phenotypic causation (gateway) model and (ii) the correlated liabilities model. We used data from 1191 male and 934 female same-sex twin pairs to test 13 genetically informative models of comorbidity. Models were fit separately for use and abuse/dependence in both sexes. The correlated liabilities model provided a good fit to the data for cannabis and other illicit drug use, as well as abuse/dependence. The relationship between the use or abuse of cannabis and other illicit drugs is not entirely phenotypic, as depicted by the random multiformity of cannabis model, which is an adaptation of the gateway model. The comorbidity appears to arise from correlated genetic and environmental influences. There is some evidence for a model in which high-risk cannabis users may be at increased risk for other illicit drug use. For abuse/dependence, a model with causal pathways between the liability for cannabis and other illicit drug abuse/dependence also fits well. Overall, our results suggest that the use and abuse/dependence of cannabis and other illicit drugs are strongly linked via common risk factors that jointly influence their individual liabilities.

Using the Mplus computer program to estimate models for continuous and categorical data from twins

Historically, the focus of behavior genetic research was to obtain estimates of the sources of familial resemblance for a single phenotype. Current research strategies have moved beyond heritability estimates to the search for physiological and behavioral mechanisms by which genetic risk is translated into individual differences in behavior and disease liability. Such research questions often require multivariate designs and complex analytic models, including the analysis of continuous and categorical dependent variables within the same model. Recent advances in computer software for categorical data analysis have increased the tools available for researchers in behavior genetics. This paper describes how to use the Mplus software program (Muthén and Muthén, 1998, 2002) for the analysis of data obtained from twins. Example analyses include two- and five-group twin models for univariate and bivariate continuous and categorical variables. Data on alcoholism and age at first drink drawn from the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders are used to illustrate how Mplus can be used to analyze multiple-category variables, recode and transform variables, select subgroups for analysis, handle subjects with incomplete data, include constraints to ensure non-negative loadings, include model covariates, model sex differences, and test alternative hypotheses about mediation of genetic risk by measured variables.

A Population Based Twin Study of Sex Differences in Depressive Symptoms

Depressive symptoms reflect depressed mood over a relatively short period of time and are measured using symptom checklists such as the SCL-90. There is some evidence that depressive symptoms are associated with major depression (MD), which is a clinically diagnosed psychiatric illness. Genetic studies of depressive symptomatology suggest a role for genetic factors as well as unique environmental influences. While epidemiological research suggests that depressive symptoms may be influenced by sex-specific factors, few genetically informative findings support this result entirely. We used data from male and female same sex and opposite-sex twin pairs to assess the extent to which genetic, shared and unique environmental factors influence depressive symptoms. Furthermore, we tested for the presence of qualitative and quantitative sex differences in depressive symptoms. Our results suggest that similar to other studies, depressive symptomatology is moderately heritable (31%) with no evidence for shared environmental factors. Our best fitting model suggests that there are no qualitative or quantitative sex differences in depressive symptoms. Our analyses suggest that while there may be mean differences in the levels of depressive symptoms across sexes, the genetic and environmental factors that predispose males and females to depressive symptoms are not different.

The interrelationship of neuroticism, sex, and stressful life events in the prediction of episodes of major depression

OBJECTIVE

Three potent risk factors for major depression are female sex, the personality trait of neuroticism, and adversity resulting from exposure to stressful life events. Little is known about how they interrelate in the etiology of depressive illness.

METHOD

In over 7,500 individual twins from a population-based sample, the authors used a Cox proportional hazard model to predict onsets of episodes of DSM-III-R major depression in the year before the latest interviews on the basis of previously assessed neuroticism, sex, and adversity during the past year; adversity was operationalized as the long-term contextual threat scored from 15 life event categories.

RESULTS

In the best-fit Cox model for prediction of depressive onsets, neuroticism, female sex, and greater adversity all strongly increased risk for major depression. An interaction was seen between neuroticism and adversity such that individuals with high neuroticism were at greater overall risk for major depression and were more sensitive to the depressogenic effects of adversity. An interaction was also seen between adversity and sex, as the excess risk for major depression in women was confined to individuals with low stress exposure.

CONCLUSIONS

Psychosocial adversity interacts both with neuroticism and with sex in the etiology of major depression. The impact of neuroticism on illness risk is greater at high than at low levels of adversity, while the effect of sex on probability of onset is the opposite--greater at low than at high levels of stress. Complete etiologic models for major depression should incorporate interactions between risk factor classes.