Overlapping genetic and environmental influences among men's alcohol consumption and problems, romantic quality and social support.

BACKGROUND: Alcohol consumption and problems are associated with interpersonal difficulties. We used a twin design to assess in men the degree to which genetic or environmental influences contributed to the covariance between alcohol consumption and problems, romantic quality and social support. METHOD: The sample included adult male-male twin pairs (697 monozygotic and 487 dizygotic) for whom there were interview-based data on: alcohol consumption (average monthly alcohol consumption in the past year); alcohol problems (lifetime alcohol dependence symptoms); romantic conflict and warmth; friend problems and support; and relative problems and support. RESULTS: Key findings were that genetic and unique environmental factors contributed to the covariance between alcohol consumption and romantic conflict; genetic factors contributed to the covariance between alcohol problems and romantic conflict; and common and unique environmental factors contributed to the covariance between alcohol problems and friend problems. CONCLUSIONS: Recognizing and addressing the overlapping genetic and environmental influences that alcohol consumption and problems share with romantic quality and other indicators of social support may have implications for substance use prevention and intervention efforts.

Evidence for multiple genetic factors underlying the DSM-IV criteria for alcohol dependence.

To determine the number of genetic factors underlying the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) criteria for alcohol dependence (AD), we conducted structural equation twin modeling for seven AD criteria, plus two summary screening questions, in 7133 personally interviewed male and female twins from the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders, who reported lifetime alcohol consumption. The best-fit twin model required three genetic and two unique environmental common factors, and criterion-specific unique environmental factors. The first genetic factor was defined by high loadings for the probe question about quantity and frequency of alcohol consumption, and tolerance criterion. The second genetic factor loaded strongly on the probe question about self-recognition of alcohol-related problems and AD criteria for loss of control, desire to quit, preoccupation and activities given up. The third genetic factor had high loadings for withdrawal and continued use despite the problems criteria. Genetic factor scores derived from these three factors differentially predicted patterns of comorbidity, educational status and other historical/clinical features of AD. The DSM-IV syndrome of AD does not reflect a single dimension of genetic liability, rather, these criteria reflect three underlying dimensions that index risk for: (i) tolerance and heavy use; (ii) loss of control with alcohol associated social dysfunction and (iii) withdrawal and continued use despite problems. While tentative and in need of replication, these results, consistent with the rodent literature, were validated by examining predictions of the genetic factor scores and have implications for gene-finding efforts in AD.

Accounting for the association between childhood maltreatment and alcohol-use disorders in males: a twin study.

BACKGROUND: An association between childhood maltreatment and subsequent alcohol abuse and/or dependence (AAD) has been found in multiple studies of females. Less is known about the association between childhood maltreatment and AAD among males, and the mechanisms that underlie this association in either gender. One explanation is that childhood maltreatment increases risk for AAD. An alternative explanation is that the same genetic or environmental factors that increase a child's risk for being maltreated also contribute to risk for AAD in adulthood. METHOD: Lifetime diagnosis of AAD was assessed using structured clinical interviews in a sample of 3527 male participants aged 19-56 years from the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders. The sources of childhood maltreatment-AAD association were estimated using both a matched case-control analysis of twin pairs discordant for childhood maltreatment and bivariate twin modeling. RESULTS: Approximately 9% of participants reported childhood maltreatment, defined as serious neglect, molestation, or physical abuse occurring before the age of 15 years. Those who experienced childhood maltreatment were 1.74 times as likely to meet AAD criteria compared with males who did not experience childhood maltreatment. The childhood maltreatment-AAD association largely reflected environmental factors in common to members of twin pairs. Additional exploratory analyses provided evidence that AAD risk associated with childhood maltreatment was significantly attenuated after adjusting for measured family-level risk factors. CONCLUSIONS: Males who experienced childhood maltreatment had an increased risk for AAD. Our results suggest that the childhood maltreatment-AAD association is attributable to broader environmental adversity shared between twins.

The Norwegian Institute of Public Health twin study of mental health: examining recruitment and attrition bias.

All Norwegian twin pairs born 1967-1974 and still living in Norway in 1992 were invited to a health questionnaire study (Q1). 2,570 pairs (65%) participated. These cohorts and the twin cohorts born 1967-1979 were invited to a new questionnaire study (Q2) in 1998. This time 3,334 pairs (53%) participated. Almost all pairs having participated in the 1998 study were invited to an interview study of mental health (MHS), taking place 1999-2004. 1,391 complete pairs (44%) participated. The questionnaire studies included extensive data on somatic health with fewer items on mental health and demography. Health-related and demographic information available from the Medical Birth Registry on all invited twins was applied to predict participation to the first study. A few registry variables indicating poor health predicted nonparticipation in Q1. Health information and demography from Q1 were tested as predictors of participation in the follow-up study (Q2). Monozygosity, female sex, being unmarried, having no children, and high education predicted participation, whereas few indicators of poor mental and somatic health and unhealthy lifestyle moderately predicted nonparticipation in Q2. No health indicators reported in Q2 predicted further participation. Standard genetic twin analyses of indicators of various mental disorders from Q2, validated by diagnostic data from the MHS, did not indicate differences in genetic/environmental covariance structures between participants and nonparticipants in MHS. In general the results show a moderate selection towards good mental and somatic health. Attrition from Q2 to the MHS does not appear to affect twin analyses of mental health related variables.

Prospective, randomized, single-blind, controlled study to compare two methods of performing adenoidectomy.

OBJECTIVE: To compare adenoidectomy using suction-diathermy ablation to curettage adenoidectomy with respect to operative time and adenoid regrowth at 6 months after surgery. STUDY DESIGN: A prospective, randomized, single-blind, study to compare two methods of performing adenoidectomy. A group of 100 children, undergoing adenoidectomy alone or in combination with tonsillectomy, were randomized into two groups and underwent either suction diathermy or curettage adenoidectomy by a single surgeon. SETTING: A tertiary care Paediatric Hospital. METHOD: Indication for surgery, adenoidal size, duration of surgery and complications were recorded and compared. Six-month follow-up was conducted and adenoidal size and symptom status were recorded and compared. Statistical analysis was performed using Microsoft Excel. RESULTS: One hundred patients participated in this study and underwent adenoidectomy alone or adenotonsillectomy. Ninety-two patients returned for follow-up and 91 patients completed the study. The two treatment groups were well matched for age and gender. The main indications for both groups were snoring, nasal obstruction and obstructive sleep apnoea. For adenoidectomy alone there was no significant difference in duration of surgery between the curette and suction diathermy groups. When performing tonsillectomy and adenoidectomy together suction diathermy took significantly longer to complete than curettage (P<0.001). Overall 96% of patients' symptoms had either improved or resolved. The post-operative comparison at 6 months showed a significant difference in the residual adenoidal size between the two groups, the suction diathermy group being generally smaller than the curettage group. CONCLUSIONS: Suction diathermy was better at reducing the adenoidal size 6 months after surgery. Although the difference in size was statistically significant it did not seem to be of clinical significance.

An investigation of the significance of Actinomycosis in tonsil disease.

OBJECTIVES: The objective of the study was to establish the incidence of Actinomycosis in the tonsils of children undergoing tonsillectomy or adenotonsillectomy, and to evaluate its role in clinical tonsillar disease. METHODS: This was a prospective controlled study done at the Red Cross Children's Hospital in Cape Town, South Africa over an 8-month period and included all children undergoing tonsillectomy or adenotonsillectomy. All resected tonsils were examined for the presence of Actinomycosis and any signs of significant cryptitis or active tonsillitis. A comparison was made in the incidence of Actinomycosis in children with obstructive sleep apnoea, recurrent tonsillitis or obstructive sleep apnoea and recurrent tonsillitis. The data was further analysed to determine the statistical significance of the association between Actinomycosis of the tonsils and age, sex and histopathological and clinical diagnosis. RESULTS: A total of 344 tonsils were analysed on 172 patients. We found 20 patients (11.6%) with Actinomycosis in the tonsils. The mean age of patients with Actinomycosis was 7.25 years and without Actinomycosis was 5.4 years (p=0.002). Most specimens (16) had no evidence of tissue reaction to Actinomyces, and their presence was found to be due to colonisation of the tonsils only. Actinomycosis was present in 11% of patients with obstructive sleep apnoea, 11% of patients with recurrent tonsillitis and in 9% with obstructive sleep apnoea and recurrent tonsillitis. The difference in incidence of Actinomycosis between these three groups (p=0.94), and between the recurrent tonsillitis group alone compared to the obstructive group (p=0.83), was not statistically significant. There was therefore no statistical significance found between Actinomyces and OSA+/- recurrent tonsillitis. CONCLUSIONS: There was no correlation found between the presence of tonsillar Actinomycosis and recurrent tonsillitis and/or obstructive tonsillar hypertrophy. Histopathologic findings showed no evidence of tissue reaction to Actinomyces and its presence was found to be due to colonisation of the tonsils only. The series did however show a statistically significant correlation between Actinomycosis colonisation and age with Actinomycosis being more common in older children, especially those over 5 years of age.

Tracheostomy in human immuno-deficiency virus infected children at the Red Cross War Memorial Children's Hospital, Cape Town, South Africa.

UNLABELLED: Tracheostomy in adults with HIV/AIDS has been reported to be associated with both high and early mortality of 47-100%. There is minimal data regarding the role of tracheostomy in HIV infected children. We did a retrospective analysis of HIV positive children that underwent tracheostomy at our institution over a 5-year period, 2002-2006. A total of 70 tracheostomies were done during the period and 15 (21.4%) of these children were confirmed as HIV infected. The average age at presentation for HIV infected children with upper airway obstruction resulting eventually in tracheostomy was 9.4 months and 60% were under 1 year of age. Only three (20%) were on Anti-Retroviral Therapy (ART) prior to presentation. The cause of upper airway obstruction was croup in 14 (93%) of these 15 children. Following tracheostomy all were treated with ART. To date six children have been successfully decannulated (40%) and there have been three deaths (20%) which were unrelated to tracheostomy. CONCLUSION: Tracheostomy in HIV positive children is not associated with the high mortality that has been reported in adults provided such children are started on treatment with antiretroviral therapy.

Is topical local anaesthesia necessary when performing paediatric flexible nasendoscopy? A double-blind randomized controlled trial.

OBJECTIVE: To evaluate the effectiveness of lignocain 2% and oxymetazoline 0.025% compared to oxymetazoline 0.025% alone when administered prior to fibreoptic nasendoscopy in paediatric patients. STUDY DESIGN: Prospective, randomized controlled, double-blind study. A group of 56 children, undergoing nasendoscopy to determine adenoidal size, were randomized into two groups and received either lignocain 2% and oxymetazoline 0.025% or oxymetazoline 0.025% alone prior to fibreoptic nasendoscopy. SETTING: A tertiary care Paediatric Hospital. METHOD: The endoscopist recorded the ease of performance of the procedure, cooperation of patient and quality of the view achieved using a visual analogue scale (VAS). The pain and anxiety levels of the child were recorded before, during and immediately after the procedure, using a VAS. The duration of performing the procedure was recorded from insertion of the endoscope into the nostril until removal. RESULTS: All 56 children were able to undergo the endoscopy and the full anxiety and pain assessment was done. Three children were excluded because they have undergone nasendoscopies before. Of the 53 patients included, 27 children received solution A (oxymetazoline 0.025%) and 26 children received solution B (oxymetazoline 0.025% and lignocain 2%). There was no statistical difference between the two groups regarding the duration of the endoscopy, quality of view, ease of performance and cooperation of the patients. The median pain and anxiety scores were not significantly different between the two groups. CONCLUSIONS: This study concludes that the use of a decongestant (oxymetazoline) for paediatric nasendoscopy is just as effective as the use of oxymetazoline with lignocain. Pain and anxiety is not increased in the absence of lignocain.

A population-based twin study of lifetime major depression in men and women.

BACKGROUND: Women report higher rates of major depression (MD) than men. Although genetic factors play an important etiologic role in MD, we are uncertain whether genetic factors are of equal importance in men and women, and whether the same genetic factors predispose men and women to MD. METHODS: We obtained, by telephone interview, a lifetime history of MD, defined by the DSM-III-R, from 3790 complete male-male, female-female, and male-female twin pairs, identified through a population-based registry. Results were analyzed using probandwise concordance, odds ratios, and biometrical twin modeling. RESULTS: The odds ratios (plus tetrachoric correlations) for lifetime MD were as follows: (1) male-male monozygotic, 3.29 (+0.37); (2) male-male dizygotic, 1.86 (+0.20); (3) female-female monozygotic, 3.02 (+0.39); (4) female-female dizygotic, 1.59 (+0.18); and (5) male-female dizygotic, 1.39 (+0.11). In the best-fitting twin model, the heritability of liability to MD was the same in men and women and equal to 39%, while the remaining 61% of the variance in liability was due to individual-specific environment. We rejected, with only modest confidence, the hypothesis that the genetic risk factors for MD were the same in men and women. The best-fitting model estimated the genetic correlation in the liability to MD in the 2 sexes to be +0.57. While we found no evidence to suggest a violation of the equal environment assumption, MD was less common in women from opposite-sex vs same-sex twin pairs. CONCLUSIONS: Major depression is equally heritable in men and women, and most genetic risk factors influence liability to MD similarly in the 2 sexes. However, genes may exist that act differently on the risk for MD in men vs women.

The genetic epidemiology of irrational fears and phobias in men.

BACKGROUND: Much of our knowledge of the role of genetic factors in the etiology of phobias comes from one population-based sample of female twins. We examined the sources of individual differences in the risks for phobias and their associated irrational fears in male twins. METHODS: In personal interviews with both members of 1198 male-male twin pairs (707 monozygotic [MZ] and 491 dizygotic [DZ]) ascertained from a population-based registry, we assessed the lifetime history of agoraphobia and social, animal, situational, and blood/injury phobias as well as their associated irrational fears. Twin resemblance was assessed by means of probandwise concordance, odds ratios, tetrachoric correlations, and univariate and multivariate biometrical model fitting. RESULTS: The suggestive results obtained by analysis of phobias only were supported by analyzing both fears and phobias. All 5 phobia subtypes aggregate within twin-pairs. This aggregation is due largely or solely to genetic factors with heritability of liabilities ranging from 25% to 37%. Multivariate analysis revealed a common genetic factor, genetic factors specific to each subtype, and a common familial-environmental factor. CONCLUSIONS: In male subjects, genetic risk factors, which are partially common across all subtypes and partially subtype specific, play a moderate role in the etiology of phobias and their associated irrational fears. Family environment probably has an impact on risk for agoraphobia and social phobia. The genetic liability to blood/injury phobias is not distinct from those of the more typical phobias.

Sex-specific genetic influences on the comorbidity of alcoholism and major depression in a population-based sample of US twins.

BACKGROUND: Alcoholism and depression frequently co-occur, but the origins of this comorbidity remain uncertain. Most previous family, twin, and adoption studies of these disorders have used cases ascertained through treatment settings, who may differ from cases in epidemiological samples. We studied the importance of genetic influences on risk for lifetime comorbidity of major depression and alcoholism by means of a population-based twin sample. METHODS: Lifetime major depression (MD), alcohol abuse, and alcohol dependence were assessed by structured interview for both members of 3755 twin pairs from the Mid-Atlantic Twin Registry. Pair resemblance was analyzed by means of structural equation models. RESULTS: Individuals with MD were at significantly increased risk for alcohol dependence and for a combined diagnosis of alcohol abuse and/or dependence. History of MD in a twin significantly increased the risk of cotwin alcohol dependence and alcohol abuse and/or dependence among identical male pairs and for alcohol abuse and/or dependence in identical female pairs, but not among male or female fraternal pairs. Results of structural modeling indicate that comorbidity occurs because the genetic and specific environmental sources of liability to MD overlap with those underlying alcohol dependence and alcohol abuse and/or dependence. This overlap was significant only within sex, not across sexes. CONCLUSIONS: In this population-based twin sample, the familial transmission of MD and alcohol dependence was largely disorder specific. Comorbidity appears to be due to sex-specific genetic and environmental risk factors. The factors underlying depression in women do not appear to arise from the same factors underlying alcoholism in men.

Clinical characteristics of major depression that predict risk of depression in relatives.

BACKGROUND: Major depression (MD) is both clinically and etiologically heterogeneous. We attempt to relate clinical and etiologic heterogeneity by determining those features of MD that reflect a high familial liability to depressive illness. METHODS: Our sample, 3786 personally interviewed twin pairs from a population-based registry, contained 1765 people with a lifetime history of MD by DSM-III-R criteria, of whom 639 (36.2%) had affected co-twins. We examine, using Cox proportional hazard models, the clinical features of MD in affected twins that predicted the risk for MD in the co-twin. Control variables were zygosity, age at interview, and sex of the twin and co-twin. RESULTS: The best-fitting model contained 4 significant predictors: number of episodes, duration of longest episode, recurrent thoughts of death or suicide, and level of distress or impairment. These 4 clinical features were similarly predictive of the risk for MD in the co-twins of male and female twins and predicted risk of illness more strongly in monozygotic than in dizygotic twins. Variables that did not uniquely predict risk of MD in the co-twin included age at onset and number of depressive symptoms. For number of episodes, the best-fitting model indicated an inverted U-shaped function with greatest co-twin risk for MD with 7 to 9 lifetime episodes. CONCLUSIONS: The clinical features of MD in epidemiologic samples can be meaningfully related to the familial vulnerability to illness. Familial MD is best characterized by intermediate levels of recurrence, long duration of episodes, high levels of impairment, and recurrent thoughts of death or suicide. These clinical features probably reflect a high genetic liability to depressive illness.

Temperance board registration for alcohol abuse in a national sample of Swedish male twins, born 1902 to 1949.

BACKGROUND: Temperance boards were established in Sweden to register and follow up individuals who were seen in legal or medical settings with problems of alcohol abuse. These records, available in a large epidemiologic twin population, have provided an objective and validated measure of alcohol abuse. METHODS: We examined Swedish temperance board registrations from 1929 to 1974 (n = 2516 individual twins) in all male-male Swedish twin pairs of known zygosity from the population-based Swedish Twin Registry; these twin pairs were born from 1902 to 1949 (n = 8935 pairs). RESULTS: The lifetime prevalence and probandwise concordance rates for temperance board registrations were 13.2% and 47.9%, respectively, in monozygotic twins and 14.6% and 32.8%, respectively, in dizygotic twins. Model fitting suggested that genetic and familial-environmental risk factors accounted for 54% (95% confidence interval [CI], 47%-61%) and 14% (95% CI, 8%-19%) of the liability to temperance board registration, respectively; these estimates were stable across birth cohorts. High genetic liability was reflected by large numbers of temperance board registrations and registrations for criminal alcohol use. Elevated familial-environmental liability was indicated by an early age at first registration. CONCLUSIONS: Genetic factors are of major etiologic importance for alcohol abuse in men, while familial environmental factors play a significant but less important role. The etiologic importance of these factors has remained constant in Sweden for men who were born in the first half of the 20th century.

Childhood sexual abuse and adult psychiatric and substance use disorders in women: an epidemiological and cotwin control analysis.

BACKGROUND: Women who report childhood sexual abuse (CSA) are at increased risk for developing psychiatric disorders in adulthood. What is the diagnostic specificity and cause of this association? METHODS: In a population-based sample of 1411 female adult twins, 3 levels of CSA were assessed by self-report and cotwin report: nongenital, genital, and intercourse. Interviews with twins and parents assessed family background and diagnoses of psychiatric and substance dependence disorders. Odds ratios (ORs) were calculated by logistic regression. RESULTS: By self-report, 30.4% reported any CSA and 8.4% reported intercourse. Self-reported CSA was positively associated with all disorders, the highest ORs being seen with bulimia and alcohol and other drug dependence. The ORs were modest and often nonsignificant with nongenital CSA and increased with genital CSA and especially intercourse, where most ORs exceeded 3.0. A similar pattern of findings was seen with CSA as reported by the cotwin, although many ORs were smaller. Controlling for family background factors and parental psychopathology produced a small to modest reduction in ORs. In twin pairs discordant for CSA, the exposed twin was at consistently higher risk of illness. CONCLUSIONS: Women with CSA have a substantially increased risk for developing a wide range of psychopathology. Most of this association is due to more severe forms of CSA and cannot be explained by background familial factors. Although other biases cannot be ruled out, these results are consistent with the hypothesis that CSA is causally related to an increased risk for psychiatric and substance abuse disorders.

Illicit psychoactive substance use, heavy use, abuse, and dependence in a US population-based sample of male twins.

BACKGROUND: In order to develop informed approaches to prevention and treatment of illicit psychoactive substance use, abuse, and dependence, we need to understand the sources of individual differences in risk. METHODS: In personal interviews with 1198 male-male twin pairs (708 monozygotic and 490 dizygotic) ascertained from a population-based registry, we assessed lifetime use, heavy use, and abuse of and dependence on cannabis, sedatives, stimulants, cocaine, opiates, and hallucinogens. Twin resemblance was assessed by probandwise concordance, odds ratio, tetrachoric correlations, and biometrical model fitting. RESULTS: Twin resemblance for substance use, heavy use, abuse, and dependence was substantial, and consistently greater in monozygotic than in dizygotic twins. For any drug use and for cannabis and hallucinogen use, model fitting suggested that twin resemblance was due to both genetic and familial-environmental factors. Twin resemblance for sedative, stimulant, cocaine, and opiate use, however, was caused solely by genetic factors. With 2 exceptions (cocaine abuse and stimulant dependence), twin resemblance for heavy use, abuse, and dependence resulted from only genetic factors, with heritability of liability usually ranging from 60% to 80%. No consistent evidence was found for violations of the equal environment assumption. CONCLUSIONS: In accord with prior results in studies of women, the family environment plays a role in twin resemblance for some forms of substance use in men. However, twin resemblance for heavy use, abuse, and dependence in men is largely caused by genetic factors, and heritability estimates are high.

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.

Similarity in saliva cortisol measures in monozygotic twins and the influence of past major depression.

BACKGROUND: Some studies suggest that cortisol may be under genetic control. The aims of our study were to investigate the familial resemblance in morning and evening cortisol secretion as assessed by saliva cortisol and to assess the influence of history of major depression. METHODS: Women for this investigation were selected from an ongoing study in female-female twin pairs ascertained from the Virginia Twin Registry. Telephone screening assured that current inclusion/exclusion criteria were met. Subjects were asked to collect AM samples within 45 min after awakening, and evening samples immediately before bedtime for 14 days. RESULTS: There was a high degree of correlation across weeks in both the AM and PM cortisol values, indicating significant stability across individuals. There was significant correlation between AM and PM cortisol in monozygotic twins. In twins with a history of major depression (n = 30), compared with the twins without past major depression (n = 28), there was a trend towards higher cortisol (p = .056). CONCLUSIONS: These results suggest that around 40-45% of the total variance in salivary cortisol is shared by monozygotic twins. Although the increase in baseline cortisol in twins with a history of major depression is only significant at the trend level, the effect size is comparable to an "in episode" depressed population.

Cannabis use, abuse, and dependence in a population-based sample of female twins.

OBJECTIVE: The rate of cannabis use by women has been increasing in recent decades. This study examined the etiology of cannabis use and abuse among women and the possible role of genetic risk factors. METHOD: Unselected individual twins (N=1,934) from female-female pairs ascertained through a population-based registry, including both members of 485 monozygotic pairs and of 335 dizygotic pairs, were interviewed by telephone to assess lifetime cannabis use, heavy use, abuse, and dependence as defined by DSM-IV criteria. Biometric model fitting was performed with the Mx computer package. RESULTS: The prevalences of lifetime cannabis use, heavy use, abuse, and dependence were 47.9%, 6.7%, 7.2%, and 2.2%, respectively. Model fitting suggested that twins' resemblance for liability to cannabis use was due to both genetic and familial-environmental factors, while twins' resemblance for heavy cannabis use and abuse and symptoms of dependence resulted solely from genetic factors, with heritabilities ranging from 62% to 79%. The frequency of adolescent social contact between co-twins, which was greater among monozygotic than among dizygotic twins, predicted the twins' resemblance in cannabis use. However, further analyses suggested that the heritability of cannabis use was at most modestly inflated by such social factors. CONCLUSIONS: In women, genetic risk factors have a moderate impact on the probability of ever using cannabis and a strong impact on the liability to heavy use, abuse, and, probably, dependence. By contrast, the family and social environment substantially influences risk of ever using cannabis but plays little role in the probability of developing heavy cannabis use or abuse.

Genetic and environmental contributions to alcohol abuse and dependence in a population-based sample of male twins.

OBJECTIVE: Most twin and adoption studies of alcoholism have ascertained cases through treatment settings or archival data; these subjects may differ from affected subjects identified epidemiologically. The authors studied the importance of genetic influences on risk of alcohol-related disorders in a new population-based twin sample. METHOD: Structured personal interviews were used to assess DSM-III-R-defined and DSM-IV-defined alcohol abuse and dependence among 3,516 twins from male-male pairs born in Virginia between 1940 and 1974. RESULTS: The magnitude of resemblance among twin pairs was similar across several definitions of alcoholism and was substantially higher among 861 identical pairs than among 653 fraternal pairs. On the basis of a liability threshold model, 48%-58% of the variation in liability was attributed to additive genetic factors, with the remainder attributed to environmental influences not shared by family members. When a treatment-based proband concordance model was used, evidence for shared environmental as well as genetic influences emerged. CONCLUSIONS: In this first population-based study of male twins from the United States, it was found that genetic factors played a major role in the development of alcoholism among males, with similar influence for alcohol abuse and alcohol dependence. Prior findings implicating the influence of common environment may be attributable to sampling strategy; in this population-based sample, environmental factors shared by family members appear to have had little influence on the development of alcoholism in males.