Genetic factors have historically been thought of as important in the development of obsessive–compulsive disorder (OCD). For the estimation of the relative importance of genetic and environmental factors, twin studies are an obvious approach. Twin studies of OCD have a long history, starting in 1929. In this review, over 70 years of twin research of OCD is presented, using four different approaches that represent the steps in the twin research of OCD from past to present. These steps include (1) case-studies of twins with OCD from the old literature; (2) twin studies of OCD using Diagnostic and Statistical Manual of Mental Disorders (DSM) criteria; (3) twin studies of OCD using a dimensional approach, comparing resemblances in monozygotic and dizygotic twins; and (4) twin studies of OCD using a dimensional approach, analyzing the data with Structural Equation Modeling. It is concluded that only the studies using the last method have convincingly shown that, in children, obsessive–compulsive (OC) symptoms are heritable, with genetic influences in the range of 45% to 65%. In adults, studies are suggestive for a genetic influence on OC symptoms, ranging from 27% to 47%, but a large twin study using a bio- metrical approach with continuous data is still needed to provide conclusive evidence. Strategies for future twin studies of OCD are discussed.

Background: This study focuses on sex dierences in depression of the widowed. Previous research showed dierent results in sex dierences and in depression after bereavement. We assessed the eects of widowhood on depressive symptoms for men and women and examined whether environmental strain like social support, ®nances and housekeeping concerns explain these eects. Methods: Data were used from a large communitybased study of older people in three regions of the Netherlands. Our study sample consists of 2626 widowed and married subjects in the age group of 55-85 years. Depression was measured using the CES-D scale; the various strains were obtained by structured interviews. Multiple linear regression, performed for men and women separately, were used. Results: The results show that widowhood is associated with higher levels of depressive symptoms and that this association is stronger for men than for women. The eect of widowhood is mediated by dierent types of environmental strain for men and women. However, a strong direct main eect of widowhood on depression remains. The dierence in depression rates between men and women is most evident among those widowed for a longer period of time. Conclusions: It appears that, over time, women adapt to widowhood more successfully than men. From a clinical point of view this is important, as it suggests that men who remain alone after losing their partner are at a higher risk of developing symptoms of chronic depression.

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To investigate environmental factors that protect against or exacerbate obsessive-compulsive (OC) symptoms, we selected 25 monozygotic (MZ) twin pairs discordant, 17 MZ twin pairs concordant high and 34 MZ pairs concordant low on OC symptoms from a large longitudinal Dutch sample of adult twin pairs and their family members, applying stringent criteria for OC symptomatology. Data were collected on psychopathology, family structure, health, lifestyle, birth complications and life events. Unique environmental factors were studied using within-discordant MZ pair comparisons, whereas betweenconcordant MZ pair comparisons were used to study environmental factors that are shared by the twins of an MZ pair. The high-scoring MZ twins of the discordant group reported more life events (especially sexual abuse) than their lowscoring twin-siblings. The between-pair comparisons showed lower birth weight in the discordant MZ pairs than in the concordant MZ pairs. Further, the concordant high MZ pairs as well as their spouses had a lower educational level than the two other groups. On scale scores of anxiousdepression, neuroticism, and somatic complaints, concordant high MZ pairs showed highest scores, and the discordant MZ pairs scored intermediate, except for neuroticism, on which the high-scoring twins of discordant MZ pairs were equal to the concordant high pairs. Discordance on psychological scale scores between the concordant MZ pairs was evident from 1991 onward, and within the discordant MZ pairs from 1997 onward, confirming previous reports of an association of early-onset OC symptoms with higher genetic load. Parent scores of OC symptoms and anxious-depression suggested intermediate genetic load in the discordant MZ group. In conclusion, this study reports on both unique and shared environmental factors associated with OC symptomatology. Whether these factors operate in addition to or in interaction with genetic disposition is to be elucidated in future studies.

Background. The contribution of genetic factors to obsessive-compulsive (OC) symptoms has not been examined using a large population-based sample of adults. Furthermore, the extent to which there are qualitative and quantitative differences in genetic architecture between men and women with OC symptoms has not been elucidated.