Introduction 1994 1998 1995 2000 2004 2000 2002 2007a b 2007a b 2004 1999 2005 1999 2004 2006 2004 1994 2004 2007 2002 2004 2006 2001 1990 1997 unique 2005 shared 1993 2007 2000 1996 2000 2005 Method Sample selection 2002 1988 1992 1995 2004 1995 1 1995 n n n 1995 p t p 1996 1999 Table 1 The Padua Inventory-Revised abbreviated (PI-R ABBR)  PI-R ABBR Original factor 1 In certain situations, I am afraid of losing my self-control and doing embarrassing things Impulses 2 I check and recheck gas and water taps and light switches after turning them off Checking 3 I feel obliged to follow a particular order in dressing, undressing and washing myself Precision 4 When I see a train approaching I sometimes think I could throw myself under its wheels Impulses 5 I return home to check doors, windows, drawers etc., to make sure they are properly shut Checking 6 When I start thinking of certain things, I become obsessed with them Rumination 7 I feel I have to repeat certain numbers for no reason Precision 8 Unpleasant thoughts come into my mind against my will and I cannot get rid of them Rumination 9 My thoughts constantly go astray, therefore I find it difficult to attend to what is happening around me Rumination 10 I sometimes have to wash or clean myself dimply because I think I may be dirty or ‘contaminated’ Washing 11 I get upset and worried at the sight of knives, daggers and other pointed objects Impulses 12 If I touch something which I think is ‘contaminated’, I immediately have to wash or clean myself Washing Fig. 1 The 2002 NTR wave using the PI-R abbreviated  2005 1 Of the final 76 MZ twin pairs selected for this study, 18 pairs participated in wave 2002 only, 28 pairs participated in two waves, 4 pairs in three, 8 pairs in four, 13 pairs in five, and 4 pairs in all six waves. Measures and instruments The NTR survey contains a broad range of longitudinal measurements taken at six time points between 1991 and 2002, as well as cross-sectional measurements. Information is obtained on life events, perinatal adversities, physical and mental health, lifestyle factors such as physical activity, religiosity, drinking, smoking and drug behavior, and on demographic variables such as relationships, number of children, level of education, living situation, and work status. Since this is an exploratory study, all available information was taken into account. Religiosity was assessed by asking whether the respondent had had a religious upbringing (yes/no), the person’s current religion, and whether the respondent currently was an active church member. 1987 1992 1970 1979 2000 1997 2001 2006 1970 1964 1995 1965 1982 Socio Economic Status (SES) in 2002 was assessed using a full description of the occupation of the twins according to the descriptions provided by the Central Office for Statistics in the Netherlands. The work level was coded into three levels based on the mental complexity of the work, ranging from low skilled (1) to academic work (3). Living situation was coded between 1 and 3 (1 = with parents; 2 = alone; 3 = with partner). Finally, we used child-derived information on their parents’ level of education. Direct parent information was used to collect information on their smoking and drinking behavior and on their scores of OC behavior, anxiety and depression. Statistical analyses Within-pair analyses t t χ 2 Between-pair analyses χ 2 U To adjust for correlated error in the between-group comparisons of common environment variables, separate regression analyses (multiple regression for continuous measures and logistic regression for categorical measures) were conducted in STATA 9.2 for these variables (StatCorp, College Station, TX, USA). The robust cluster option was used to account for nonindependence of the twin pairs on the variables that reflected common environmental influences (i.e., caesarean section, birth weight, and religious upbringing of the twin; parental death and divorce, death of a sibling, and level of education, alcohol use and smoking behavior of the parents). Alpha was set at 0.05. Results Within-pair analyses of discordant pairs Twenty-five MZ twin pairs discordant on OC behavior were included, of whom 18 pairs were female. Their mean age was 29.6 years (SD 6.8 years). Mean PI-R ABBR OC scores in the high-scoring twins of the discordant pairs were 21.4 (SD 5.9), in the low scoring twins 4.5 (SD 2.0). Health and lifestyle characteristics p p p p p p p p p p p 2 Table 2 2002 wave of data collection: within-discordant twin pair characteristics  Low risk twin, mean (SD) High risk twin, mean (SD) Test statistic* p Birth weight (g) 2,189 (806) 2,028 (667) 0.9 n.s. Birth order (first born) n n 1.7 n.s. General health (1–5) 4.1 (0.7) 3.8 (0.6) 2.3 0.03 Mental health contacts ever yes n n 0.5 n.s. Sumscore impediments physical activity 43.5 (12.5) 53.9 (12.4) −4.2 <0.001 Number of persons drinking ever n n 0 n.s. Number of drinks per week (past 12 months) 11.1 (1.4) 13.4 (0.7) 0.19 n.s. CAGE score alcohol dependence 4.0 (0.2) 4.3 (0.7) −1.4 n.s. Duration current relation (years) 5.9 (7.4) 3.0 (0) 1.97 n.s. Number of children 1.1 (1.2) 0.7 (0.6) 1.87 n.s. Education level self (1–13) 7.9 (3.0) 8.3 (2.6) −0.8 n.s. Education level partner (1–13) 8.3 (3.6) 8.1 (3.1) −1.2 n.s. Living situation (1–4) 2.8 (0.8) 2.7 (0.8) −1.3 n.s. PI-R ABBR OC scale 4.5 (2.0) 21.4 (5.9) −13.7 <0.001 YASR anxious depression scale 4.3 (2.8) 11.6 (4.3) −6.4 <0.001 ABV extraversion 51.0 (16.7) 45.6 (12.5) 1.1 n.s. ABV neuroticism 48.6 (23.1) 85.3 (27.4) −5.8 <0.001 ABV somatic complaints 16.6 (5.1) 24.4 (10.3) −4.0 <0.001 STAI-trait 31.6 (4.7) 46.4 (10.8) −6.5 <0.001 Satisfaction with life scale scores 27.4 (4.1) 23.8 (7.0) 2.3 0.02 Happiness scores 22.7 (3.9) 17.9 (5.8) 23 0.001 Self-efficacy scores 31.3 (4.0) 27.5 (4.7) 2.9 0.006 PI-R ABBR, Padua Inventory-Revised Abbreviated; OC, obsessive-compulsive; YASR, young adult self-report; ABV, Amsterdamse Biografische vragenlijst; STAI, State trait Anxiety Inventory; SBL, Spannings behoefte Lijst (sensation seeking list); CI, confidence interval; n.s., not significant Unique environment influences p 3 Table 3 Within-discordant twin comparisons—unique life events  Low OC twin, mean (SD) High OC twin, mean (SD) p Birth weight (g) 2,189 (806) 2,028 (667) n.s. Disease self (0–2) 0.08 (0.4) 0.26 (0.6) n.s. Disease child (0–2) 0.09 (0.4) 0.6 (0.1) n.s. Disease partner (0–2) 0.09 (0.4) 0 n.s. Disease significant other (0–2) 0.82 (0.9) 0.82 (0.9) n.s. Death child (0–2) 0 0 n.s. Death partner (0–2) 0 0 n.s. Death significant other (0–2) 1.1 (0.9) 1.0 (0.9) n.s. Sexual abuse* (0–2) 0.17 (0.5) 0.43 (0.8) 0.08 Violence (0–2) 0.17 (0.6) 0.17 (0.6) n.s. Relationship termination (0–2) 0.52 (0.8) 0.82 (0.9) n.s. Theft (0–2) 0.47 (0.8) 0.56 (0.8) n.s. Traffic accident (0–2) 0.52 (0.8) 0.38 (0.7) n.s. Dismissal (0–2) 0.30 (0.7) 0.48 (0.7) n.s. Total score life events 2.35 (1.9) 2.76 (1.7) n.s. Longitudinal data p p p p p Between-pair analyses of concordant and discordant pairs Seventeen MZ twin pairs were included who were concordant high on OC behavior, of whom 14 pairs were female. Their mean age was 30.0 years (SD 11.2 years), mean PI-R ABBR OC scores were 23.7 (SD 6.7). Thirty-four MZ twin pairs were included who were concordant low on OC behavior, of whom 28 pairs were female. Their mean age was 30.0 years (SD 11.3 years), mean PI-R ABBR OC scores were 3.8 (SD 2.2). Health and lifestyle characteristics p p 4 Table 4 2002 wave of data collection: between concordant and discordant twin pair health and lifestyle characteristics  Concordant Low twin pairs, mean (SD) Concordant high twin pairs, mean (SD) Discordant twin pairs, mean (SD) p p p General health (1–5) 4.4 (0.7) 3.6 (1.2) 4.1 (0.7) <0.001 0.007 n.s. Mental health contacts ever yes n n n <0.001 0.006 0.009 Impediments physical activity 41.3 (12.6) 53.0 (17.1) 43.5 (12.5) 0.001 n.s. 0.03 Specialized medical treatment ever yes n n n 0.002 n.s. 0.04 Currently active in church (1–3) 0.8 (0.7) 0.5 (0.7) 0.9 (0.8) n.s. n.s. n.s. Number of persons drinking ever n n n n.s. 0.003 n.s. Number of drinks per week (past 12 months) 2.7 (1.4) 2.3 (1.5) 1.9 (1.2) n.s. 0.01 n.s. CAGE score alcohol dependence 4.1 (0.5) 4.5 (0.8) 4.2 (0.5) 0.02 n.s. 0.04 Number of persons smoking ever n n n n.s. n.s. n.s. Number of cigarettes per day (1–7) 4.1 (1.1) 4.7 (1.1) 3.8 (1.0) n.s n.s. 0.07 Number of persons with partner n n n n.s. n.s. 0.04 Children yes n n n n.s. 0.003 0.015 Education level self (1–13) 9.4 (2.3) 8.27 (2.8) 8.24 (2.8) 0.02 0.02 n.s. Education level partner (1–13) 9.1 (2.8) 6.7 (3.7) 8.20 (3.4) 0.006 n.s. n.s. Living situation (1–3) 2.4 (0.8) 2.2 (1.0) 2.8 (0.8) n.s. 0.04 0.01 PI-R ABBR OC scale 3.8 (2.2) 23.7 (6.7) 12.9 (9.5) <0.001 <0.001 <0.001 YASR anxious depression scale 2.9 (4.7) 14.5 (19.3) 8.7 (12.6) <0.001 <0.001 <0.001 ABV extraversion 62.8 (16.2) 46.4 (17.7) 48.3 (17.7) <0.001 <0.001 0.001 ABV neuroticism 36.2 (18.9) 92.8 (19.7) 66.6 (31.1) <0.001 <0.001 n.s. ABV somatic complaints 15.7 (3.7) 27.5 (8.3) 20.3 (8.8) <0.001 0.003 <0.001 STAI-trait 29.4 (6.4) 36.9 (7.9) 37.6 (7.5) <0.001 <0.001 <0.001 Satisfaction with life 28.7 (4.0) 19.8 (7.2) 25.6 (5.9) <0.001 0.01 0.001 Happiness 24.2 (3.2) 16.2 (5.7) 20.4 (5.4) <0.001 <0.001 0.01 Self-efficacy 33.7 (3.9) 25.5 (3.9) 29.4 (4.7) <0.001 <0.001 <0.001 p n n n p p p p p p p p p p p p Shared environment influences p p p p p 5 Table 5 Between twin-pair comparisons: comparison of common environment characteristics (after correction for interrelatedness)  Concordant low twin pairs, mean (SD) Concordant high twin pairs, mean (SD) Discordant twin pairs, mean (SD) p p p Caesarean section (yes) n n n n.s. 0.008 <0.001 Birth weight (g) 2,650 (876) 2,685 (795) 2,109 (736) n.s. 0.004 0.009 Religious upbringing yes n n n n.s. n.s. n.s. a 7.5 (4.0) 5.5 (3.7) 5.7 (3.5) n.s. n.s. n.s. a 6.3 (3.7) 5.2 (3.4) 4.7 (3.0) n.s. n.s. n.s. Death mother (0–2) yes n n n n.s. n.s. n.s. Death father (0–2) n n n n.s. n.s. n.s. Death sibling (0–2) n n n 0.05 n.s. n.s. b n n n n.s. n.s. n.s. b 91% 100% 74% 0.06 n.s. 0.04 b 3.5 (4 drinks/week) 2.7 (2–3 drinks/week) 2.3 (1–2 drinks/week) n.s. 0.03 n.s. b 71% 89% 48% n.s. 0.06 0.07 b 4 (6–10 cig/day) 5(11–20 cig/day) 5 (11–20 cig/day) n.s n.s. n.s. a b Longitudinal data p p p p p p Parent data 6 Table 6 2002 wave of data collection: between-parents comparisons of psychological scales  Parent concordant low, mean (SD) Parent concordant high, mean (SD) Parent discordant, mean (SD) p p p PI-R ABBR OC scale 5.2 (3.8) 11.7 (3.8) 9.9 (5.7) 0.002 0.002 n.s. YASR anxious depression scale 4.5 (3.5) 11.7 (4.2) 7.7 (4.3) <0.001 0.02 0.04 ABV extraversion 53.4 (17.5) 44.9 (12.9 62.4 (13.3) n.s. n.s. 0.02 ABV neuroticism 36.3 (25.3) 79.8 (26.5) 62.7 (13.3) <0.001 <0.001 n.s. ABV somatic complaints 16.3 (4.5) 21.2 (6.6) 26.9 (2.3) 0.01 <0.001 0.005 STAI-trait 29.5 (6.7) 44.1 (7.1) 47.7 (4.1) <0.001 <0.001 n.s. Satisfaction with life 28.1 (4.9) 18.6 (8.5) 25.5 (7.1) 0.001 n.s. 0.03 Happiness 23.5 (3.5) 16.3 (6.9) 15.1 (3.3) <0.001 <0.001 n.s. Self-efficacy 32.1 (3.8) 28.0 (4.2) 25.3 (2.6) 0.01 <0.001 n.s. PI-R ABBR, Padua Invetroy-Revised Abbreviated scale; OC, obsessive-compulsive; YASR, young adult self-report; ABV, Amsterdamse Biografische vragenlijst; STAI, State trait Inventory Discussion The most important aim of this MZ twin study has been to explore unique and shared environmental factors involved in OC symptoms. Unique and shared environmental factors unique 2004 shared 2002 2006 2004 2007 2001 1999 Finally, level of education of the parents (as a measure of socio-economic status, a risk factor reported in OCD) was not found to be associated with OC symptoms in this study, although the parents of the twins who were concordant low on OC symptoms tended to have a higher level of education than the other groups, a difference that may have failed to reach significance due to the small sample size. Finally, between-twin pair comparisons on unique life events revealed an elevated rate of dismissal in the concordant high-scoring twin pairs compared with the other pairs. Since dismissal typically represents a unique negative environmental influence on each twin of a pair, instead of being an environmental influence shared between the twins of a pair, its elevated rate among the high-scoring concordant MZ pairs is better explained as being the consequence of OC symptomatology rather than causing OC symptoms; elevated dismissal rates in these OC twin pairs might result from over-scrupulosity and slowness in work—characteristics well known in OC symptomatology—and subsequent dysfunction. Health and lifestyle characteristics 2006 2006 2000 2004 The longitudinal data 2005 2005 The parent data As parent scores on OC symptoms and related psychopathology were expected to reflect genetic vulnerability, we expected scores to be highest in the concordant high parents, to be intermediate in the discordant parents and to be low in the concordant low parents. On most measures of psychopathology, this assumption was confirmed. Thus, the intermediate scores in the parents of the discordant twins on OC, anxious-depression and neuroticism scales may be the consequence of the intermediate amount of genetic vulnerability to OC symptoms in this group. Therefore, these parent data suggest that the symptoms in the high-scoring twins of the MZ discordant group are likely to be the consequence of a moderate genetic vulnerability to OC pathology in addition to or in interaction with environmental mediators. Limitations First, sample size is small; although we sampled from a large group of MZ twins, only a small sample was retained due to the use of rigorous criteria. Consequently, especially in the within-discordant pair comparisons, some of the negative outcomes might in fact be the result of lack of power to detect within-pair differences. Alternatively we could have relaxed the stringent selection criteria, with the disadvantage of including twin pairs not scoring in the clinical range of OCD, thus representing an unclear group of problem behavior. 1998 Finally, the database used in this study was not primarily designed to specifically inquire about environmental factors, leaving some questions unanswered, especially with respect to protective environmental mediators of OC symptomatology. Conclusion This study has been a first attempt to identify characteristics of the environment associated with OC symptoms using a twin study design. Some important environmental factors involved in OC symptomatology have been identified. Two crucial questions to be addressed in future studies are: (1) what is the differential impact of the various environmental mediators on OC symptoms, and under which circumstances and at which age are they most harmful? (2) Along which lines do the environmental factors found in this study operate? Do they add to genetic risk factors, are they causal in themselves, or do they operate through gene–environment interaction? Future studies are needed to study the differential effects of environment and genes on phenotypes (and endophenotypes), and to elucidate the nature of the interplay between genes and environment.