Obsessive-compulsive disorder (OCD) is characterized by repetitive distressing and anxiety-provoking intrusive thoughts, mostly in combination with time-consuming repetitive actions designed to reduce tension or anxiety caused by the disturbing thoughts (American Psychiatric Association 1994
). OCD can run, especially if untreated, a chronic and disabling course (Nestadt et al. 1998
). Family studies have quite convincingly shown that early-onset OCD is familial (Pauls et al. 1995
; Nestadt et al. 2000
). Studies in 7- to 12-year-old twins have indicated that between 47 and 58% of the variance in obsessive-compulsive (OC) behavior is explained by additive genetic factors (Hudziak et al. 2004
). The remaining variance is almost entirely explained by unique environment, with a small contribution of shared environmental factors (16%) at age 12. In adults, twin studies have indicated a more modest contribution of genetic factors (van Grootheest 2005 and others). One twin study in women suggested heritability of 33 and 26% respectively for obsessions and compulsions (Jonnal et al. 2000
). Further, a recent twin study in 5,893 mono- and dizygotic twins, and 1,304 additional siblings from the population-based Netherlands Twin Register (NTR; Boomsma et al. 2002
), indicated heritability estimates of 47% for both men and women (van Grootheest et al. 2007a
The course of OCD is moderately stable: longitudinal twin studies as well as epidemiological and clinical studies have indicated that on average 50% of cases remit over time (van Grootheest et al. 2007a
; Angst et al. 2004
; Skoog and Skoog 1999
). Environmental factors explain about half of persistence in boys and two-third of persistence in girls. Thus, environmental factors are of substantial importance in the likelihood to obtain and persist or remit with respect to OC symptomatology.
To date, only few studies have addressed the specific nature of these environmental factors in OC phenomenology. Which environmental influences can be detected from the literature? Family studies have revealed that parents of children with OCD suffer from poorer mental health and have fewer coping strategies than parents of healthy children (Derisley et al. 2005
). A-specific risk factors for (the persistence of) OCD include: earlier age at onset, presence of co-morbid conditions and low socio-economic status (Skoog and Skoog 1999
; Stewart et al. 2004
; Angst et al. 2004
). Further, OCD patients report more often than healthy controls to have been overprotected or emotionally neglected by their parents (Cavedo and Parker 1994
). Patients with the hoarding subtype of OCD in particular, report a lack of parental emotional warmth (Alonso et al. 2004
). Perinatal risk factors, such as prolonged labor and edema during pregnancy, have been reported to increase the risk of later OCD (Vasconcelos et al. 2007
). Childhood sexual abuse appears to be an important mediator for later OCD, especially in women (Lochner et al. 2002
). The relationship between religiosity and OCD is unclear. Some authors find increased frequencies of religious obsessions and hand washing among highly religious protestants in comparison with less or non-religious subjects (Abramowitz et al. 2004
), while others find no relation between religiosity and an increase in OC symptoms (Assarian et al. 2006
), and argue that religiosity is merely a form in which OC symptoms can be displayed (religious obsessions) (Tek and Ulug 2001
). Finally, β-hemolytic streptococcal infections have been reported to be associated with OC symptom exacerbation (March et al. 1990
The comparison of monozygotic (MZ) twins who score high on a trait with their low-scoring co-twins, comprises a powerful method to identify environmental factors involved in a disorder (Martin et al. 1997
). MZ twins have identical genomes and are born and raised at the same time in the same family, thus sharing a very similar family environment. Consequently, discordance on the trait is mostly explained by differences in the non-shared (i.e., unique
) environment that act either directly on the phenotype, or by epigenetic mechanisms (Fraga et al. 2005
). Environmental factors that are shared
by both members of a twin pair (such as maternal smoking during pregnancy, or parental divorce) can be studied by comparing MZ twins who are concordant high on the trait with MZ twins who are concordant low.
Comparisons within discordant MZ pairs or between concordant MZ pairs have not been employed to study environmental factors involved in OC phenomenology. In other psychiatric disorders, such as schizophrenia and ADHD (Stabenau and Pollin 1993
; Lehn et al. 2007
), as well as in somatic disorders such as diabetes mellitus (Bo et al. 2000
), this method has been successfully used. One twin study on a disorder related to OCD, i.e., Gilles de la Tourettes’ Syndrome, has studied basal ganglia D2-receptorbinding in five MZ twins who were discordant on tic severity, and found that caudate nucleus D2 receptor binding increased by up to 17% in the more severely affected twins when compared with their less severely affected twin siblings (Wolf et al. 1996
). This within-MZ twin discordance reflects unique environmental influences on D2-caudate receptor density.
In this study, we used prospective data of adult twins from the NTR, who have been followed between 1991 and 2002, and about whom information on a wide range of variables was collected every 2–3 years (Boomsma et al. 2000
). Differences between the MZ concordant and discordant groups were described using measures of anxiety and depression co-occurring with OC behavior. The aim of this explorative study was to replicate and extend the information from previous studies on both unique and shared environmental influences that might protect against or exacerbate OC behavior. Unique environmental factors were studied using within-discordant MZ twin pair comparisons. To study environmental factors shared by both twins of a pair, between-MZ pair comparisons were used. Parent data on level of education and on drinking and smoking behavior were used to compare the groups of twin pairs on these common environment influences. Further, measures of anxiety, depression and personality were compared between the parents of the concordant and discordant twin pairs, with the following reasoning: concordance between MZ twin pairs on OC behavior most likely results from genetic similarity between the twins of a pair. Thus, the contrasts between twin pairs who are concordant high and low reflect differences in genetic vulnerability to OC behavior. As a consequence, the parent scores on OC symptoms, on anxious depression and on neuroticism (the latter characteristics are known to be related to OC symptoms) are expected to reflect these differences in genetic vulnerability and therefore to be highest in the parents of the concordant high MZ pairs, to be intermediate in the parents of the discordant MZ pairs and to be low in the parents of the concordant low MZ pairs.
Finally, longitudinal measures of psychopathology were studied to investigate age at onset of OC symptoms, anxiety and depressive symptoms in the concordant and discordant groups. Family studies have suggested that lower age at onset is associated with higher familiarity, possibly reflecting higher genetic load (Delorme 2005
). We hypothesized that the concordant high MZ twin pairs, in whom the OC symptoms are theoretically more genetically determined, would show lower age at onset than the high-scoring twins of the discordant group in whom unique environmental factors might be more important.