240 Obsessive-Compulsive and Related Disorders
may develop the sudden onset of obsessive- compulsive symptoms, which has been asso-
ciated with different environmental factors,  including various infectious agents and a
post-infectious auto immune syndrome.
Genetic and physiological. The rate of OCD among first-degree relatives of adults with
OCD is approximately two times that among fi rst-degree relatives of those without the
disorder; however, among first-degree relatives of individuals with onset of OCD in child-
hood or adolescence, the rate is increased 10-fold. Familial transmission is due in part to
genetic factors (e.g., a concordance rate of 0.57  for monozygotic vs. 0.22  for dizygotic twins).
Dysfunction in the orbitofrontal cortex, anteri or cingulate cortex, and striatum have been
most strongly implicated.
Culture-Related Diagnostic Issues 
OCD occurs across the world. There is substant ial similarity across cultures in the gender
distribution, age at onset, and comorbidity of OCD. Moreover, around the globe, there is a
similar symptom structure in volving cleaning, symmetry, hoarding, taboo thoughts, or
fear of harm. However, regional variation in symptom expression exists, and cultural
factors may shape the content of obsessions and compulsions.
Gender-Related Diagnostic Issues
Males have an earlier age at onset of OCD than females and are more likely to have co-
morbid tic disorders. Gender differences in the pattern of symptom dimensions have been
reported, with, for example, females more likel y to have symptoms in the cleaning dimen-
sion and males more likely to have symptoms in the forbidden thoughts and symmetry di-
mensions. Onset or exacerbation of OCD, as well as symptoms that can interfere with the