Se Joo Kim and Chan-Hyung Kim concordance rate of obsessional neurosis among ten pairs of Japanese MZ twins, as compared to a 50% concordance rate among four pairs of DZ twins. Carey and Gottesman5 found an 87% con- cordance rate of obsessive symptoms and features between MZ twins (47% in DZ twins) from the Maudsley Twin Register. In addition, the response to clomipramine6 or sertraline7 has appeared to be more similar in MZ twins than for DZ twins. Most recently, van Grootheest et al.8 reviewed past twin researches. They concluded that only the studies which used a dimensional approach and analyzed the data with Structural Equation Modeling have convincingly shown that obsessive-compulsive (OC) symptoms are heritable in children, with genetic influences in the range of 45% to 65%. In adults, studies suggest a genetic influence on OC symptoms, ranging from 27% to 47%. In sum- mary, although most twin studies have important limitations, such as inconsistent diagnostic criteria and small sample sizes, all the results consistently found higher concordance rates for MZ twins than for DZ twins. However, the fact that the MZ concordance rate is not 100% indicates that envi- ronmental factors, such as birth complications and other physiological vulnerabilities, may have an impact on the development of OCD.9 FAMILY STUDIES Because family is not only a unit for genetic transmission, but also a transmission of environ- mental and cultural backgrounds, familial aggre- gation is not sufficient to prove that the disorder is genetically transmitted. The family study method compares the rates of illness in families with those in the general population. There are two types of methods in family study: the family history method and the family study method. Family history method The family history method is used to determine the prevalence of a disease in relatives by relying on a single informant to report diagnostic infor- mation on all first-degree relatives. Lewis et al.10 reported a rate of 32.7% for pronounced obsessive traits in a sample of 306 first-degree relatives of Yonsei Med J Vol. 47, No. 4, 2006 patients with OCD. Rasmussen and Tsuang11 presented a family history of a clinical sample of 44 patients who met the DSM-III criteria for OCD. They reported that 4.5% of parents of the OCD patients met the full criteria for OCD and another 11.4% of parents had significant OC traits. A number of first-degree relatives of these patients also had probable OCD or obsessive traits. Riddle et al.12 reported that 71% of the clinically referred children with OCD had a parent with either OCD or OC symptoms. Family study method The family study method uses a strategy of obtaining diagnostic data by directly interviewing all available relatives. This method relies on a personal assessment of all first-degree relatives to obtain diagnostic information. In 1992, Bellodi et al.13 reported that the morbidity risk for OCD in patients' families accounted for 3.4%. When 21 patients with an age of onset under 14 years were examined, the morbidity risk in first-degree rela- tives reached 8.8%, compared with 3.4% among the relatives of 71 later-onset probands.13 Using a structured interview, Paul et al.14 directly inter- viewed all available first-degree relatives of 100 probands with OCD. They found that the rates of OCD and subthreshold OCD were significantly greater among relatives of probands with OCD (10.3% and 7.9%, respectively) than among the comparison subjects (1.9% and 2.0%, respectively). Nestadt et al.15 also reported that the lifetime pre- valence of OCD was significantly higher in cases with OCD relatives when compared with control relatives (11.7% vs. 2.7%). Probands with tics or OC personality disorder were not more likely to have relatives with OCD than those without these features. Recently, two direct interview family studies of OCD were published. Fyer et al.16 reported that a significantly higher risk for OCD, but not other anxiety disorders, was found in relatives of OCD probands when compared to relatives of controls. There was no relationship between the proband age at onset of OCD and the strength of familial aggregation. However, the same group17 found that, when relative diagnoses were derived without the benefit of proband informant reports, no evidence of familial OCD