Twin and Family Studies
One of the main ways that researchers try to figure out whether a disorder or disease has a genetic basis is by using twin studies. In a twin study, researchers compare the data of a large group of identical/monozygotic (MZ) twins with data from fraternal/dizygotic (DZ) twins. MZ twins have identical genes, whereas DZ twins share only as many genes as regular siblings do. Therefore, if an MZ twin of someone with OCD is at a greater risk of having the disorder than a DZ twin, there is probably a strong genetic component to it.
In the 1980s, Carey and Gottesman published a study in the Archives of Genetic Psychiatry, which studied pairs of MZ and DZ twins, in which one twin of each pair had been diagnosed with obsessive tendencies. This study found a heritability estimate of around 80%, which means that the disorder has a very strong genetic tendency. Other studies followed with similar findings, but these were of limited value for two reasons. First of all, they had no standardized diagnostic criteria to define OCD, and secondly the researchers knew exactly what they were searching for, which prevented them from being "blind" from the results of the study.
Although the diagnostic criteria developed as the years progressed, it wasn't until decades later that a study was published in Psychological Medicine in which this concern was addressed. This study looked at twins in a national registry and discovered that MZ twins had a significantly higher correlation than DZ twins. Family studies, which are slightly less accurate than twin studies, also seem to show a strong genetic component, although it seems that early-onset OCD is more likely to be familial than late-onset OCD.