For each outcome of interest reported by ≥2 twin studies, we transformed the correlation coefficients to Fisher Z values and performed a random-effects meta-analysis to derive pooled effect estimates. This transformation was beneficial because the standard error of a correlation depends on the correlation itself, making larger correlations appear more precise and thus receiving more weight. In contrast, the Fisher transformation only depends on the sample size. We conducted these analyses twice: once assuming that the standard error was that of the Pearson correlation and once assuming that it was that of the tetrachoric correlation. The latter assumes that the presence of OCS represents latent variables that follow a bivariate normal distribution. To use the standard error of the tetrachoric correlation, we first estimated the number of concordant and discordant twin pairs for the presence of OCS according to a 2.3% lifetime prevalence of OC [61] then using the “polychor” function to derive the standard error [62] and finally calculating the “effective” sample size to perform the meta-analysis, as described in Polderman et al. [63].
