Most of the original twin studies used a model-fitting approach whereby non-significant parameters were sequentially dropped from the model. In such cases, it is standard practice to present reduced models with only the significant parameters. Accordingly, the model parameters that are reported in the original articles are typically not the full statistical model that would be required to reconstruct the data for meta-analysis, but the most parsimonious model for the specific study. However, most studies reported the tetrachoric correlations or odds ratios (ORs) and the prevalence rates, which allowed us to reconstruct the raw data with equivalent population parameters. Tests of heterogeneity within the twin models relied upon this recreated data (effectively resulting in a mega-analysis). Among the adoption studies, some presented ORs while others presented the percent affected in the sample. To put everything on the same metric and to make the adoption studies comparable to the twin studies, we generated raw data from contingency tables from the adoption studies and then computed tetrachoric correlations.
