The exploratory factor analysis of the genetic correlation matrix produced four eigenvalues above unity: 9.88, 3.19, 1.85, and 1.53. A scree plot was consistent with an inflection break at either three or four factors. The fourth factor identified a coherent factor of five disorders (genetic factor 4 below) and so merited retention. By contrast, a fifth factor included only one syndrome with a substantial loading (eating disorders)—a clear sign of overextraction. Furthermore, the four-factor solution provides a reasonable summary of the matrix of genetic correlations seen in Table S3 in the online data supplement.
