Models 7, 8 and 9 set to zero, respectively, the single shared environmental, genetic and individual-specific environmental common factors. Models 8 and 9 fitted very poorly, indicating the need for genetic and individual-specific influences common to all the AD criteria. Surprisingly—given prior evidence of the absence of shared environmental effects for AD as a categorical diagnosis in this sample10,11,36— model 7 also fitted more poorly than model 6, suggesting the importance of shared environmental effects.
