Twin models, in Mx (Neale, 2004), were used to estimate the influence of latent risk factors on AD symptoms. These latent factors include: A or additive genetic risk factors, C or shared environmental risk factors and E or non-shared environmental risk factors (Fisher, 1918; Jinks and Fulker, 1970; Neale and Cardon, 1992). ‘A’ refers to the extent to which genetic factors influence the total population variation in a trait. MZ twins who share all their genes have a genetic correlation of 1.0 whereas DZ twins who, on average, share only half their segregating genes, have a genetic correlation of 0.5. Similarly, ‘C’ refers to the latent environmental risk factors that make members of a twin pair that are reared together similar to each other – C is correlated 1.0 across MZ and DZ twins (Eaves et al., 2003; Kendler and Gardner, Jr., 1998).
