To determine whether maternal and paternal maximum consumption have independent effects, we examined interactions between them. Only one of 17 tests was significant – the interaction for early drug use in the older cohort – and then only marginally, χ2 = 3.85, p = .050 with 1 df. The coefficient was opposite in sign to what would be expected and was accompanied by inflated coefficients for the main effects, indicating that estimates were likely unstable. All other effects produced χ2 values less than 1.
