Although we have limited power, because of its substantial theoretical interest, we explore qualitative sex effects in our models—that is are the same genetic and environmental factors influencing risk of CB in both sexes? Qualitative sex effects are captured by the parameters rg and rc that reflect, respectively, the degree to which the genetic or shared environmental risk factors are correlated in the two sexes. In our models, we examine separate estimates for rg in Ac and As and rc in Cc and Cs, termed, respectively: rac, ras, rcc, and rcs. (Note that three values of ras and rcs have to be estimated, one for each phenotype.) We also examine quantitative sex effects—that is whether the genetic and environmental parameter estimates in the model can be constrained to equality or significantly differ between the sexes.
