The statistical significance of the genetic and environmental covariances was assessed by comparing twice the log-likelihood of the full Cholesky model to that of a sub-model in which the genetic or environmental covariation between morphometric measures and pack-years was set to zero (df = 1). Under certain regularity conditions, such differences are asymptotically distributed as chi-squared with one degree of freedom. A third sub-model tested the significance of the phenotypic correlation by setting both the genetic and unique environmental covariances between nicotine use and cortical volumes to zero (df = 2).
