- Methods for estimating heritability and investigating the genetic architecture of complex traits. Recent work focuses on the heritability and genetic architecture of human psychiatric disorders and substance use and dependence. This includes identifying genetic variants that contribute to disorder liability, integrating whole genome and transcriptome data, understanding gene-by-environment interactions, and testing how these factors influence risk prediction. Selection and adaptation. Identifying quantitative and molecular signatures of selection, both recent positive selection and purifying selection. Using large-scale reciprocal transplant experiments to test gene-by-environment interaction hypotheses and to understand how environments shape complex traits in humans and in other species. Much of this work has been with the ecologically and economically important tree genus, Populus, but also with Salix.