An example of how genetic confounding works (note, only one parent drawn, for simplicity). Parents share genes related to reading ability with their children, and also control the number of books in their home. This creates gene-environment interplay. It is important to note that the environmental effect may still have a causal role, even with gene-environment interplay. If genes play a role but are not modeled (as in Figure 1), the correlation between the environmental measure and the child’s trait is genetically confounded. Here, the role of genes is modeled, allowing for an estimation of the genetic effect and the environmental effect.