A growing body of science shows the critical effects of an extreme and sustained stressful environment for children on their developing brain architecture and the expression of genes in later life. Toxic stress can shift the brain into surviving in a way that's more rigid and less adaptive. For example, as a result of biologically altered brain processing, children who have suffered physical abuse may be predisposed to imagine anger in otherwise ambiguous faces. The implications of the science for public-policy decisions are far-reaching. Science reinforces the urgent need to focus on early childhood development, and not to shortchange it, thinking we can compensate later.