Most of the individual difference variance in the population is found "within" families, yet studying the processes causing this variation is difficult due to confounds between genetic and nongenetic influences. Quasi-experiments can be used to test hypotheses regarding environment exposure (e.g., timing, duration) while controlling for genetic confounds. To illustrate, two studies of cognitive self-regulation in childhood (i.e., working memory, effortful control, attention span/persistence) are presented. Study 1 utilized an identical twin differences design (N = 85-98 pairs) to control for genetic differences while using relative twin birth weight difference to predict relative twin difference in working memory and effortful control. Larger relative twin difference in working memory and effortful control was predicted by the combination of shorter gestation and larger relative birth weight difference. Study 2 utilized an adoptive sibling relative difference design (N = 123 same-sex pairs) to control for genetic similarity while using relative sibling difference in age at time of adoption to predict relative sibling difference in attention span/persistence. Larger relative sibling difference in attention span/persistence was predicted by the combination of larger relative difference in time in the adoptive home and age at adoption. Within-family quasi-experimental designs allow stronger inferences about hypothesized environmental influences than between-family designs permit.