1. in animal behavior studies, the exchange of offspring between litters as a means of separating the effects of genetics from early experience. Wild rats reared by laboratory rats display less aggressive behavior, and mice from a polygynous species with low levels of territorial aggression that are cross-fostered to monogamous territorial mice display increased aggression and have patterns of brain neuropeptides more similar to their foster parents than to their natural parents. See also sexual imprinting.

2. a similar technique used for investigating the effect of genetic factors in the development of a disorder. It involves either (a) having the offspring of biological parents who do not show the disorder reared by adoptive parents who do or (b) having offspring of parents who show the disorder reared by parents who do not. Children cross-fostered in this manner are called index adoptees, whereas control adoptees are children whose biological parents and adoptive parents do not show the disorder.