The differences between rats that got licked a lot and those that got licked only a little do not emerge from differences in their genes. Meaney found that out in an experiment involving newborn rat pups. He took pups whose mothers who didn’t lick much and placed them with foster mothers who licked a lot, and vice versa. The pups’ experience with their foster mothers—not the genes they inherited from their biological mothers—determined their personality as adults.
The Brain: The Switches That Can Turn Mental Illness On and Off | Genes & Health | DISCOVER Magazine