Biologically, this group of monkeys had lifelong elevated levels of a major stress hormone in their spinal fluid, indicating an abnormality in their stress apparatus.5 That also adds to the propensity for addiction, since both animals and humans use substances or other behaviors to modulate their experience of stress.6 Obviously it’s not a question of the mothers in the other two groups having been “better” parents but of the stresses afflicting the variable-foraging mothers as they were nursing their infants—uncertainty being a trigger for physiological and emotional stress.
