Learning through reinforcement and punishment. If a behaviour produces a pleasant consequence, is likely to be repeated. Explains why babies cry for comfort. Crying leads to result from caregiver (e.g., feeding), so crying reinforced. Reinforcement = two-way process. Same time as baby is positively reinforced, caregiver receives negative reinforcement because the crying stops (escaping something unpleasant). This interplay of mutual reinforcement strengthens an attachment.