According to Bowlby, in these circumstances children will display a relatively clear set of behaviours from protest, such as crying, lashing out, screaming and clinging, to despair, where the protest behaviours subsides and the infant becomes calmer and apathetic.
They internalise their anger and respond little to offers of comfort. They opt to comfort themselves by, for example, sucking their thumb.
The last stage is detachment, where the infant resumes their responses to other people but treats them warily. They might reject the caregiver on return and continue to display signs of anger.
Generally referred to the PDD model (Protest, Despair, Detachment). Barrett (1997) has emphasised the role of individual differences. Barrett argues that infants who are securely attached and more mature may well cope better with separation.