Attachment theory describes the dynamics of long-term relationships between humans. Its most important tenet is that an infant needs to develop a relationship with at least one primary caregiver for social and emotional development to occur normally. Attachment theory explains how much the parents' relationship with the child influences development. Attachment theory is an interdisciplinary study encompassing the fields of psychological, evolutionary, and ethological theory. Immediately after World War II, homeless and orphaned children presented many difficulties, and psychiatrist and psychoanalyst John Bowlby was asked by the UN to write a pamphlet on the issue which he entitled maternal deprivation. Attachment theory grew out of his subsequent work on the issues raised. Infants become attached to individuals who are sensitive and responsive in social interactions with them, and who remain as consistent caregivers for some months during the period from about six months to two years of age, this is known as sensitive responsiveness. When an infant begins to crawl and walk they begin to use attachment figures (familiar people) as a secure base to explore from and return to. Caregivers' responses lead to the development of patterns of attachment; these, in turn, lead to internal working models which will guide the individual's perceptions, emotions, thoughts and expectations in later relationships. Separation anxiety or grief following the loss of an attachment figure is considered to be a normal and adaptive response for an attached infant. These behaviours may have evolved because they increase the probability of survival of the child.