Robertson and Robertson, who were collaborators of Bowlby's, researched the effects of young children separated from their parents under circumstances such as the child spending time in hospital or being sent to a residential nursery while the mother or main caregiver was in hospital. After having done detailed observations, they suggested that when children are first separated from their mother or mother substitute they go through three stages of reaction known as the syndrome of distress: protest, despair and detachment.
During the protest stage children attempt to follow the mother, scream and do everything they can to recover her. They will continue looking for her long after she has gone.
During the despair stage they will often sob but in a more helpless way making fewer attempts to find the mother. They tend to show a distinct loss of hope.
Finally during the detachment stage they appear calm and even settled. However, this apparent calm often masks underlying distress. The child appears to be emotionally flat, not really responding very much to anything. After the mother returns and they are reunited, the detachment may persist for a while but it is then likely to be followed by ambivalence to the mother, clinging and anxiety and hostility whenever she leaves. These were classed as the short term affects.
If the child experiences many separations in his childhood then the behaviour patterns such as detachment or despair may persist into life as intellectual retardation or depression. The most serious consequence of separation is the development of an affectionless character. This is seen in children or adults who, although at first glance are sociable, have no real concern for others or capacity to care for people. They fail to develop a conscience or feelings of guilt, and tend to be persistent liars, thieves or psychopaths. These were classed as the long term affects.
As well as Robertson and Robertson, Bowlby had many other sources of evidence to back up his theory.