Adopted child syndrome is a set of symptoms that are associated primarily with children who have been adopted or separated from their parent/s at a very early age. Children who have been adopted have a higher incidence of behavioural disorders, criminality, and mental illness than those who have not been adopted.

This theory states that the relationship that a child has with his or her mother is the most important relationship in the world, and when the mother is parted from her child, it causes a psychological wound that can only be healed through the reunion of mother and child. The trauma caused by the severing of the individual from his or her mother lies at the core of what is peculiar to the psychology of the adopted child.

Marshall D. Schechter observed in many case studies on adopted children symptoms relating to such things as fantasies and "act out|acting out" regarding the real parents, i.e. their appearance, their names, and killing and murder, especially towards their real mother. Adopted children may suffer symptoms of depression, feelings of incompleteness, phobic fear of abandonment, anxiety, aloofness, and distancing themselves as to make close interpersonal relationships impossible. Adopted children, according to this theory, quite often have superficial relationships, which are dominated by a driving need to have their impulses satisfied immediately. Adopted boys in particular had problems with lying, stealing, and lack of integration with others according to Schechter.

Michael Humphrey found the adopted children he studied suffered from varying degrees of parental deprivation, neglect, parental rejection, or at the opposite extreme, over-indulgence, mental or physical illness sufficient to impair the quality of parental love, and jealousy of a sibling born before or too soon after adoption.

This theory asserts that many adoptees go on to develop borderline personality disorder, with symptoms including impulsivity or unpredictable behaviour, unstable and intense personal relationships with idealisation, devaluation, manipulation and inappropriate intense anger. Quite often these individuals manifest uncertainty about their identity, intolerance of being alone, affective instability, physically self-damaging acts, and chronic feelings of boredom and emptiness, according to this theory.

Kirshner D. Nagel noted that adoptees tend to exhibit more anti-social behaviour than non-adoptees. It was also found that adopted children showed a greater occurrence of overt destructive acts and sexual acting-out. Some adoptees set fires or behave promiscuously. Truancy and academic underachievement are common amongst these children.

Kaplan S. Silverstein noted the seven core issues of adoption, which are:

1. Loss
2. Rejection
3. Guilt and shame
4. Grief
5. Identity
6. Intimacy
7. Control

Bowlby noted that some adopted children never totally bonded with their adopted mothers because the adoptive mothers felt as if they had accepted the infant, but the infant had not quite accepted them.

Abandoned child syndrome is a very closely related phenomenon that applies to children who were not adopted in the strict sense of the word, but felt abandoned.