Scientists believe that the behaviour, which is characterized by an inappropriate willingness to approach adults, including strangers, is rooted in brain adaptations associated with early-life experiences.
The UCLA group used functional magnetic resonance imaging (fMRI) to demonstrate that youths who experienced early maternal deprivation, specifically, time in an institution such as an orphanage prior to being adopted, show similar responses to their adoptive mother and to strangers in a brain structure called the amygdala.
For children never raised in an institutional setting, the amygdala is far more active in response to the adoptive mother.
This reduced amygdala discrimination in the brain correlated with parental reports of indiscriminate friendliness.
First author Aviva Olsavsky, a resident physician in psychiatry at the Semel Institute for Neuroscience and Human Behavior at UCLA, said that the findings suggest that even for children who have formed attachments to their adoptive parents, this early period of deprivation has led to changes in the brain that were likely adaptations and that may persist over time.
Senior author Nim Tottenham, an associate professor of psychology at UCLA, said that the stranger anxiety or wariness that young children typically show is a sign that they understand their parents are very special people who are their source of security.
The study is published in the peer-reviewed journal Biological Psychiatry.