These findings from researchers at King's College in London, published in the November/December issue of the journal Child Development, suggest that the environment in which teenagers find themselves has an impact on their risk of depression independent of their genetic background, which is already known to be a strong predictor of depression.
The findings are important because teenage depression is a major contributor to teenage suicide. While previous studies found that individual environmental aspects in a teen's life (which may include favoritism in parenting, bullying of one child but not a sibling, and different friendship circles), are more important than shared environmental factors between siblings (which may include family poverty) in determining the risk of depression, very little research has considered the influence of genetics compared to environmental aspects on the development of depression.
To investigate this question, the researchers asked 328 identical twins aged 12 to 19 and their parents to complete information about depressive symptoms, parenting measures, life events and peer group characteristics.
Questions assessed the use of punitive discipline (i.e., do the parents yell at their children when the child has done something wrong) vs. constructive discipline (i.e., do parents talk to their child when the child has done something wrong), adverse events the child could not control (such as the death of a parent or close friend, or a parent's loss of employment) and adverse events over which the child had some control (such as breaking up with a boy/girlfriend and suspension from school).
They found that the twin with the greatest number of adverse events over which they had some control (twin A) was most likely to have higher levels of depressive symptoms, particularly if their co-twin (twin B) had suffered multiple uncontrollable adverse life events.
One possible explanation for this finding is that twin A may be blaming themselves for their role in generating the adverse event, while twin B had no control over the event. Alternatively, the family may be more sympathetic to a child who has had a run of bad luck compared to a child who has caused his or her own problems. This lack of sympathy from the parents may contribute to the depression of twin A.
The researchers also found that after episodes of depression adolescents experienced increased levels of punitive discipline from their parents, particularly their mother. This, in turn, may increase the risk of another round of depression.
The researchers also learned that having a pro-social peer group (for instance, having peers who were focused on attending college and had other positive goals) appeared to protect the teens against the development of depression, regardless of their genetic background.
"These findings are useful as they show that not only can parenting and life events cause depression, but that depression can lead to changes in life events and parenting," said lead author Holan Liang, MA, MBBChir, MRCPsych, a researcher at King's College in London.
Additionally, she notes, it suggests areas for future research. Specifically, helping teens learn to reduce adverse life events and promote prosocial peer relationships may help prevent adolescent depression.
Summarized from Child Development, Vol. 76, Issue 6, A Monozygotic Twin Differences Study of Nonshared Environmental Influence on Adolescent Depressive Symptoms by Liang H and Eley TC, (King's College, London). Copyright 2005 The Society for Research in Child Development, Inc. All rights reserved.