Carried out at the University of Toronto with researchers from McMaster University and the University of Rochester, the study appears in the journal Child Development.

"Past studies have looked at the effects of differential parenting on the children who get more negative feedback, but our study focused on this as a dynamic operating at two levels of the family system: one that affects all children in the family as well as being specific to the child at the receiving end of the negativity," explains Jennifer M. Jenkins, Atkinson Chair of Early Child Development and Education at the University of Toronto, who led the team.

The study looked at almost 400 Canadian families with children whose average age ranged from 2 to 5. Most previous studies of differential parenting have included only sibling pairs, making it difficult to determine the dynamics that affect the whole family and those that affect individual children; this study included up to four children per family and used special statistical techniques to differentiate between dynamics operating across the whole family and those specific to individual children. Information came from mothers' reports and observation in the home.

The researchers also constructed a cumulative risk index to gauge the number of stressful circumstances in the mother's current or past life, such as single parenting, low income, past abuse, and safety in the home. Mothers with a lot of risk factors were found to be more differential in how they treated their children than moms whose lives were less stressful. Mothers with more risk factors showed a wider range in the amount of warmth and affection they showed and how harsh and irritable they were with different children in the family. Such cumulative risk has been associated with increased mental health problems in children, such as aggression, attention, and emotional problems.

Differential parenting had a stronger effect at the family level than in the way it affected individual children, the study found. When siblings in families were parented very differently, all children in those families showed more mental health problems. "In all likelihood, this occurred because differential parenting sets up a dynamic that is very divisive," Jenkins notes.

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