Jenae M. Neiderhiser, a professor of psychology at Pennsylvania State University, says adoptive families were used in the study because the researchers could focus on environmental factors, since there are no shared genetic factors if a child does not share genes with a parent.
"It is important to understand how parenting comes in to play here," Neiderhiser says. "Looking at the marital relationship is not direct parent-child interaction, but it is an index of stress in the family."
The researchers interviewed 357 sets of adoptive parents both together and separately, assessing their habits and emotions as well as their children's behaviors. The parents were interviewed twice -- first when their children were 9 months old, and again at 18 months.
Parents were asked a series of questions, including, "Have you or your partner seriously suggested the idea of divorce?"
The study, published in the journal Child Development, showed marital conflict during the first survey at 9 months predicted a child would be more likely to have sleep problems at the time of the second survey at 18 months.
"Our study suggests that marital instability is impacting change in the child's sleep patterns over time, and it could be that this is setting the child up for a pattern of problematic sleep," Neiderhiser says.