Researchers say that women struggling with long-term pressures, such as depression or poverty, are more likely to be hostile, insensitive and neglectful toward their toddlers.
The study showed that mothers with higher depressive symptoms exhibited overactive stress responses, while moms who struggled with poverty had underactive responses, with each extreme being associated with distinct types of problematic parenting.
"Stress gets under your skin," explains Melissa Sturge-Apple, assistant professor of psychology at the University of Rochester and lead author.
"Stress is not just in our heads, it's in our bodies.
"It literally changes the way a mother's body responds to the normal demands of small children and those changes make it much harder to parent positively," she added.
Fred Rogosch, research director at the University of Rochester's Mt. Hope Family Centre and a fellow author on the paper measured participants' response to stress using a novel wireless electrocardiograph (ECG) monitor developed for the study by University of Rochester engineers Zeljko Ignjatovic and Wendi Heinzelman.
The unobtrusive device allowed the team to analyze subtle changes in participants' heart rhythms as they were happening.
In the study, the researchers observed 153 mothers and their 17-to-19-month-old children in individual two-hour sessions.
Although the popular image of depression is of someone who is listless and sad, Sturge-Apple points out that the study confirms what clinicians have long observed: that depression in mothers sometimes is linked to harsh, highly reactive parenting, not subdued mothering.
By contrast, study participants who struggled with poverty and lived in high-crime neighborhoods exhibited underactive, or "hypoactive," stress response systems. Their heart rates patterns began lower and rose little during their child's distress, researchers found.
The study will be published in the journal Development and Psychopathology.