Preschoolers whose parents are depressed get stressed out more easily than kids with healthy parents, but only if their mothers have a negative parenting style, according to a new study.

The research, set to be published in an upcoming issue of the journal Psychological Science, measured the levels of the stress hormone cortisol in kids' saliva after mildly stressful experiences, such as interacting with a stranger. The researchers found that cortisol spikes were more extreme in kids whose parents had a history of depression and also exhibited a critical, easily frustrated parenting style.

"It's actually quite hopeful, because if we focus on the parenting, we could really intervene early and help parents with chronic depression when they have kids," study author Lea Dougherty, a psychologist at the University of Maryland, said in a statement.

Stressful strangers
Earlier studies have found that people with depression often have abnormal cortisol spikes in response to stress, suggesting that problems with the body's stress-regulation system are a risk factor for — or at least a hallmark of — depression. Several studies have found these abnormal reactions in very young babies of depressed mothers, which could mean the system is disrupted either in utero or very early in life.

But it's difficult to tease out the early influences on the body's stress hormone system. Genetics are likely partially to blame, Dougherty and her colleagues wrote. The changes could come about because of biochemical influences in the womb or because of the way depressed moms interact with their babies. Most likely, it's a combination of all of these factors.

To find out whether parenting style matters, the researchers recruited 160 3- and 4-year-olds and their parents. Half of the children were boys and half were girls, and most were white and middle class.

First, the researchers evaluated the moms and dads for a history of depression. Next, they scheduled two lab visits for the parents and children. During one, the parent played with the child while the researchers observed the interactions for signs of criticism, frustration and anger on the part of the parent. In 96 percent of cases, the mother was the one who brought the child to the lab, so the researchers collected little data on father-child interactions.

During the other lab visit, the child played a variety of fun games with an experimenter, interspersed with a few activities designed to elicit stress and frustration. In one activity, the experimenter left the room and an adult male stranger came in to talk to the child. In another, the experimenter gave the child a transparent locked box with a toy in it, along with a key that didn't fit the lock. The final stress-inducing activity involved promising the child a gift but instead giving him or her an empty box.

After each stressful experience, the child's feelings were soothed, Dougherty said.

"After each one of these, you come back in, and you're like, 'Oh, my gosh, I forgot the present! Here it is,'" she said. "Everything's kind of remedied."

During the experiment, the researchers used cheek swabs to measure the children's levels of cortisol. More cortisol indicates a higher level of stress.

Cortisol spikes and parenting style
Just having a depressed parent didn't make kids more prone to cortisol spikes, but having a depressed mother with a hostile parenting style did. The study was just a one-time snapshot of stress response, so researchers can't say for sure that hostile parenting by depressed parents causes the spikes, just that there is a correlation.

Longer-term studies are needed to establish causation, the researchers wrote. The study also included few depressed fathers, leaving the relationship between paternal depression and child stress largely unexplored.

Nonetheless, the findings are important, the researchers wrote, because early stress is a risk factor for later depression. If parenting style interacts with genetic and other environmental influences to send kids' stress sky-high, early treatment may help, Dougherty said. Helping parents interact positively with their kids might be especially important early in life, the researchers wrote, because the stress regulatory system is still developing.

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