That children growing up in poverty are at a much greater risk for a variety of developmental issues as they become older is a well-known, if tragic, reality.

Earlier this year, a study from the University of North Carolina found that as many as 25 percent of children experiencing homelessness are in urgent need of mental health services as they approach elementary school, rates much higher than among the general population.

Slippier though is our understanding of how exactly these children’s brains and bodies respond to the burdens of financial scarcity. Now, researchers in Child Development may have stumbled upon at least a partial answer.

Their study of 201 low-income children over a three-year period was able to identify three distinct profiles of cortisol levels — cortisol being a hormone believed to be strongly tied to the regulation of stress. They also found that children with relatively low or high levels of cortisol were more likely to test poorly in measures of cognitive functioning. These impaired levels were likewise associated with greater family instability or reported adversity, according to a press statement released by The Society for Research in Child Development, which oversees the journal.

In the first year of the study, the researchers observed how the children’s mothers interacted with them alongside extensively documenting their family situation, including the prevalence of domestic violence. In the third year, at age four, they tested the children’s cognitive abilities. And along the way, each year, they collected saliva swabs, which allowed them to measure the children’s cortisol levels as they grew.

About 30 and 40 percent of the children had either relatively high or low levels of cortisol respectively, compared to the moderate group. "We found that children's cortisol levels remained relatively stable across the three years," said lead author Jennifer H. Suor, a doctoral student in clinical psychology at the University of Rochester. "And we discovered that exposure to specific forms of family adversity when children were two years old predicted their cortisol profile, which in turn was linked with notable differences in children's cognitive functioning at age four."

More specifically, children who were treated worse by their mothers appeared to have higher levels. High or low though, it was those in the middle who were more likely to experience less adversity and maintain high cognitive ability.

We’re still not sure what role cortisol specifically plays in ensuring proper developmental skills, though the authors note there are some prevailing theories. "The exact mechanisms through which too much or too little cortisol affects cognitive functioning aren't fully understood," said co-author Dr. Melissa L. Sturge-Apple, also of the University of Rochester. "Researchers hypothesize that too much cortisol can have toxic effects on parts of the brain that are important for cognitive functioning, and too little might hinder the body's ability to recruit the biological resources necessary for optimal cognitive functioning."

Dr. Sturge-Apple believes their study’s conclusions may lead to further clarification of the risk factors that disproportionately affect poor children, as well as illuminating what sort of outreach programs can best help them. "The findings can inform preventive interventions, especially those that can reduce family stress and strengthen parent-child relationships, because these may promote healthy cortisol levels in children and, in turn, may result in positive cognitive outcomes,"she said.

Source: Suor, JH, Sturge-Apple ML, Davies PT, et al. Tracing differential pathways of risk: associations among family adversity, cortisol, and cognitive functioning in childhood. Childhood Development. 2015