The group’s findings were published Monday in the Proceedings of the National Academy of Sciences.

“Originally, I think we thought that we’d see it with the mothers of these kids,” said Colter Mitchell, lead author of the study and a faculty research fellow at the University of Michigan’s Population Studies Center and Survey Research Center. “But I think more surprising is that we see it as young as 9.”

The scientists were surprised to find significant associations between the shortening of the boys’ telomeres and low family income, low levels of maternal education, family instability and a harsh parenting style, compared with boys who came from higher-income and more stable and nurturing backgrounds. In addition, disadvantaged boys who had a genetic sensitivity to dopamine and serotonin — neurotransmitters connected with happiness and feeling pleasure — experienced accelerated shortening of their telomeres, pushing them farther down the road toward stress and sickness.

Telomeres are repetitive sequences of DNA found at the ends of chromosomes that function as a sort of cap to protect the genetic information when the DNA replicates. The telomeres become shorter each time DNA replicates, and studies have shown that stress accelerates that shortening , serving as a sort of genetic weathering that’s similar to aging.

Researchers examined the DNA of a small group of 9-year-old African-American boys who had experienced chronic stress as a result of growing up in families with poor socioeconomic status. They found that the boys’ telomeres were shorter than those of boys the same age and ethnicity who came from advantaged families.

The stress of growing up in a poor and unstable household affects children as young as 9 years old on a genetic level, shortening a portion of their chromosomes that scientists say is a key indicator of aging and illness, according to a study released Monday . The researchers say their findings are the first that document this type of genetic change among minority children and make a strong case for the importance of early-childhood intervention in vulnerable communities.

Poverty’s physical toll

Scientists have already documented how the stress of being poor can have an intense physical effect on people, literally making them sick.

But the relationship between poverty and telomeres is a relatively new field of research. Elizabeth Blackburn, a biology and physiology professor at the University of California at San Francisco, pioneered modern telomere research by deciphering their molecular structure and was awarded the Nobel Prize in 2009 for her work.

Subsequent studies have shown that shortened telomeres can serve as a genetic indicator for signs of aging and disease in adults. Recent research has examined the connection between stressful environments and telomere length in children, finding that exposure to violence, for example, meant shorter telomeres in 10-year-olds and that babies and toddlers who spent more time in Romanian orphanages had shorter telomeres than those who had briefer stays.

But most of this research has been conducted on white children, Mitchell said. He and his team wanted to know if telomere length could serve as a biomarker for the stressful effects of poverty and family instability before disadvantaged children feel those effects as grown-ups in the form of diseases.

To find out, the researchers looked at a group of nearly 3,000 black 9-year-old boys and their mothers who had been part of the Fragile Families and Child Wellbeing Study at Princeton and Columbia universities, a project tracking a large population of children born from 1998 to 2000 in urban areas of the U.S., mostly to unmarried parents.

They selected a small sample of 40 of the boys: 20 who were the most disadvantaged in terms of family income, the mother’s level of education, whether the mother had ever suffered from depression, the harshness of parenting style and the stability of the family structure — factors they considered highly influential on the stress of a child’s environment — and 20 who were the most advantaged in terms of those factors.

During their analysis, they found significant associations between the disadvantaged boys and shortened telomere length, compared with their advantaged counterparts. For example, doubling a family’s income was associated with telomeres that were 5 percent longer. Kids whose mothers had completed high school had telomeres that were 32 percent longer; if the mothers had attended some college, the boys’ telomeres were 35 percent longer.

Furthermore, the researchers took saliva samples from the boys to test for the presence of genetic markers for sensitivity to dopamine and serotonin and discovered that that genetic tendency exacerbated the telomere shortening effect among disadvantaged kids.

To illustrate what that genetic tendency means, Mitchell turned to a common analogy made by scientists. Some people are dandelions; no matter the environment, they turn out the same and are resilient when faced with stressful circumstances.

Other people are orchids; they’re highly affected by their environment. In good circumstances, they flourish, but, as Mitchell explains, “If anything seems to go wrong, they just crumble.”

Those people, the orchids, Mitchell said, are the ones who have the genetic marker for sensitivity to dopamine and serotonin. “It amplifies whatever signal they’re getting from the environment,” he told Al Jazeera. “If it’s a good environment, then it’s great, but if you’re constantly in a disadvantaged environment, then you have the worst result.”

In terms of evolution and survival of the fittest, having that biomarker isn’t such a good thing when a person is living in a stressful environment. “In general, you’d want people to be insensitive to all sorts of environments,” Mitchell explains, so they can adapt and survive in any circumstances.

However, he also points out that the genetic sensitivity — as well as his team’s findings — suggests an early intervention in the lives of poor children can have a profound effect.

“Those kids can also benefit the most from any intervention,” he said, adding that the results are “continued support for the idea that we need to have early interventions in early childhood that help alleviate the effects of these kinds of negative environments. And some of those kids will benefit much more than we could realize.”

In fact, Mitchell added, because those kids are so affected by their environments, “it may be the kids that have the worst outcomes that benefit the most from any intervention.”