A study has shown that growing up in a stressful environment affects a child’s DNA, modifying their development through disadvantage.

Stress, fear and disadvantage leaves imprints on the genes of children as young as age 9, according to research from Princeton University and Pennsylvania State University.

Such chronic stress during youth leads to physiological weathering similar to aging.

A study of 40 9-year-old boys has shown that those who grow up in disadvantaged environments have shorter telomeres - DNA sequences that generally shrink with age - than their more advantaged peers.

The researchers report that boys with genetic sensitivities to their environment have shorter telomeres after experiencing stressful social environments than the telomeres of boys without the genetic sensitivities.

These sensitivities are based on gene variants related to the serotonin and dopamine pathways - neurotransmitters essential for relaying information between the brain and body.

The sample size seems low, but the 40 participants were chosen from data collected by the Fragile Families and Child Wellbeing Study, which tracked 5,000 children from a wide range of social backgrounds. They were interviewed at regular intervals between birth and age 9. Saliva DNA samples were collected when the children were 9 years old.

The sample was restricted to 40 boys who participated in the Fragile Families study and met criteria including having provided saliva samples at age 9 and giving complete information about their social environments.

Half of the children studied came from households deemed to be disadvantaged. 

Disadvantage was characterised by factors such as a low household income, low maternal education, an unstable family structure and harsh parenting.

To measure telomere length, the researchers analysed the saliva DNA provided by the boys.

Telomeres at the end of each chromosome protect the ends from damage. Telomeres vary in length by person, shrinking with age.

Through their analysis, the researchers found that living in a disadvantaged environment was associated with 19 per cent shorter telomeres by age 9.

For boys predisposed to being sensitive to their environment, this negative association was even stronger.

Arline Geronimus, a pioneer in research on physiological weathering but who was not involved in the study, said the work adds to the growing body of research related to the role of chronic stress in health inequalities, especially among the poor.

“I think it's very striking that these findings are in children at age 9, because you are talking about accelerated aging or stress-mediated wear and tear on your body, which make you more vulnerable to all kinds of illnesses and diseases,” Geronimus said.

“To say that you can see this by 9 years old is a very strong statement.”

The full report is accessible in the latest Proceedings of the National Academy of Sciences.