Children with attention deficit hyperactivity disorder (ADHD) are more likely to grow up in socially and economically impoverished environments, according to a new long-term study conducted by the University of Exeter Medical School.

Researchers drew data from the Millennium Cohort Study — a massive multi-disciplinary project following around 19,000 British teenagers who were born in 2000. The team found that, on average, children with ADHD grew up in families who took home £324 each week, compared to £391 for those whose children weren’t affected. Among other findings, an important takeaway from this difference is understanding the true strains of ADHD. Not only does the disorder make education and socialization difficult for children, but it also reflects deeper fractures within the socioeconomic status (SES) of the family.

ADHD has been researched in nearly 1,800 studies thus far, and has currently been diagnosed 6.4 million times in the United States. Science has compiled a wealth of data to support the disorder’s genetic link, which researchers roughly chalk up to 70 to 80 percent of a person’s risk. But heritability isn’t the only factor, according to study leader, Dr. Ginny Russell, whose team developed four key theories as to why, for instance, children below the poverty line were two to three times more likely to develop mental health illnesses.

"There is a genetic element to ADHD,” Russell said in a statement, “but this study provides strong evidence that ADHD is also associated with a disadvantaged social and economic background.”

She and her colleagues hypothesized that poorer families tended to have kids with ADHD because:

  1. “there are risk factors (e.g. smoking in pregnancy, diet, under-resourced parenting etc.) that are associated with both low SES and ADHD;”
  2. “that parents who themselves suffered from ADHD as children had difficulties as adults in getting good jobs /sustaining relationships;”
  3. “that doctors over-diagnose ADHD in low SES groups (a clinical labeling bias);”
  4. “that ADHD in children causes their families to suffer socially and economically.”

“We tested '1' and '2' and '3,' and found that there was no evidence of clinical labelling bias — and no evidence of decreased income,” Russell said. “In fact, increase in income for families with ADHD children was pretty much the same as for families without kids with ADHD over the seven-year study period.”

The team also found that smoking during pregnancy didn’t act as a mediator between SES and ADHD. However, they did find that parent-child conflict acted as a mediator. Russell was hesitant to draw any specific conclusions about this relationship because it could simply be the case that overactive children cause conflict, and then respond to the parent’s reaction with more conflict, cyclically. Russell and her colleagues could not test for heritability because they didn’t have a sufficient number of twins with whom to do so.

In addition to testing for weekly income, the researchers also found that younger mothers were far more likely to have a child with ADHD than were other mothers. Mothers without a high school or college degree were more than twice as likely to have a child with ADHD than mothers who had earned their degrees. And single parents were more likely to have a child with an ADHD diagnosis than families with two live-in parents.

The present study adds to an already sizable bank of research that argues that poverty significantly impairs a child’s mental and cognitive development. A recent Northwestern University study found that children who grew up in poorer areas actually experienced worsening auditory processing in the brain, due to their neighborhoods being louder than more socioeconomically advantaged areas. What’s more, researchers from Harvard, Princeton, other universities in North America, and Britain's University of Warwick, showed that people in poverty experienced reduced cognitive function when they had to make tough financial decisions. The research team called this a limiting of their “mental bandwidth.”

Russell explained that her team’s present study echoed many of these prior findings. But it didn’t find, for example, that “ADHD in children causes disadvantage to the economic situation of their family.” It’s far more likely that poverty puts children at risk for the disorder, which may indeed have environmental roots that science has yet to account for.

The largest obstacle, by Russell’s measure, is conferring the dangers of poverty for childhood cognitive development. Mental illness still lags behind physical disability in the U.S. in how seriously it’s received, so understanding how children come to develop disorders such as ADHD may provide the concrete footing science needs to tackle mental illness at large.

“It's important to discover more about the causes of this disorder so that we can look toward prevention, and so that we can target treatment and support effectively,” Russell said. “The bottom line is, if we want to tackle mental health and behavioral issues in children we must also tackle child poverty.”

Source: Russell G, Ford T, Roseberg R, et al. The association of attention deficit hyperactivity disorder with socioeconomic disadvantage: alternative explanations and evidence. Journal of Child Psychology and Psychiatry. 2013.