Does a diagnosis of attention deficit hyperactivity disorder increase one's risk of dying young? Though some studies have suggested there may be a link, they weren't large enough to tell for sure.

Now, a new, well-designed study in the Lancet gets pretty close to demonstrating this disturbing correlation.

A small group of Danish researchers gathered data from nearly two million people on national medical registers, tracking them from their first birthdays until 2013, for a maximum of just more than 30 years. In this cohort, 32,000 people had been diagnosed with ADHD.

When the researchers followed the group for the next three decades to learn about how they fared, they came to some startling conclusions. Compared to people without ADHD, those who had the disorder were twice as likely to die prematurely — and much more accident prone.

Of the 107 people with ADHD who died, they were able to get information on the causes of death in 79 cases. Of these people, 25 people died from natural causes and 54 passed away from unnatural causes. The overwhelming majority (78 percent) who died prematurely and unnaturally had been involved in accidents.

They also found out that people who were diagnosed with ADHD in adulthood had higher mortality rates than those who had been diagnosed early, and that women with ADHD had a higher risk of death than men.

"This study provides the first evidence of increased all-cause mortality in individuals with ADHD," the study authors concluded. "The excess mortality in ADHD was mainly driven by deaths from unnatural causes, especially accidents."

Why may ADHD people be accident prone?

In an accompanying commentary, researchers pondered the question of why ADHD people seem to be dying younger and disproportionately from accidents. They note that while we don't yet fully understand the cause, there is a well-documented link between ADHD and antisocial and substance abuse disorders, which can lead to aggression, violence, and crime.

They also suggest there may be other pathways: "inattention and impulsivity, would seem to be risk factors for accidents," they said, citing several studies that have found associations between ADHD and more risky behaviors.

Finally, another major contributor to early death among those with ADHD seemed to be insufficient treatment. The researchers found that the mean age at diagnosis was about 12 years. So many people in the study were only getting treated long after the disease sprang up. And because the risk for premature death increased with the age at diagnosis, under-treatment may be at least partially driving the trend.

Why you shouldn't freak out about this study

While the study is important — and really big, and well conducted — you shouldn't freak out about dying early because of an ADHD diagnosis.

First of all, this was an observational study — not an experiment — and observational studies can only ever tell us about correlations between things, not that one caused the other. (For more on the difference between the two types of studies and their limitations, see here.)

Secondly, the study was only done in one country — Denmark. For whatever reason, there may be some quirk in the health-care setting here or the way Danes are diagnosed and treated with this disorder that makes the findings not applicable to other settings.

Finally, though the relative risk of premature death in ADHD folks was higher than those without ADHD, the absolute risk is quite low.

As the study's lead author, Søren Dalsgaard from the Aarhus University in Denmark, told Vox, "Among two million individuals in the total cohort, 5,580 died during follow-up. Among those 32,000 with ADHD within this cohort 107 died." In other words, the group of ADHD people who died is very small so we're not talking about thousands of deaths.

To put it into more concrete terms, Dalsgaard explained, "When following 10,000 patients with ADHD for one year, 5.8 died, compared to only 2.2 deaths among 10,000 individuals without the disorder within one year." That means that while the relative risk is much higher among the ADHD group, it's still a small risk overall.