People with a traumatic brain injury are more likely to be diagnosed with dementia even 30 years later, according to a new study that looked at data spanning nearly half a century.

Traumatic brain injury (TBI) has long been associated with dementia, but there have been few long-term studies of this link. In today’s study, published in the journal PLOS Medicine, researchers looked at a dataset of all Swedes that were at least 50 as of December 2005 — totaling about 3.3 million people in all. They checked this against diagnoses of dementia and TBI from national databases that spanned 1964 to 2012. The results show that the risk of developing dementia is increased by four to six times in the first year after a TBI. Overall, the risk increased by 80 percent in the 15 years after the injury. Thirty years later, there was still a 25 percent increased risk. This is an observational study, so it doesn’t establish that TBI causes dementia, but it does strengthen the link between the two.

There were three parts to the study. First, the scientists looked at people diagnosed with a TBI over this period — about 165,000 people — against people who hadn’t. During an average follow-up period of 15 years, 6.3 percent of people with TBI were diagnosed with dementia, compared to 3.6 percent without. Next, instead of looking at people with TBI to see if they had dementia, they reversed and looked at 136,233 individuals with dementia compared to people who hadn’t been diagnosed. Finally, they looked at about 50,000 pairs of siblings in which one had a brain injury and the other didn’t. Risks were similar for men and women and, unsurprisingly, having a severe brain injury or multiple injuries had a stronger link to developing dementia.

As usual, there are several caveats to the study. It’s likely that in some cases, the onset of dementia causes these types of accidents to begin with. Plus, people who have had brain injuries are likely to be monitored more closely by relatives afterward, which probably increases the chance of being diagnosed with dementia. And dementia is very common — most people develop it if they live long enough — and this could distort the data. Still, the study uses an enormous dataset over a long period of time, and should be taken seriously as yet another reason to look closely at how brain injury affects brain health in the long run.