Your zip code may shape your aging brain’s health. New research out today shows that people’s odds of being diagnosed with dementia differ significantly across different parts of the U.S.

Scientists at the University of California, San Francisco, led the study, published Monday in JAMA Neurology. They analyzed the medical records of veterans, finding that dementia rates were noticeably higher in the Southeast, Northwest, and other regions, even after accounting for some possible factors like income. The findings suggest that deep-seated regional differences can contribute to dementia risk, the researchers say.

Dementia is a growing public health issue, particularly among the elderly. More than 6 million Americans are living with dementia currently and a government-funded study this February projected that 42% of Americans over 55 will develop it in their lifetime.

It’s a complex condition, with most cases caused by a mix of environmental and/or genetic factors. But according to senior study author and neurologist Kristine Yaffe, there’s been little research looking at how the risk of dementia can vary geographically, at least on a national level. Yaffe and her team had access to a dataset that had the potential to shed light on that: deidentified records from people enrolled in the Veterans Health Administration, the largest integrated healthcare system in the U.S.

“We realized the VA national data would allow for such an investigation as the VA has a uniform way of capturing data across the U.S.,” Yaffe, who is also chief of neuropsychiatry at the San Francisco VA healthcare system, told Gizmodo in an email. “There are no other national healthcare systems that have this.”

Yaffe’s team studied the health of more than 1.2 million randomly selected VA patients 65 years and older who had no pre-existing dementia. These people were followed for an average length of 12 years.

After adjusting for age, dementia rates were lowest in the Mid-Atlantic region, covering states like Pennsylvania, Maryland, and Virginia. Using this as a baseline, the researchers found that dementia rates were 25% higher in the Southeast (Kentucky, Tennessee, and Alabama); 23% higher in both the Northwest (Idaho, Oregon, and Washington) and the Rocky Mountains (Colorado, Montana, and the Dakotas); 18% higher in the South (Texas, New Mexico, and Louisiana); 13% higher in the Southwest (California, Nevada, and Arizona); and 7% in the Northeast (New York).

“This is a very large difference, especially given that these are all veterans with care at the VAHS,” Yaffe said. “It was really surprising we saw such big differences.”

The researchers reasoned that factors such as a person’s average level of education, how rural a state was, or the rate of other health conditions like heart disease within these states might explain most of the variance they found. But even when they adjusted for these variables, the patterns barely budged at all. That could mean there are other reasons—reasons not so easily captured through medical records alone—why someone in New Jersey will tend to have a lower risk of dementia than a similar person in Kentucky.

“It’s possible that the differences are explained by lifelong differences in things like education quality (vs quantity) and social determinants of health might be driving some of the differences,” Yaffe said.

The findings now leave open more questions to be answered, which the team plans to start digging into. With any luck, the lessons they learn might help us find new ways to better prevent dementia.