Links between depression and Alzheimer’s disease have been the subject of research for decades. But now Dutch researchers are making one of the boldest assertions to date: Depression might more than double the odds of developing Alzheimer’s.

Nearly 500 people without dementia, ages 60 to 90, were studied. During an average six-year follow-up, 33 of the participants who had had at least one episode of depression developed Alzheimer’s. In fact, they were 2.5 times more likely to develop the disease than individuals without depressive symptoms, said lead researcher Dr. Monique M.B. Breteler of the Erasmus University Medical Center in Rotterdam.

In addition, the risk of developing Alzheimer’s was four times more likely for individuals who had experienced depression before the age of 60, investigators noted in Tuesday’s edition of the journal Neurology. They acknowledge that more study is needed to determine what cause-and-effect may truly exist.

Their research may be especially noteworthy because investigators said they did not detect signs that individuals with depression and Alzheimer’s suffered shrinkage of certain parts of the brain. Some researchers have theorized that shrinkage was an indicator of further problems.