Researchers found that people with Alzheimer’s disease are more likely to have had less mentally stimulating careers from their 30s on than their peers who do not have Alzheimer’s, according to a study in the journal Neurology.

Researchers at University Hospitals of Cleveland, Ohio, and Case Western Reserve University School of Medicine studied 122 people with Alzheimer’s and 235 people without the disease, all of whom were more than 60 years old. They examined the career history of each subject over 40 years, from ages 20 through 60.

Researchers classified each job as mentally stimulating or not based on complexity, variety of tasks, and whether it involved creative thinking or manipulation of data.

The mental demands of occupations during a subject’s 20s did not appear to be linked to later Alzheimer’s developments, according to the study. Beginning in the 30s, those who had not developed Alzheimer’s had more stimulating careers than their peers who had the disease, the study said.