More than half of clinical trials in the United States exclude the elderly from their studies, even though 34% of personal medical expenditures come from this group of people, new research points out.

In a review of clinical trials published in major medical journals, researchers from the University of Michigan found more than 50% of trials did not include senior citizens or people with age-related illnesses. The remaining studies also excluded frail participants and those with cognitive problems. Individuals over the age of 65 currently comprise 12.5% of the population.

“There’s a critical need to ensure that research findings are relevant for our most complex and vulnerable older patients,” lead author Dr. Donna Zulman, told Medical News Today. “Our findings suggest a need for policy change by government agencies like the Food and Drug Administration and the National Institutes of Health to increase the representation of typical older adults in clinical trials.” The findings were published online in the Journal of General Internal Medicine, on Feb. 2.