If we knew more, would we opt for different kinds and amounts of health care? Despite the existence of metrics to help patients appreciate benefits and harms, a new systematic review suggests that our expectations are not consistent with the facts. Most patients overestimate the benefits of medical treatments, and underestimate the harms; because of that, they use more care.

The study, published in JAMA Internal Medicine and written by Tammy Hoffmann and Chris Del Mar, is the first to systematically review the literature on the accuracy of patients’ expectations of benefits and harms of treatment. They examined over 30 studies that assessed whether patients understood the upsides or downsides of certain treatments. To a great extent, patients didn’t.

In the 34 studies that assessed understanding of benefits, patients overestimated their potential gain in 22 of them, or 65 percent. For instance, a 2002 study published in the Journal of the National Cancer Institute asked women who had undergone prophylactic bilateral (double) mastectomy to estimate how much the procedure reduced their risk of breast cancer. On average, the women thought they had reduced that risk from 76 percent to 11 percent, an absolute risk reduction of 65 percentage points.

For the more than 80 percent of the women in the study who did not have a BRCA genetic mutation — which drastically increases the risk of breast cancer — the real risk before surgery of developing breast cancer was 17 percent, meaning they greatly overestimated their risk reduction. Even the women with a BRCA mutation overestimated their risk reduction, but to a lesser extent.