Bias, which takes many forms, affects how doctors think and the treatment decisions they make. Racial biases in treatment decisions by physicians are well documented. But a growing body of scientific research on physician decision-making shows that doctors exhibit other biases as well—cognitive ones—that influence the way they think and treat patients.

Sometimes these cognitive biases are simple overreactions to recent events, what psychologists term availability bias. One study found that when patients experienced an unlikely adverse side effect of a drug, their doctor was less likely to order that same drug for the next patient whose condition might call for it, even though the efficacy and appropriateness of the drug had not changed.

When comparing patients who had a heart attack in the weeks leading up to their 80th birthdays with those who’d recently had an 80th birthday, we found that physicians were significantly less likely to perform a coronary artery bypass surgery for the “older” patients.

Awareness of these cognitive biases has prompted efforts to reduce them in clinical decision-making. Given our growing understanding of the errors that doctors can make, these biases are too costly to ignore.