More than 70% of the Fortune 500 companies offer some form of mentoring to their employees, hoping to boost performance and bolster retention, among other things. However, hard evidence of firms’ accruing those benefits has been scarce. New research finds that mentorship programs can indeed produce valuable gains—for employees and their firms—but only when they are mandatory. That’s because if mentoring is optional, the people most in need of it tend to decline the opportunity.

A version of this article appeared in the September–October 2022 issue of Harvard Business Review.