Vanguard, the mutual fund company known for its low-cost index funds, frequently shows up on lists of organizations with the most-loyal customers—and that’s no accident. During his tenure as CEO, from 1996 to 2008, Jack Brennan emphasized what he calls the virtuous circle of attracting loyal clients who stick around and create new ones through word of mouth. Now Vanguard’s chairman emeritus, he is a leading voice on corporate disclosure issues and has held positions at the Financial Accounting Foundation, the Financial Industry Regulatory Authority, and the Investment Company Institute. Brennan spoke with HBR about why companies should want to tell investors more about their customers—and how soon they may be required to. Edited excerpts follow.