During a Q. and A. session at Facebook’s Menlo Park, Calif., headquarters last year, Mark Zuckerberg fielded a range of queries: How do you deal with obstacles? How accurate is the movie“The Social Network”?

But it was a question about his attire that generated the most headlines: Why do you wear the same T-shirt every day?

Dressed in his usual gray T-shirt, Mr. Zuckerberg offered a rationale that was equal parts Zen and Type A. “I’m in this really lucky position where I get to wake up every day and help serve more than a billion people,” he said. “I really want to clear my life to make it so that I have to make as few decisions as possible about anything, except how to best serve this community.”

Mr. Zuckerberg may not seem like a fashion role model, but he is arguably the most visible member of a self-selective group of men who have chosen to adopt a personal uniform.