It looks so easy from the outside. An entrepreneur with a hot technology and venture-capital funding becomes a billionaire in his 20s.

But now there is evidence that venture-backed start-ups fail at far higher numbers than the rate the industry usually cites.

About three-quarters of venture-backed firms in the U.S. don't return investors' capital, according to recent research by Shikhar Ghosh, a senior lecturer at Harvard Business School.

Compare that with the figures that venture capitalists toss around. The common rule of thumb is that of 10 start-ups, only three or four fail completely. Another three or four return the original investment, and one or two produce substantial returns. The National Venture Capital Association estimates that 25% to 30% of venture-backed businesses fail.

Mr. Ghosh chalks up the discrepancy in part to a dearth of in-depth research into failures. "We're just getting more light on the entrepreneurial process," he says.