Some investors are finding that companies with more women in top positions outperform. Now they're starting to wonder: How much does a female CEO add to market returns?

On one hand, a company led by a woman has reached the pinnacle of corporate diversity efforts, but on the other hand, some investors worry that women are too frequently appointed to helm struggling companies, running off the "glass cliff." Market returns bear out both points, showing that female CEOs frequently deliver strong performance, but are often handed a difficult job to start.

"It's the cynical view," said Alison Cook, a management professor at Utah State University, who has studied the glass cliff. Women may take a tough job because it's one of their only opportunities, while men may avoid it because they know "their chances of succeeding and being the hero are slim," she said.