For women CEOs, danger lurks atop the corporate ladder

In one recent  24-hour span, two women were pushed out of leadership roles at major US corporations, spotlighting how investor pressure can frustrate efforts to broaden diversity in the boardroom.

Ellen Kullman, Meg Whitman, Mary Barra, Indra Nooyi, Marissa Mayer, Irene Rosenfeld, Sheri McCoy and Ursula Burns  all have led large, publicly-traded companies. And Burns was the first black woman to serve as CEO of a major company.

These women led DuPont, HP, General Motors, PepsiCo, Yahoo!, Xerox, Mondelez and Avon Products, but they also all have been in the crosshairs of influential Wall Street investors, activists who succeeded in pushing Kullman, Burns, Rosenfeld and McCoy to the exit.

To save their jobs, others gave in to the critics: Whitman agreed to split HP in two, while Barra agreed to dividend payments for GM shareholders.

Do male CEOs really outperform their female counterparts or are the women simply easy targets for investors trying to throw their weight around in search of bigger returns?

That question has taken on greater importance at a time when Silicon Valley tech companies face accusations of sexism and are under pressure to increase their racial and gender diversity.

Of the 500 corporations included in S&P's Wall Street stock index, just 27 firms, or 5.4 percent, are led by women.

"Above and beyond all other factors we might use to explain why these firms are being targeted, we found very large and significant gender effect," said Christine Shropshire, a professor of management at Arizona State University.

Shropshire studied the demands that so-called activist investors made of corporate leadership between 2003 and 2013.

Among companies with similar financial results...