Many reasons have been given for why only 19 percent of corporate board directors are women -- a number that has barely budged over the last decade. There aren't enough directors retiring to make room for new ones. There aren't enough outside pressures to prompt real change. Boards can't find enough women to take new seats.

But a new survey offers a potentially simpler explanation. Male directors, who occupy the vast majority of board seats, apparently don't think it's very important.

PricewaterhouseCoopers released its annual directors report Tuesday, which surveyed nearly 800 corporate board members. It reveals a sharp and unsettling divide between the way the male and female directors who were queried view the issue of diversity.

The numbers are striking. Just 35 percent of the male directors said diversity on the board is "very important," compared with 63 percent of the women. Eighty percent of the women surveyed said they "very much" agree that diversity leads to more effective boards, compared with just 40 percent of the men. The male directors were also less likely than women to view racial diversity as very important, and less likely to believe there were enough qualified diverse candidates for board seats.

Meanwhile, 74 percent of female directors said they strongly agree board diversity leads to better  company performance, compared with just 31 percent of the male directors. That's despite a growing body of research that shows a link between the two. More diverse boards have been tied to higher average growth, better priced mergers and acquisitions, and higher returns on equity, sales and invested capital.

Until more male directors view the issue as a critical imperative, diversity will fall behind the many other pressing matters that compete for boards' time. It's hard for change to happen when so many of the people actually in a position to do something about it don't view it as very important.

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