Nicholas Kristoff wrote about the subconscious racism that seems to be present in all of us. He also wrote about gender bias:
Women now hold 55 percent of top jobs at American foundations but are still vastly underrepresented among political and corporate leaders — and one factor may be that those are seen as jobs requiring particular toughness. Our unconscious may feel more of a mismatch when a woman competes to be president or a C.E.O. than when she aims to lead a foundation or a university.
Women face a related challenge: Those viewed as tough and strong are also typically perceived as cold and unfeminine. Many experiments have found that women have trouble being perceived as both nice and competent.
Perhaps this explains why some Americans have turned away from Hillary Clinton - perhaps it explains why I don't think she's the stronger candidate. Though my problem with Clinton (and being a non-U.S. voter renders my problem irrelevant), or at least, the problem I say I have with her is that I don't think she can defeat John McCain - Republicans hate the Clintons more than Barrack Obama - and I'd like the Democrats to win the presidency.
But unrelated to U.S. politics, the idea a woman cannot be nice and competent is ridiculous; I've worked with more women who were nice and competent than the opposite.