S(h)ee you on the board of directors?

Despite all the research proving that women are excellent leaders, corporate boards of directors and clubs are mostly reserved for male members. Does that mean that corporations don't trust women, even when their participation in decision-making brings better results, more creativity and innovation? Are women leaders in corporations still an exception and how much does boards formation rely on social prejudices? What else needs to be done to increase the number of women in leading positions in corporations?