Another International Women's Day has come and gone, with too many celebrating small scraps of progress and too few focusing on the real issues women face. Some people think gender inequality is no longer an issue in the workplace - they are wrong. The reality is that women aren't winning at work, and we're all losing from it. It's true some progress has been made: there are now no FTSE 100 boards with no women on them, and women are now close to occupying half of the non-executive director roles. This is exactly as it should be, women are 50% of the world's population, intelligent and capable, so there is no reason they shouldn't represent 50% of its leadership.

Encouraging as this seems, the truth is that companies are not run by boards but by executive teams and women are still seriously under-represented there: just 20% of executive directors are women and fewer than 10% of CEOs. A man is 10 times more likely to make partner in a law firm. Only 13% of fund managers are women, and only 2% of venture funding goes to female-founded start-ups. In my book, Why Men Win At Work, I explored the fundamental question of why men hold more than 90% of top leadership positions, even though, based on population and capability, women should occupy half of them. The answer is, on one level, quite simple: men get the big jobs because they are perceived to be better performers.

But this can't be right, because all the data shows that women are at least as intelligent and competent. The question, then, is why do we perceive a man to be better than a woman even in the 50% of situations when he isn't?

There are many reasons, many of them invisible and unconscious. The most fundamental: the male-dominant culture in the workplace gives men more of a sense of belonging, so they're more comfortable and able to perform. Women, meanwhile, are often held back from focusing on their strengths and contributing their best by the sense of not "fitting in".

Then there is the "competence versus confidence equation": we all value confidence (if someone is confident in themself, we feel confident in them), but women are, in general, less confident than men, even when they are at least as competent. And so the confident man is usually rewarded ahead of the competent, but less confident, woman.

In addition, women tend to believe in the myth of meritocracy and that their work will speak for itself, whereas men are generally more savvy about "The Umbrella Theory": the need to make sure your boss is seeing under the umbrella and that you are visible, your work is visible, and you are known.

Men also generally have more time for self-marketing and networking because women with families are more likely to take on unpaid household work, even when their careers are equal to or exceed their partners'. This creates a time gap, meaning most men have more hours in the day to win at work.

All of these forces combine to convince us that the man is the stronger performer, when sometimes he is not, he is simply more comfortable in the culture, more confident, more visible and better known.

Why should we all care that women aren't winning at work?

Some men do because they have wives, daughters and sisters, but that's not the biggest reason to. This isn't charity, it's business: having women in leadership drives business. The Fortune 500 companies with women CEOs drive stock prices up 50% versus an average of 25%.

Companies with the highest percentage of women in leadership outperform the rest. This isn't because women are better than men; it's because diversity is better than a dominant culture: it means more diverse and better input, insight, decisions, strategies, plans, and better results.

Perhaps most important of all, men aren't winning at work either. Gender inequality works both ways, and most dads will tell you that they wish they could benefit from flexible working and parental leave policies for a better work-life balance without risking their career.

We're all losing because women are not winning at work.

Gill Whitty-Collins is a former senior vice-president at Procter & Gamble and author of The How to Win at Work Book