Yesterday's International Women's Day saw leaders from across the world pledging to take action as champions of gender parity. The 2016 theme for the day was “Planet 50-50 by 2030: Step It Up for Gender Equality”. Events happened across the planet, media was buzzing, hashtags were trending, and generally there was a great deal of excitement about the day. But, as with so many of these events, the big worry is we wake up the next morning and go back to the status quo. Here's why the world cannot afford for this to be the case. Simply put, the cost of not reaching parity is just too high. McKinsey Global Institute has put a price on it. They found that if women were to participate in the economy identically to men, they could add as much as $28 trillion, or 26 percent to annual global GDP in 2025. This is roughly the combined size of the economies of the United States and China today. So, there is a very real economic imperative for yesterday's talk to increase the economic participation of women to be turned into action. Women need to be equal partners in society and in the economy. The world cannot afford for it not to be so.