At the heart of the issue is an extreme imbalance in supply and demand. Population growth, the continuing trend toward urbanization, and rising global incomes are all fueling steady demand increases. In 1950, New York City and Tokyo were the only two cities on earth with populations of more than 10 million; today there are more than 20 cities of that size. The world’s urban population has been rising by an average of 65 million people a year over the last three decades, led by breakneck urbanization in China and India. The housing stock of expensive urban centers around the world has not expanded quickly enough to keep up with this surge in demand. Research from the McKinsey  Global Institute has examined the scope of the housing affordability gap. California, for instance, added 544,000 households but only 467,000 net housing units from 2009 to 2014. Its cumulative housing shortfall has expanded to two million units. With home prices and rents hitting all-time highs, nearly half of the state’s households struggle to afford housing in their local market. In New York City, MGI estimates that 1.5 million households cannot afford the cost of what we define as a decent apartment at market rates. This puts the city’s total “affordability gap” at $18 billion a year, or 4 percent of the city’s GDP.