Nearly 900 million people around the world live in slums, lacking access to adequate water and sanitation or adequate housing. And by 2025 it is estimated that 1.6 billion people—a fifth of the world’s population—will lack access to secure, adequate, and affordable housing.

As in the U.S., the dominant approach to the global housing crisis is to build large-scale subsidized housing programs in slums relegated to the fringes of cities. But there is increasing evidence that this is having a perverse effect.

A new study by the World Resources Institute (WRI) lays out how bad policies on public housing, an emphasis on home ownership, and problematic land use policies have worsened conditions in slums and made the global urban crisis worse.

More than 330 million households in the world suffer from a lack of secure, adequate, and affordable housing, a figure that will grow by 30 percent to 440 million households by 2025. And although the percentage of the world’s population that lives in global slums has declined over the past couple of decades, the absolute number of people living in urban slums worldwide has grown from less than 700 million in 1990 to 880 million in 2014 and will expand further in the coming decades. In India and China, roughly a quarter of the population lives in slums, and in Africa, more than half of the population is trapped in substandard living conditions. Women and minority groups worldwide are disproportionately concentrated in slums.