If you can't afford to own, you can rent. But what if you can't afford to rent? Millions of Americans are in precisely that situation, according to a study released today by the Joint Center for Housing Studies at Harvard University. The availability of apartments, especially cheaper ones, hasn't kept up with demand, and the problem has worsened since the 2007-09 recession, the study says. "In 1960, about one in four renters paid more than 30 percent of income for housing. Today, one in two are cost burdened," according to the study, "America's Rental Housing."