From Zillow:

An increasing number of American adults are moving in together to share expenses as rental rates continue to outpace income, according to a new Zillow analysis.

As of 2012, more than a third (32 percent) of adults live in doubled-up households, or homes where two or more working-aged adults live together but aren?t married or partners. The share of doubled-up households has steadily risen over the past decade, up from 25.4 percent in 2000 and 30.8 percent in 2010.

This rise in doubled-up households coincides with rental prices that are increasingly unaffordable nationwide. Americans making the national median income ($53,216) should currently expect to spend nearly 30 percent of their monthly income on rent, the highest rate ever. Rather than moving to a smaller home or to a less expensive area, many people are choosing to live with roommates.

Large metro areas with the highest share of adults living with roommates include Los Angeles (47.9 percent), Miami (44.5 percent), New York (42.5 percent) and San Diego (39.7 percent).

Not surprisingly, the markets with the largest increases in doubled-up households are also the most expensive places to rent. In San Francisco, for example, the median rent payment went from 25 percent of income in 2000 to 40 percent in 2012, while doubled-up households increased from 25 percent to 39 percent over the same period.