As growth in rents outpaces growth in incomes, rent affordability (the share of the typical American’s income that goes to rent) has steadily deteriorated over the past five years – from a historic average of 26 percent, to roughly 30 percent currently. But focusing only on medians can sometimes hide more than it reveals. In this case, there are sharply different trends at the top and at the bottom of the market.

Nationwide, rent appreciation has been much stronger in the bottom third of the apartment market than in the top third, and 11 of the 15 housing markets Zillow analyzed saw double-digit rent appreciation among lower-end apartments. Trends at the top of the apartment market have been more mixed. In places like San Francisco, appreciation among high-end apartments has exceeded rent for lower-end apartments. Elsewhere – in places like Boston and Chicago – high-end rents have stagnated.

Some of these diverging trends in rent affordability are driven by income growth, which has largely been stronger for higher-earning workers than for lower-income workers – particularly in booming cities like San Francisco. Perhaps more importantly, a broader socio-economic swath of Americans are renting than at any time in recent history as the homeownership rate has dropped to generational lows. And as long as incomes rise and more affluent Americans are choosing to rent, rising rents are not necessarily a problem.

For the top-third of earners in markets analyzed, the share of income spent on rent remains below the historic rule of thumb that says a household should spend no more than 30 percent of their income on housing costs. In notoriously pricey San Francisco, the top third of earners are spending 27.8 percent of their income to rent a pricier top-tier apartment – up from about 20 percent at the start of 2012 (the largest percentage point increase among the 15 markets studied), but still within a prudent range. But the average household earning a bottom-third salary in San Francisco would need to spend 86.6 percent of their income to afford the typical bottom-third apartment, up sharply from about 65 percent at the start of 2012. (Of course, at those astronomical levels, very few low-income San Franciscans are actually able to rent without adding housemates, securing scarce public housing or moving farther away from the expensive urban core.)

In five of the 15 metros studied, rent affordability for upper-income households has been flat (Chicago) or declined (Boston, Philadelphia, St. Louis, and Washington, D.C.) compared to 2012 (figure 1). In places like Boston, upper-income households spend 18.2 percent of income on rent, down by about a percentage point since 2012, and lower than middle-income households (which spend 33.8 percent of income on rent), or low income households (which spend 80.8 percent of income on rent).

These diverging rent affordability trends also highlight some of the growing inequality experienced between high-income and low-income Americans over the past few years (figure 2). At the end of 2011, the average gap across all markets analyzed between the share of income devoted to rent by top earners and bottom earners was 39.7 percentage points. Today, that gap has widened to 45 percentage points. Metros with the widest current (as of Q2 2015) gaps between high-end affordability and low-end affordability are Miami-Fort Lauderdale (70.4 percentage points), Los Angeles (69.8 percentage points) and Boston (62.6 percentage points). Metros with the smallest gaps are St. Louis (30.2 percentage points), Washington, D.C. (32.4 percentage points), and Charlotte (34 percentage points).