Luxury apartment supply is already overloading the market in New York City and San Francisco. According to Zumper, median apartment rents fell not just in the two most expensive markets, but in half of the top 20 most expensive markets, including, San Diego, Miami and Honolulu.

As Business Insider notes, it's not because the economy is hurting - it's because oversupply at the top of the market is bringing down the most expensive rents.

The median asking rent – half are lower, half are higher – for a one-bedroom apartment fell 7.9% year-over-year to $3,380, according to Zumper, which analyzes rental data from over 1 million active rental listings in multi-family buildings (it does not include single-family houses on the rental market). For a two-bedroom, asking rent fell 6.6% year-over-year to $4,670. The fourth month in a row of year-over-year declines.

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