Monthly payments on U.S. homes jumped 21 percent in the fourth quarter of 2012 from a year earlier, a survey of 325 counties found. Online housing marketplace and research firm RealtyTrac said Thursday that the payment bump was the result of a 10 percent average gain in the median price of homes and a 33 percent increase in the interest rates for the average 30-year, fixed rate mortgage. Although home payments have soared, home ownership remains more affordable than renting a home in most markets, said Daren Blomquist, vice president of RealtyTrac. On the other hand, he said, home affordability is losing ground due to "still-stagnant median incomes." In contrast to the housing market collapse that triggered the recession, the current imbalance in the market is a result of "investors and other cash buyers who are not tethered to the typical affordability constraints," Blomquist said. Buyers who are speculating on the market to flip a home -- to resell it quickly for a profit -- or to turn a family home into a rental property have been given credited with kick-starting the housing market's recovery in 2013. But speculative buyers now appear to be part of the reason families looking to buy a home for themselves are being boxed out of the market, RealtyTrac said. "One simply needs to look at the minimum income needed to qualify for a median-priced home in some markets to realize the extent of the disconnect between prices and income," Blomquist said. By example, the minimum income required to buy a median-priced home in Los Angeles County was $68,000 a year ago. That has climbed to $95,000 this year, Blomquist said. By the numbers, a 30-year, fixed rate loan at 4.46 percent interest, assuming a 20 percent down payment, for a median-priced three-bedroom home cost an average of $865 per month in the fourth quarter of 2013. A year earlier, all details the same, but assuming a 3.35 percent interest rate, and the average monthly payment came to $714 per month, RealtyTrac said.