A new working paper from Jacob Cosman and Luis Quintero of the Carey Business School at Johns Hopkins University cites concentration of developers as a reason for the dearth of affordable housing. From 2006 to 2015, the number of builders who controlled 90 percent of a typical market dropped by a quarter, which has cost the country approximately 150,000 additional homes a year and decreased the annual value of housing production nationwide by $106 billion — all else being equal. The researchers analyzed data from 137 local markets with at least 25,000 people stretching from the southern suburbs of DC to the northern suburbs of New York City. By 2015, most of the 137 housing markets they studied were concentrated in the hands of just a few builders.