An unfinished construction project in Houston, TX. (Photo by Spencer Platt/Getty Images)

Several years after the Great Recession, it’s safe to say that America’s housing market is back. Since 2009, when the number of new authorized housing units dipped to 583,000, they have ticked up every year since, and are now around 1.2 million annually. The National Association of Home Builders/Wells Fargo Housing Market Index has also increased every year since then, almost returning to the levels found in the late-1990s boom years. And national median home prices, while bottoming out in 2012, have risen back up to their 2007 levels, creating a nearly perfect symmetrical, upside-down bell curve on Zillow’s 10-year graph.

But one element of the housing industry has not returned: the workers. And this appears to have increased construction costs, becoming one of the under-reported factors behind America’s rising home sale prices.

This problem began surfacing in 2012, as construction activity picked up and the unemployment rate dropped. It has worsened in the years since, according to numerous reports, and analysts I’ve spoken with. Most notable has been the annual labor shortage survey published by NAHB, which tallies answers from the nation’s single-family home builders. According to Paul Emrath, an economist for the organization, “the share of builders reporting either some or a serious shortage has skyrocketed from a low of 21 percent in 2012, to 46 percent in 2014, 52 percent in 2015, and now 56 percent in 2016.”

The shortages have harmed builders’ abilities to both form their own workforces, and hire subcontractors, such as bricklayers and electricians. The net result, according to the 2016 survey, is that 75% of builders say they’ve had to pay higher wages and bids, 64% have delayed projects, and 68% have raised home prices.

National Association of Home Builders

This graph shows that labor shortage complaints from developers have ticked up every year since 2012.

According to Jerry Howard, CEO of the NAHB, the cause for the shortage was workers leaving the industry after jobs dried up during the recession, and never returning. Much of the domestic construction workforce either pursued higher education, or found other, less-cyclical professions. Meanwhile much of the foreign-born construction workforce returned to their home countries. Many of them haven’t come back to America, thanks to improved economic conditions in their homeland–and tougher immigration enforcement. This has particularly hurt markets with disproportionately high immigrant populations, such as Texas and California, where roughly 40% of the construction workforce is foreign-born.

“The recession we experienced 6 years ago is the biggest factor because a lot of people left the workforce,” said Rick McGuire, a Lubbock, Texas-area developer, by phone. “But certainly I do think immigration has something to do with it.”

The labor shortage continues at a time when home prices keep going up. Between April 2012 and April 2017, the median price went from $151,000 to $196,500, or a 30% increase. That is far faster than the rate of inflation since that period.