Did you move last year? If so, you took part in a declining ritual—just 11 percent of the U.S. population changed residences last year, down from 12 percent in 2013 and 13 percent in 2006.

Americans are no longer the footloose strivers of popular imagination. It’s not just about moving between states, a long-running and well-documented phenomenon that has recently leveled off. Most of the current decline is from Americans not making local moves, either. Three million fewer local moves in 2016 vs. 2006, in fact, compared with just 200,000 fewer interstate moves. Eleven percent is the lowest mobility rate on record, and as the annual report from Harvard’s Joint Center for Housing Studies shows, it’s stuck in a feedback loop with several features of the housing market.

One of those features driving down the mobility rate: America’s aging population. Old people are most likely to own their houses and don’t want to leave them. In some places—New York City, California—cost-freezing mechanisms like rent stabilization and property-tax relief penalize moving and create a lock-in effect. (Both renters and owners are moving less.) The Boomer cohort entering old age is so big it’s dragging the overall rate down. And the vast number of Boomers staying in place has another consequence: An epic shortage of homes for sale.

It’s not just your parents’ fault. After the recession, nearly 4 million single-family homes, many purchased in foreclosure auctions, became rentals. (That’s about three percent of the total U.S. housing stock.) Big-time landlords who own dozens or even hundreds of properties are a rising force in U.S. cities, and their purchases have taken a big chunk of homes off of the market. There are fewer single-family homes for sale than at any time since 1982. Adjusted for the nation’s population gains in that time, the shortage is even more severe.

There’s scarcely more slack if you’re looking for an apartment.

As you might expect, this has driven home prices through the roof. Prepare for sticker shock when you Zillow that neighborhood you’ve been picturing yourself in. But those high prices haven’t produced the housing-construction boom that your Econ 101 professor would predict. On the contrary, fewer homes are being built per capita than at almost any time in history. The Harvard report cites a few reasons for this: fewer vacant lots, land use restrictions, building material costs, and a skilled worker shortage. What’s clear is that even as America’s (very bad) sprawl model continues its slow decline, nothing has emerged to replace it. New construction in most major cities remains vastly outpaced by demand.

Thus, the bulk of the mobility decline is attributed to young people, who are supposed to move the most. Just 24 percent of 20–24 year-olds moved last year, down from 34 percent in 1996. The low mobility rate is linked to delayed household formation. It’s harder than it has been in decades to move into your own place. Some of this is cultural: The rise of working women has pushed marriage and childbirth back, and for the first time, American women have more kids in their 30s than in their 20s.

But it’s also economic: The median income for young adults has risen just 5 percent in the past three decades, far outpaced by rising rents and housing prices. Young adults living with their parents puts both groups in a holding pattern. According to the report, household formation among millennials falls by 10 percentage points as you move from the 25 most affordable cities to the 25 least affordable cities.

This mobility drop holds for renters as well as would-be owners. Except at the very high end, tenants are still under historic levels of financial stress, which may discourage them from making expensive moves, even if a better neighborhood or apartment might be out there.

There’s nothing necessarily wrong with staying in place. But choosing to do so is different than not having the option to leave.