The fantasy home of a Millennial daydreaming in his parents' basement — or just a photo of Ikea furniture courtesy of Flickr user Hillary .

Several years ago, when the economy was still looking grisly, it was easy to explain the single most popular trope about millennials — that an unusually high number of them were still living in their parents' basements. Obviously, they didn't have jobs. Where were they supposed to go?

"The standard explanation was, 'it’s a crummy job market,'" says Richard Fry, a senior researcher at the Pew Research Center. He reasoned, as just about everyone did, that as the job market improved, Millennials would move out. A new Pew analysis this week muddles that picture: The unemployment rate has fallen significantly since the recession for 18-to-34-year-olds. But the number of them heading their own households has not budged at all.

"That expectation," Fry recalls of the old job-market hypothesis, "at least so far has proven wrong."

About 42 million 18-to-34-year-olds lived independently of their families on the eve of the recession. About 42 million of them live on their own today, even as the size of this age bracket has grown. That means a young adult is even more likely to live at home in 2015 than back in 2008.

So now we have a modest, more perplexing new trend: Millennials are finding work, but they still seem stuck at home.

That pattern raises some much tricker questions. For one: Why? And how long will this last? Is this a sign of financial savvy or gun-shy caution? And what are these people doing with their newfound income if they're not spending it on rent?

Some of this may be explained by student debt. But the Pew analysis, based on data from the Census Bureau's Current Population Survey, suggests that young adults are less likely than at any point since the recession to live on their own, whether they have a college degree or just a high school diploma.

Another likely factor: While young adults have been sitting at home, housing beyond their parents' basements has been getting more expensive. So even as their incomes are rising, housing costs are rising, too. And other data show that many of the metros where the job market is the strongest are also places where rents are rising quickly.

San Francisco, Seattle and Washington may be great places to find a job. But they're not great places to find an $800 apartment.

A 25-year-old who stays at home for a little while longer may, in fact, be making a smart choice. It's hard to know right now, Fry says, if this trend is good or bad for Millennials, or even good or bad for their parents.

"What I do know is the number of households being run by young adults is stuck at 25 million. It was 25 million in 2010, and it’s still 25 million in 2015," he says. "That does have serious larger economic ramifications. A lot of money gets spent when you set up your house, whether you’re a renter or an owner. Realtors care. Home Depot cares. The cable company cares."

It's possible other sectors of the economy are benefiting — maybe restaurants and bars — from the money these particular Millennials aren't spending on build-it-yourself bed frames. But it's also possible they're saving it. Or maybe they're paying rent at home. Back in a 2011 Pew survey, about a third of 18 to 34-year-olds living at home said they pay rent there, and three-quarters said they contribute to household expenses. Maybe their parents could use that money.

Either way, if there's a loser here, it is clearly Ikea.