Stop us if you’ve heard this one: There isn’t enough inventory in the housing market.

It’s a well-worn refrain, but a new analysis from Trulia offers a conclusion so simple it seems silly. Too little fresh supply — new homes being built — is far and away the single biggest contributor to the meager level of housing inventory.

Anyone keeping tabs on the housing market is familiar with the low-construction theme. While the pace of new-home sales has perked up over the past few years, they still represent only about 69% of their long-run sales pace.

Read:New-home sales tread water in June as supply-starved recovery lurches ahead

Trulia’s analysis suggests that in the largest 100 metros of the U.S., a one-percentage-point increase in home-building over the past five years — if builders had increased housing supply by 2% more homes rather than 1%, for example — would mean existing inventory would stand 13% higher in 2017 — in addition to the actual number of homes that were constructed.

That’s because newly-constructed homes are usually purchased by people who already own their homes and who are now buying pricier properties. If there’s new supply for them, they sell, freeing up inventory all the way down the housing food chain and creating a multiplier effect that goes beyond the number of new homes constructed.

Put another way, we’d now have more than 81,000 more homes in the market across the U.S. Or consider Charlotte as a local example. The overall number of housing units increased by 51,458 between 2010 and 2016. But if builders had built just 1 percentage point more homes, the multiplier effect of having more supply in the market would mean additional inventory of 901 more homes in the second quarter of 2017 than those that were constructed, according to McLaughlin’s analysis.

Here’s how that might look in practice:

A new million-dollar home is purchased by a couple trading up from a $750,000 home, which has been bought by a family trading up from a $500,000 home, which has been bought by an owner trading up from a $400,000 home, which was bought by someone who just sold his first home, which cost $250,000, and so on.

Read:A money manager struggles with the social pressure for ‘more house’

In contrast, McLaughlin found that another housing-market development that’s often blamed for tight inventory — investor purchases of homes to rent — has a much smaller effect throughout the market: a 2.5% decline in inventory for a 1 percentage-point increase in investor ownership. That’s because investors are typically buying at the lower end of the market. If a $400,000 home is purchased as a rental, it will only quash one or two lower-priced trade-ups.

McLaughlin’s findings corroborate an argument that the National Association of Realtors has been making for years. Discussing existing-home sales in April, NAR Chief Economist Lawrence Yun blamed, in part, “the home-building industry’s struggle to meet the dire need for more new homes.”

That need has only become more “dire” as the spring selling season has wound on. Despite strong demand, sales of existing homes faltered in June, the Realtors said Monday.

What may be a little more tricky to untangle is just why new-home construction has been so muted.

“First and foremost is land use regulation in some of our biggest, most productive markets,” McLaughlin said. Tight local control over things like zoning — specifying that builders must construct single-family homes rather than apartments, for example — has held back not only a stronger pace of building, but also construction of the types of homes that would satisfy the most demand.

Another reason is that home builders are still picking up the pieces from the bursting of the housing bubble in 2007. Builders shed 1.5 million jobs — and have only replaced barely half those jobs.

More recently, an increasingly protectionist White House is helping nudge up the cost of inputs like lumber from Canada. The builders’ association estimates that tariffs add as much as $1,236 to the cost of an average single-family home.

Also read: Builder sentiment stumbles to 8-month low on higher lumber costs