"Everyone Here Is From California." We Checked.

You hear it at every backyard barbecue in the Treasure Valley, usually delivered as settled fact.

We went looking for the numbers. Here's what they actually show — including the parts that don't flatter the skeptics.

First, an honest problem with the data

Anyone citing "recent Census data" about which counties Boise newcomers come from is citing something old.

The Census Bureau's American Community Survey county-to-county migration flow series stops at 2016–2020. Newer releases are state-to-county only. There is no current county-to-county file. That's not a minor footnote — it means the granular version of this question genuinely cannot be answered with fresh federal survey data.

The IRS publishes county-to-county migration derived from tax returns, and the most recent release covers moves between 2022 and 2023 filings. It counts only people who filed returns in both years, so it systematically undercounts non-filers, retirees, students, and very low-income movers. IRS and ACS should not be expected to match, and where they differ it's usually methodology rather than error.

We'd rather tell you that up front than pretend to a precision nobody has.

What the last real county-to-county data showed

In the 2013–2017 ACS, gross in-migration to Ada County broke down like this: 9,260 people from other Idaho counties. 5,593 from California. Canyon County alone sent 3,820 — more than any single California county.

You'll see that reported locally as "62% Idaho, 38% California," but that math divides only by the Idaho-plus-California subset rather than all in-migrants. Against total in-migration, California was closer to 30%. The defensible statement either way: in-state movers outnumbered Californians roughly 1.7 to 1.

In the 2016–2020 ACS net flows, the largest single origins for Ada County were Orange County, California (1,023), Sacramento County (575), Lubbock County, Texas (553), Benton County, Washington (474), and Twin Falls County, Idaho (436).

And then there's the number that reframes the whole conversation. In that same 2016–2020 net data, Ada County lost a net 3,197 people to Canyon County.

The largest single migration relationship in the Treasure Valley isn't Californians moving into Boise. It's Boise-area residents moving west.

The California share is falling

This is the part that's changed most and gets talked about least.

Californians were nearly one-third of Idaho's newcomers in 2021. By 2023 they were just over one-fifth — roughly 17,000 people.

The origin-side data agrees. California's Legislative Analyst's Office, working from the same 2023 IRS release, found California's net out-migration roughly doubled from about 170,000 a year in 2019 to about 300,000 during 2020–2022, then fell to about 200,000 in 2023 — a drop of a third from the peak.

So the California wave was real, it was concentrated in 2020–2022, and it has receded.

Where Idahoans go

The traffic runs both directions, which almost never comes up.

Using 2019 ACS data, the top destinations for people leaving Idaho were Washington (12,600), Utah (6,700+), Oregon (6,623), California (4,372), and Arizona (2,961). California is the fourth most common destination for people leaving Idaho. That same year 17,772 Californians moved to Idaho, so the exchange ran about four to one in Idaho's favor — a real net gain, and a two-way street.

Growth is slowing, and it isn't slowing in Boise

Ada County went from 392,365 residents in 2010 to 494,967 in 2020 to roughly 546,600 at the start of 2026. Idaho's Department of Labor reports that about three of every four new Idaho residents in 2025 came from in-migration — down from 88% in 2022 and 91% in 2020.

The distribution is the interesting part. As of April 2025, Boise itself grew 1.4%. Meridian grew 3.16%, Nampa 3.36%, Caldwell 5.71%, Kuna 7.18%, and Star 9.82%. Canyon County was the fastest-growing county in the state at 2.9%.

Whatever is happening to the Treasure Valley, it is not primarily happening inside Boise city limits.

One caution: COMPASS, the regional planning organization, publishes its own Ada-plus-Canyon series showing roughly 3% annual growth and does not show the same slowdown that Census-based sources do. Different methods, different answers. Don't blend the two.

Did out-of-state buyers drive prices?

The honest answer, from Idaho's own economists, is: far less than people assume.

The Idaho Department of Labor's housing market analysis found that existing housing inventory has by far the greatest effect on prices — a 1% decrease in inventory correlates with roughly a 0.5% price increase the following year. A 1% increase in out-of-state migrants correlates with about a 0.005% increase. That's roughly a hundredfold difference.

And the inventory decline itself was driven largely by falling mobility — people staying put — not by buyers arriving.

Rates did the rest. Mortgage rates went from 3.37% in December 2021 to 6.8% a year later, and Boise Regional REALTORS® attributed the subsequent price reversal largely to the loss of bidding-war pressure.

The Ada County single-family median tells the story plainly: $345,000 in 2019, a record $602,250 in April 2022, $524,900 for 2023, and $575,900 in May 2026 — down 0.7% year over year. Four years after the peak, the median is still below it.

So what's true?

The claim is substantially false as stated, and not baseless.

True: California is consistently the largest single out-of-state source of Boise-area newcomers, and Ada County receives more California migrants than any other Idaho county.

Not true: that everyone is from California. Movers from other Idaho counties outnumbered Californians roughly 1.7 to 1 in the last period with real county-level data. Ada County has been losing net population to Canyon County. California's share of Idaho newcomers fell from about a third to about a fifth between 2021 and 2023. And Idaho's own labor economists put migration's effect on home prices at roughly a hundredth of inventory's.

Boise State's 2025 Idaho Public Policy Survey adds a footnote worth having: among Idahoans who thought the state was on the wrong track, "too many people moving to Idaho from out of state" ranked only fourth, at 7.2%. Meanwhile 49% reported financial strain from housing costs.

Which suggests the thing people are actually feeling isn't who their neighbors are. It's what a house costs. Those are different problems with different solutions, and only one of them gets solved by arguing about license plates.

Sources: IRS Statistics of Income county-to-county migration (2022–2023) · U.S. Census Bureau ACS migration flows · Idaho Department of Labor · COMPASS · Boise State University Idaho Public Policy Survey 2025 · Boise Regional REALTORS® · California Legislative Analyst's Office · HUD