July 28, 2026
What hermit crabs know about the housing shortage
Let me tell you about hermit crabs. Stick with me, because this goes somewhere.
A hermit crab does not grow its own shell. It borrows one. It finds an empty snail shell, backs into it, and carries it around as a house. The problem is that the crab keeps growing and the shell does not. So every hermit crab spends its life needing a bigger place to live.
Here is the part I love. When a new, bigger shell washes up on the beach, the crabs do not brawl over it. They line up. Biggest to smallest, in a queue, behind the shell. The biggest crab checks it out, and if it fits, it climbs in and leaves its old shell behind. The next crab in line takes that one and leaves its shell behind. The crab behind takes that one. Down the whole line, each crab moves up one house, in a matter of seconds. One new shell, and a dozen crabs all end up better housed.
Scientists have a name for this. They call it a vacancy chain.
Now, when there are not enough shells to go around, it gets bleak. A crab with nothing better will move into whatever it can find. A bottle cap. An old bottle. A chunk of plastic junk washed up on the sand. Not because it wants to, but because the real homes ran out and you have to live somewhere.
Hold that picture, because we do the same thing.
We are the hermit crabs
When a family finally moves up into a bigger or newer house, they leave one behind. Somebody moves into that one and leaves theirs behind. Somebody moves into that one. The chain runs down the line the same way it does on the beach. One new house, built anywhere, sets off a string of moves, and the home that opens up at the bottom of the chain is an older, cheaper, already-existing home. A real home a real family can actually afford, that never could have bought new.
This is not something I am guessing at. An economist named Evan Mast went and traced it. He followed tens of thousands of people backward and forward through their moves and mapped the chains. He found that building 100 new homes, even expensive ones, opens up about 70 homes in lower-income neighborhoods, and about 40 in the poorest ones. And it does not take decades. It reaches working families within a few years.
There is a detail in his numbers that matches the crabs exactly. The bigger the new shell, the longer the chain. A bigger, pricier new house frees up more homes down the line, not fewer. Which means you do not have to build cheap housing to help the people who need cheap housing. You have to build enough housing, at any price, and let the chain do the rest.
Here is my favorite part
"Vacancy chain" is not a term biologists cooked up for crabs. Scientists coined it for us first. It described how people trade scarce things like apartments and jobs, one move opening the next. A researcher named Ivan Chase went looking for the same pattern out in nature, and found it in hermit crabs.
So when I say we act like hermit crabs lining up for shells, it is not a cute comparison I came up with. It is the same process, studied in people first and animals second. We were the original example. The crabs just make it easy to see.
What this means for one ADU in Post Falls
So back to my backyard, and the small second home I am trying to build. Why does one more little house matter? A few reasons, and they all come straight off the beach.
It adds a shell. That is the whole thing. Every home added lets the line move.
It is a small shell, for the ones just starting out. You hear all the time that starter homes do not exist anymore. It is basically true. For a generation, new houses kept getting bigger, and the small, entry-level home nearly vanished from what gets built. A young crab needs a small shell. An ADU is a small shell. It is the size the market stopped making.
Downsizing frees up a big shell. When an empty-nester moves out of the four-bedroom they raised their kids in and into a right-sized ADU, that big house opens for a young family that needs the room. That is a vacancy chain starting right here on my street. One possible future is my mother-in-law moving into the ADU I build. If she does, then her 3-bedroom house across town in Post Falls becomes available for a family with young children. Maybe my son and his girlfriend could get married and move into my mother-in-law's house.
It reuses what is already there. A hermit crab does not build a shell from scratch. It moves into one that already exists. An ADU is the same idea. The road, the water, the sewer, the schools are already in. No new subdivision, no new sprawl, and it costs the town less than building all of that again somewhere new.
Nobody loses. This is important. A vacancy chain is not a fight over one shell where somebody wins and everybody else goes home empty. Every crab in the line moves up. A new home does not take anything away from anyone. It lifts the whole line at once. As we grapple with growth and housing issues, I'm looking for win-win scenarios. Aren't you?
The shortage is the enemy
When there are not enough shells, crabs end up in bottle caps. When there are not enough homes, people end up stuck. Kids who cannot move out, like both of my adult sons and both of their adult girlfriends. Couples who cannot start. Families doubled up, waiting. The answer is not to fight harder over the shells we have. It is to add more. Every new one helps the whole line move, and an ADU is a shell almost any homeowner can add. That is why I am building one.
This is the second post on why I am doing this. If any of it rings true, or you think I have the crab science or the housing science wrong, I want to hear it. Leave a comment below, or message the Facebook page. If you would rather keep it private, use the contact form. I am learning this in the open, and the people who know more than I do are what make it better.
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