Vacancy

A hermit crab does not grow a shell. It finds one, moves in, and eventually outgrows it, at which point it needs a bigger one and there are none to be had, because every suitable shell on the beach already has a crab in it.

So the crabs wait. When a large empty shell finally appears, something remarkable happens. The biggest crab that can use it moves in and abandons its old shell. The next crab down takes that one, abandoning its own. And so on down the line, until a very small crab gets a slightly better home and the chain runs out.

Biologists have filmed this. Crabs will queue up next to a new shell in size order, sometimes for hours, waiting for the sequence to start. One shell arriving on a beach can rehouse a dozen animals.

Nothing was created. One object became available, and a dozen crabs improved their position.

The sociologist who counted the shells

In 1970 a Harvard sociologist named Harrison White published a book about promotion in organizations, and its central insight is the same one the crabs demonstrate.

White studied the clergy of several Protestant denominations, which is an ideal population for the purpose: the hierarchy is explicit, the moves are documented, and pastors move between churches in ways that are recorded going back decades.

The conventional way to study careers is to follow people. Who gets promoted, and why? What are the attributes of the ones who rise?

White turned it around. He proposed following the vacancies instead.

A senior pastor retires or dies. That leaves an empty position, and the empty position is now the thing that moves. It gets filled by someone from a somewhat less prestigious church, which creates an opening there, which gets filled from further down, and so on until the chain terminates, usually with someone entering the profession or with a position being eliminated.

The person moves up. The vacancy moves down. They are the same event described from two directions.

This reframing does something important. It shows that the number of promotions available in a system is not determined by how many talented people it contains. It is determined by how many openings appear. Talent decides who fills a vacancy. It has no influence at all on how many vacancies there are.

And openings come from exactly two sources.

The first is growth. If the organization is expanding, new positions get created, and those are genuinely new opportunities rather than recycled ones.

The second is exit. Somebody leaves, and their position becomes available.

That is the entire supply. There is no third source.

Where exits come from

Now ask what causes exits, and the answer is narrower than it first appears.

People leave positions because they die, because they retire, because they are fired, or because they move to another organization. The last of these does not create net openings across the economy, it just relocates them. Firing creates real openings but is rare at senior levels and getting rarer, for reasons of both law and social convention.

Which leaves death and retirement. And retirement, when you look at it honestly, is not an independent institution. It is a derivative of mortality.

Retirement exists because human beings decline. The specific ages got set by policy, and the policies differ across countries, but the underlying fact they are all responding to is that a person’s capacity falls off at a somewhat predictable point relative to a somewhat predictable end. Bismarck’s pension age, the standard retirement ages that spread through the twentieth century, the actuarial design of every pension system: all of it is arithmetic performed on a lifespan.

Remove the decline and the entire logic of retirement evaporates. There is no reason for a person of two hundred, in perfect health, at the height of their capability and experience, to leave a position they are extremely good at. There is no fairness argument for making them, no health argument, and no productivity argument. In fact every argument runs the other way, because they will be, by any measure we currently use, the most qualified person in the building.

So both sources of exit close at once, and what remains is growth.

The queue that has stopped

Here is where the arithmetic gets unpleasant, and where I want to insist on a distinction that is usually blurred.

People discuss this problem, when they discuss it at all, in terms of the ladder getting slower. Young people will have to wait longer. Careers will take more time to develop. Everything shifts later.

That is not what the mathematics says.

A queue is characterized by an arrival rate and a service rate. If the service rate is positive but low, you have a slow queue. Everyone eventually gets served, they just wait longer, and the system remains a functioning system with predictable, if frustrating, behavior.

If the service rate goes to zero, you do not have a slow queue. You have a queue that is not moving, and the waiting time is not long. It is undefined. Nobody at the back is going to be served eventually. They are simply never going to be served.

Openings from exit go to zero when nobody exits. That leaves growth as the only supply of positions, which means the number of career opportunities in a society becomes exactly equal to the rate at which that society is creating new institutions and expanding old ones.

And the developed world’s growth rate is not high, and its population growth is at or below replacement almost everywhere. We are already an economy where much of the opportunity comes from recycling positions rather than creating them.

The difference between a slow ladder and no ladder is the difference between an unfair society and a closed one. They feel similar to the person at the bottom for the first decade. Then they diverge permanently.

It is already visible

The best evidence for this argument is that we can watch the early version of it now, with mortality still fully intact and doing its job. Every trend line below reflects nothing more than people staying healthy and productive for a couple of extra decades.

The Congress seated in 2025 was the third oldest since 1789, and its Senate the second oldest ever. The median senator was sixty-four. Six of them were born before the end of the Second World War, the oldest was ninety-one, and the youngest person in the chamber was thirty-eight. Federal judges in the United States hold their positions for life, which means the composition of the judiciary is determined in part by the timing of deaths, and everybody involved knows it and plans around it. That is a system in which mortality is not a background condition but an explicit strategic variable, discussed openly by people making appointments.

The last chapter described what happened when American universities lost the power to enforce mandatory retirement in 1994, and I said there that the consequences are disputed. They are. But the dispute is about whether senior faculty staying on caused the damage, and it has distracted from a measurement that is not in dispute at all, which is what happened at the other end of the ladder.

That number comes from American biomedical research, and it is worth stating carefully, because it is the single most alarming statistic I encountered while writing this book. The average age at which a scientist with a doctorate wins their first major independent research grant from the National Institutes of Health rose from about thirty six in 1980 to about forty two by 2013, and has sat near forty two ever since. For those with medical degrees it went from under thirty eight to over forty five.

But the distributional figure is worse than the average. The share of principal investigators on those grants who were thirty six or younger fell from eighteen percent in 1983 to three percent by 2010.

Not a slower ladder for the young. The young, as a category, went from a fifth of the field to a twentieth of it.

I want to be careful about causation, because several things were happening at once and the end of mandatory retirement is only one of them. Funding grew more slowly, training pipelines expanded, and projects got larger and more expensive. But every one of those pressures operates through the same channel the vacancy model describes: the number of independent positions relative to the number of people qualified to hold one.

Japan has been running a version of this experiment at national scale for longer than anyone. Seniority based promotion, very low turnover in large firms, an aging population, and a cohort of graduates who entered the labor market between roughly 1993 and 2004 and never recovered from it. They have a name for that group. It translates as the employment ice age generation.

The damage was permanent rather than temporary, which is the part that matters here. Studies find a wage penalty of ten to twenty percent that persists into their forties and beyond. Under a system built around lifetime employment at a single firm, entering through the wrong door at the wrong moment is not a setback you make up later. Many never got onto the regular employment track at all, and non-regular work rose above a third of the Japanese workforce. There is a term in Japan, the 8050 problem, for households where a parent in their eighties is still supporting an unemployed adult child in their fifties.

That is what a blocked ladder does over thirty years, in a rich, orderly, high-trust country, with mortality still fully operative.

None of this is speculative. It is a live and worsening feature of several of the wealthiest societies on earth, produced by an increase in healthy lifespan of perhaps fifteen years.

Two closed doors

Now put this chapter together with Chapter 4, because they describe the two routes by which a person can come to control resources, and both of them shut.

You can inherit, or you can earn.

Chapter 4 argued that inheritance stops, because the transfer event stops occurring. Capital continues compounding in the hands of the people who already have it, and the periodic breakup of fortunes that has interrupted every accumulation in history simply does not happen.

This chapter argues that earning stops too, or at least that the ladder to significant earning stops, because the senior positions through which anyone accumulates real capital are permanently occupied.

Neither of those alone would be fatal. A society can be tolerable with entrenched inherited wealth if the career ladder is open, because there is a route. A society can be tolerable with a slow career ladder if fortunes break up and recirculate, because there is a route.

Close both and there is no route.

That is a specific and, I think, underappreciated feature of the world this book describes. It is not that it would be unequal. We already live with a great deal of inequality and we have a rough tolerance for it, sustained by the belief that positions are provisional and that the arrangement is in principle contestable.

It is that it would be sealed. And a sealed society is a different kind of object, historically speaking, with a different kind of politics. Historians who study elite dynamics have a body of work on what happens when a society produces more people who expect elite positions than it has positions to give them. The word used for the resulting instability varies. The pattern does not. Blocked mobility does not produce patient waiting. It produces a population with credentials, ambition, no prospects, and a great deal of time.

Which, in the world we are describing, they will have in unprecedented quantity.

What death was doing here

The job is the simplest of the five to state.

Death empties positions. It does so on a schedule nobody controls, without regard to whether the occupant was doing well, and without requiring anyone to make a judgment about whether they should go. That last property is the one that matters most, and it is the reason no easy substitute exists. Any replacement mechanism has to decide who leaves, and the people best placed to influence that decision will be the people the decision is about.

Death also does something subtler that I mentioned at the end of the last chapter and want to state properly here. It supplies a credible promise.

A young person entering a profession is making an enormous unsecured investment. Years of training, forgone income, deferred everything, on the expectation of a position that does not currently exist and will not exist for a long time. What makes that bet rational is the certainty that the positions ahead will be vacated. Not the hope. The certainty. It is the one thing in a career that has never required trust, because it was guaranteed by biology.

Take that away and you have not slowed anyone’s ascent. You have removed the reason to begin.

And this is the point in the book where the five jobs stop being separate problems and start being one problem, because every solution to it runs into the same wall: any mechanism that forces turnover has to be designed, adopted and enforced by the people who benefit from there being none.

I will come back to that in Chapter 12, and it is the hardest thing in this book.

First, though, there is one more place where all five of these removals meet at once, and it is the largest set of promises any society has ever made. Every commitment a state has given about old age was written against a schedule that biology used to enforce for nothing.