A friend and colleague gave a talk today about incarceration trends in the United States (she knows who she is, and I owe her a coffee). The importance of the problem — and the genuine messiness of the data — got me thinking about how difficult it is to convey the right lessons from debates about prison reform. It was only somewhere on the walk home that I remembered Maggie and I are working on a chapter right now that is almost precisely on point.
The national story about incarceration in the United States sounds, on its face, like genuine progress. The prison and jail population peaked in 2008 at roughly 2.3 million people and has declined since — down to about 1.7 million by 2020, the lowest rate since 1995. Reform advocates can point to these numbers with some justification. The trend is real. The question is whether it is the right trend to be reading.
Here is what the data actually show once you look underneath the aggregate. Urban counties — large jurisdictions housing some of the country’s biggest jails — have led the decarceration trend substantially. These counties have also, because they are very large, dominated any national population-weighted calculation. When you sum everyone in American jails and prisons and divide by total population, the enormous weight of urban America pulls the aggregate down. Rural counties tell a different story. Jail incarceration rates in rural America have risen for decades, and by 2018 the median rural jail admission rate was roughly 80 percent higher than the median urban rate — a near-complete reversal from the pattern that prevailed in the 1990s, when urban jails were the dominant feature of American incarceration. The Vera Institute, which has done the most systematic county-level analysis of these trends, documents what it calls a near-universal urban-to-rural shift in prison admissions across states — and notes that this shift persists regardless of whether a given state’s overall admissions are declining. That last clause deserves a moment’s pause.
What we have are two trends moving in opposite directions, and a national aggregate that accurately reflects neither. This is Simpson’s paradox — the phenomenon in which a trend visible within every subgroup of a dataset disappears or reverses when the groups are combined. The mechanism here is population size. Urban counties are large enough that their declining rates pull the national population-weighted average down, even as the majority of American counties — numerically, geographically — are moving in the opposite direction. The typical American lives in a jurisdiction where incarceration is falling. The typical American county is one where it is rising. Both of these sentences are true. They are true about the same data.1
The paradox is not merely a statistical curiosity. It points toward something deeper about how reform gets measured and, by extension, how it gets held accountable. Tip O’Neill’s observation that all politics is local is usually taken as a practical claim about electoral incentives — voters care about their own circumstances, so politicians had better attend to them. That reading is correct, but it undersells the formal implication. Scholars of legislative behavior (Fenno and Mayhew, especially) showed us that legislators don’t simply respond to local interests among many competing concerns — they organize their entire representational world by geographic proximity, allocating attention and credit-claiming in a hierarchy ordered by how close to home an interest sits. The practical consequence is that the politically relevant signal about prison reform, for most legislators, is not the national aggregate. It is what is happening in their district, their county, their constituent calls. A senator representing a rural state whose counties are experiencing rising incarceration is not going to be moved by a national decarceration trend — not because they are inattentive, but because the signal that reaches them is local and the accountability they face is local. The national aggregate is simply not in their information environment in any politically actionable sense.
This is where O’Neill’s maxim acquires a formal bite that goes beyond the conventional reading. If the relevant unit of democratic accountability for criminal justice is the county — and it largely is, because sheriffs, prosecutors, and jail administrators are all locally elected and locally accountable — then a national aggregate is not merely incomplete. It is measuring outcomes at a level that corresponds to almost no legislator’s actual decision environment. Reform that appears only in national aggregates has not yet reached the accountability structure that would sustain it.
The connection to what political scientists call the Ostrogorski paradox should be clear, and it is not coincidental. Ostrogorski’s observation was that a majority of voters can support a party while opposing a majority of that party’s positions, because majority preference is not transitive across issues. The spatial version of the same problem is this: you can have a national majority for reform — in the sense that most people live in jurisdictions where incarceration is falling — while simultaneously having a majority of the relevant decision-making jurisdictions moving in the opposite direction. The compound majority problem is not just about issue bundling. It is about the geometry of representation itself.
None of this is comfortable for either side of the standard reform debate. The national decarceration trend is real, and the people and organizations who produced it deserve credit. But a reform movement whose gains are concentrated in large urban counties, while rural and small-city jails keep growing largely unnoticed, has not solved the problem it set out to solve — it has solved the part of the problem that was easiest to measure in the way the measurement was already set up to reward. That is a different thing. Distinguishing between the two requires disaggregating the data, sitting with the messiness my colleague’s talk made vivid, and asking what level of aggregation the question actually demands. All politics is local. So is all accountability. The aggregate, as a rule, is neither.
With that, I leave you with this.2
1 This is not the first time this blog has encountered Simpson’s paradox lurking inside an apparently straightforward policy debate. Readers with long memories may recall a 2012 post on how the paradox was embedded in the measurement of Adequate Yearly Progress under No Child Left Behind. The structure is identical; the stakes are higher.
2 Gil Scott-Heron, “The Revolution Will Not Be Televised.” The national aggregate, like television, will not be where the real story is happening.