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Understanding Chicago’s homicide spike; comparisons to other cities

Michael Masinter writes:

As a longtime blog reader sufficiently wise not to post beyond my academic discipline, I hope you might take a look at what seems to me to be a highly controversial attempt to use regression analysis to blame the ACLU for the recent rise in homicides in Chicago. A summary appears here with followup here.

I [Masinter] am skeptical, but loathe to make the common mistake of assuming that my law degree makes me an expert in all disciplines. I’d be curious to know what you think of the claim.

The research article is called “What Caused the 2016 Chicago Homicide Spike? An Empirical Examination of the ‘ACLU Effect’ and the Role of Stop and Frisks in Preventing Gun Violence,” and is by Paul Cassell and Richard Fowles.

I asked Masinter what were the reasons for his skepticism, and he replied:

Two reasons, one observational, one methodological, and a general sense of skepticism.

Other cities have restricted the use of stop and frisk practices without a corresponding increase in homicide rates.

Quite a few variables appear to be collinear. The authors claim that Bayesian Model Averaging controls for that, but I lack the expertise to assess their claim.

More generally, their claim of causation appears a bit like post hoc propter hoc reasoning dressed up in quantitative analysis.

Perhaps I am wrong, but I have come to be skeptical of such large claims.

I asked Masinter if it would be ok to quote him, and he responded:

Yes, but please note my acknowledged lack of expertise. Too many of my law prof colleagues think our JD qualifies us as experts in all disciplines, a presumption magnified by our habit of advocacy.

I replied, “Hey, don’t they know that the only universal experts are M.D.’s and, sometimes, economists?”, and Masinter then pointed me to this wonderful article by Arthur Allen Leff from 1974:

With the publication of Richard A. Posner’s Economic Analysis of Law, that field of learning known as “Law and Economics” has reached a stage of extended explicitness that requires and permits extended and explicit comment. . . . I was more than half way through the book before it came to me: as a matter of literary genre (though most likely not as a matter of literary influence) the closest analogue to Economic Analysis of Law is the picaresque novel.

Think of the great ones, Tom Jones, for instance, or Huckleberry Finn, or Don Quixote. In each case the eponymous hero sets out into a world of complexity and brings to bear on successive segments of it the power of his own particular personal vision. The world presents itself as a series of problems; to each problem that vision acts as a form of solution; and the problem having been dispatched, our hero passes on to the next adventure. The particular interactions are essentially invariant because the central vision is single. No matter what comes up or comes by, Tom’s sensual vigor, Huck’s cynical innocence, or the Don’s aggressive romanticism is brought into play . . .

Richard Posner’s hero is also eponymous. He is Economic Analysis. In the book we watch him ride out into the world of law, encountering one after another almost all of the ambiguous villains of legal thought, from the fire-spewing choo-choo dragon to the multi-headed ogre who imprisons fair Efficiency in his castle keep for stupid and selfish reasons. . . .

One should not knock the genre. To hold the mind-set constant while the world is played in manageable chunks before its searching single light is a powerful analytic idea, the literary equivalent of dropping a hundred metals successively into the same acid to see what happens. The analytic move, just as a strategy, has its uses, no matter which mind-set is chosen, be it ethics or psychology or economics or even law. . . .

Leff quotes Posner:

Efficiency is a technical term: it means exploiting economic resources in such a way that human satisfaction as measured by aggregate consumer willingness to pay for goods and services is maximized. Value too is defined by willingness to pay.

And then Leff continues:

Given this initial position about the autonomy of people’s aims and their qualified rationality in reaching them, one is struck by the picture of American society presented by Posner. For it seems to be one which regulates its affairs in rather a bizarre fashion: it has created one grand system—the market, and those market-supportive aspects of law (notably “common,” judge-made law)—which is almost flawless in achieving human happiness; it has created another—the political process, and the rest of “the law” (roughly legislation and administration)—which is apparently almost wholly pernicious of those aims.

An anthropologist coming upon such a society would be fascinated. It would seem to him like one of those cultures which, existing in a country of plenty, having developed mechanisms to adjust all intracultural disputes in peace and harmony, lacking any important enemies, nevertheless comes up with some set of practices, a religion say, simultaneously so barbaric and all-pervasive as to poison almost every moment of what would otherwise appear to hold potential for the purest existential joy. If he were a bad anthropologist, he would cluck and depart. If he were a real anthropologist, I suspect he would instead stay and wonder what it was about the culture that he was missing. That is, he would ponder why they wanted that religion, what was in it for them, what it looked like and felt like to be inside the culture. And he would be especially careful not to stand outside and scoff if, like Posner, he too leaned to the proposition that “most people in most affairs of life are guided by what they conceive to be their self-interest and . . . choose means reasonably (not perfectly) designed to promote it.”

I’m reminded of our discussion from a few years ago regarding the two ways of thinking in economics.

Economists are often making one of two arguments:

1. People are rational and respond to incentives. Behavior that looks irrational is actually completely rational once you think like an economist.

2. People are irrational and they need economists, with their open minds, to show them how to be rational and efficient.

Argument 1 is associated with “why do they do that?” sorts of puzzles. Why do they charge so much for candy at the movie theater, why are airline ticket prices such a mess, why are people drug addicts, etc. The usual answer is that there’s some rational reason for what seems like silly or self-destructive behavior.

Argument 2 is associated with “we can do better” claims such as why we should fire 80% of public-schools teachers or Moneyball-style stories about how some clever entrepreneur has made a zillion dollars by exploiting some inefficiency in the market.

Despite going in the opposite directions, Arguments 1 and 2 have a similarity, in that both used to make the point that economists stand from a superior position by which they can evaluate the actions of others.

But Leff said it all back in 1974.

P.S. We seem to have mislaid our original topic, which is the study of the change in homicide rates in Chicago. I read the article by Casell and Fowles and their analysis of Chicago seemed reasonable to me. At the same time, I agree with Masinter that the comparisons to other cities didn’t seem so compelling.


  1. Jonathan says:

    Two comments. First, I remember the economic analysis ideas permeated law schools. Much of the lead work was done at Yale. It was attractive to the legal community at first because it generated a form of answer to difficult questions: as in, if there’s a polluter near a bay, who can sue and what might the recovery be? In traditional law, you have to establish harm, but that might mean no one can sue because you can’t necessarily prove harm from exposures that happen over years. And you can’t tie cases together when people move. And so on. But it turned out fairly rapidly, after a series of fairly high profile case (in the law community at least) that this was really being developed as a social policy tool: you can create a community of those harmed because an activity can be labeled as having a certain degree of harm, which takes care of the ‘who actually has leukemia’ problem, and further you can create a community of those affected by other forms of harm, so you can value loss of habitat or a view as an economic thing. It is this argument which is actually behind the fight over the Supreme Court: is the law a continually expanding thing which draws in bits and pieces from other fields to reach results that – and here is the nexus of difference – individual judges and then appellate judge panels review for accuracy, fairness and truth? Or is the law set by legislatures bound in this country by both the Constitution and state constitutions and is it up to the legislative process to determine such issues? To narrow in on the difference, there was an outburst of cases which generated opinions where the findings of fact run to scores if not hundreds of pages, much of it technical detail, all ‘found’ in many cases by a single judge. In the legal system, this tends to lock those findings in as fact that must be accepted as true at the appellate level (which caused (and causes) a lot of gyration by appellate judges when they don’t agree with those findings). This connects to the Leff article: as you amply demonstrate, the courts were deceived by the idea that there is a statistical truth, that arguments using data that’s less than perfect can be statistically evaluated – though you as judge have no idea how to do that – and these findings can be presented and then weighed for credibility in the traditional judicial sense, though you as judge have no idea what the witnesses are really saying. That is the conservative position in judicial interpretation. The liberal position, as it has developed, is these arguments are ways of framing social policy arguments so the judicial system can rights wrongs that the legislatures won’t or perhaps can’t. That is an actual substantial diffference in viewpoint over the role of the judiciary. One reason it developed is the recognition the judicial system maintained Jim Crow and that it took rigorous proof that separate was not equal to overturn it. You can see the mutation of Brown v Board through the years up to, for example, the district court in Michigan ordering cross-district busing – meaning they sued the state (Miliken v Bradley) and forced inclusion of cities and towns outside Detroit because the court, meaning a single judge, felt that no remedy within the City was sufficient.

    (As an aside, I still find it amazing that 4 justices voted to impose that remedy. I went to private school in the suburbs then. There was massive fear: all these people who had done nothing but try to have lives were incredibly afraid their children would be taken out of their own neighborhoods, that all the lives they had struggled to make would be disrupted, all to right wrongs that they hadn’t committed. It was social engineering on a massive scale involving millions of people. It is that kind of activism which conservatives equate to the USSR except the government wasn’t ordering this: a handful of unelected people were deciding what you could do, how you could live, all in the name of fixing something for some other group of people without even knowing if any of that would work.)

    I read the study and notes. The approach seems reasonable to me but I can’t exclude other factors combining to make the surge, including clear rises in gang activity (which could be related to other policing changes). I’m willing to say it could certainly be a factor but ‘cause’ is a stretch given this is a one-off.

    • Jonathan says:

      I was distracted by my cat and hit submit before I was done. Sorry.

      Much of what you regularly discuss touches legal doctrines. A big one is ‘proximate cause’: the idea is that when there are multiple actors and multiple actions which contribute to an outcome, you have to restrict the concept of cause legally. In the ideal, you have an obvious chain of events but in law school you get made up cases like someone is poisoned, is then knifed, is then shoved out a window, is then shot as he passes by another window, is then picked up by an ambulance and then the drunk ambulance driver smashes into a bridge. Who is guilty of what?

      So for example, if you walked into ‘my’ court and said that the ACLU caused your son’s death, then why not apply the same economic rationales used to calculate that these people get damages because maybe some contaminants might cause leukemia because maybe there’s a cluster of leukemia cases related to that company? The ACLU would then argue that they may have caused this – which they’d deny but they’d have to say even if we did then – that it was the Chicago police which agreed and that it was community groups which made them agree so they aren’t the ‘proximate cause’. To take that further, since you’re part of the community, that means you’re responsible for your own son’s death! You just have to draw the causal line around a bigger circle.

  2. Tom Passin says:

    One thing they didn’t do is to account for the weather. They did a seasonal adjustment, but they didn’t report on whether the summer weather was close to the average adjustment or not. It’s apparent from the data that the summer temperatures have a strong effect, so the actual temperatures (or their deviation after seasonal adjustment) should have been included. And there might have been more to it than just seasonal temperature. Days of rain could also have been a factor, but that is unexamined also.

    • Andrew says:


      Yes, one thing that’s not well understood in this sort of model is that, if you have an important alternative predictor like this, it’s not enough to just include it in the regression; it also has to be interacted with any other predictors of interest. A familiar example in public health studies is smoking. If you’re doing an epidemiological study and the treatment and control groups differ a lot on smoking status, you can’t just throw smoking as a predictor in your regression model and think that everything’s gonna be ok. Similarly with the age predictor in the fat-arms-and-political-attitudes study that we discussed a few years back.

    • Wonks Anonymous says:

      This is from the summary post:
      “First, as to the explanatory variables in our equations: In our most extensive model, we employ twenty variables—specifically stop and frisks (of course); temperature (since crime tends to spike in warm weather months); 911 calls (as a measure of police-citizen cooperation); homicides in Illinois excluding Chicago (as a measure of trends in Illinois); arrests for property crimes, violent crimes, homicides, gun crimes, shooting crimes, and drug crimes; homicides in St. Louis, Columbus, Louisville, Indianapolis, Grand Rapids, Gary, Cincinnati, Cleveland, and Detroit; and a time trend variable. All of these variables were based on monthly data, since we were attempting to explain homicide data reported on monthly basis.”

      So they did account for temperature (rather than simply doing a seasonal adjustment) BUT it’s monthly temperature and wouldn’t be fine-grained enough to detect variation among different days within a month. “Weather” also consists of more than just temperature, there are also things like precipitation & wind.

  3. Thanatos Savehn says:

    Speaking of the intersection of Law Street and Statistics Avenue, here’s something of interest:

  4. I use to follow the Becker-Posner Blog

    Richard Posner is a hoot. I enjoyed his scholarship. One of the most interesting thinkers I’ve come across.

  5. Jonathan says:

    So Chicago medical examiner data is available publicly. The data appear to be updated daily. Myself and some other researchers have already started analyzing it. Nothing that I can report on yet but you could check the data at a much more granular level here:

    Also its important to note that in some preliminary geospatial work with the data we are finding opioid and firearm deaths tend to be located in similar areas…

  6. Dale Lehman says:

    While not specific to law and economics, an even earlier article than Leff is Axel Leijonhufvud, 1973, “Life Among the Econ,” Western Economic Journal, 11:3.
    Here is one gem from this parody about the econ tribe:
    “They are poor- except for a tiny minority, miserably poor. Their population growth is among the highest in the world. Their land is fairly rich, but much of the natural resources that are their birth-right has been sold off to foreign interests for little more than a mess of pottage. Many of their young are turning to pot and message. In their poverty, they are not even saved from the problems of richer nations – travellers tell of villages half-buried in the refuse of unchecked modl-making and of the eye-sores left on the once pastoral landscape by the random strip-mining of the O’Metrs. It is said that even their famous Well Springs of Inspiration are now polluted.”

  7. Terry says:

    The relationship is so strong you don’t even need statistics. You just need to look at the graphs of arrests versus murders. The causal link is likewise obvious. And this happened in a number of cities hit by BLM-style activism.

    Recently there was a post about the moral cowardice of not admitting when we are wrong. Many of us have been moral cowards for not facing up to our culpability here. A symbolic way for us to do this would be to apologize to Officer Wilson who was unjustly maligned for being attacked by Michael Brown.

    “We were wrong Officer Wilson, and we apologize”.

    Ta Nahasi Coates should be shunned by polite society until he has the moral courage to do this.

  8. oncodoc says:

    Isn’t the ACLU nationwide? I am not a law enforcement scholar, but wouldn’t the pernicious effects of the ACLU be more widespread? In 1991 there were 922 homicides in Chicago and 760 in 2016. New York City had 2245 in 1990 with a dramatic drop to 290 in 2017. I would think that these differences would reflect local factors rather than a national organization. Of course, it is good that our entire nation has seen trends toward decreasing murders and other violent crimes over the past 25-30 years.

  9. Mike Maltz says:

    I read Cassell’s paper and commented on it when he presented it in Chicago. I’ll be happy to send whoever wants to see the video of his presentation and my comments to anyone. It’s easy to find my email address. But prior to that (by about a year) I criticized the restrictions placed on police stops in a post on (Google “police stops” to find it). Basically, by restricting police activity in high-crime neighborhoods you are reducing the protection given the residents of those neighborhoods, over 95% of whom live in fear of getting shot while walking near their home.

  10. Mike Maltz says:

    Further, the gang problem in NYC, which reduced stops and frisks without any consequence, is very different from the gang problem in Chicago, which is why the same tactics won’t work there. Ceteris ain’t always paribus.

  11. Alex says:

    I know you kind of came back around in the P.S., but man this post took a heck of a turn.

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