Large models, small models and Brexit

Non-economists with no interest in modelling techniques can skip to paragraph starting 'How is this all related to Brexit'.

I promised to look at some of the other papers in the OxREP volume “Rebuilding macroeconomic theory” besides my own, but as usual other things - including Brexit - got in the way. In this post I want to talk about the paper by Haldane and Turrell, which is about Agent Based Models, or ABMs. Right at the end of this post, however, I will come back to Brexit.

As a result of the microfoundations hegemony, any paper talking about a different modelling strategy often feels it must start by describing some drawbacks of that hegemony, and this paper is no exception. I might talk about that some other time, but instead I want to recommend what I think is one of the most realistic discussions of what ABM can or cannot do I have read.

As you might guess from the name, ABMs model the economy as a collection of a large number of different agents, each of which behaves in a specified way. The authors generalise the idea of a choice between internal and external consistency that I talk about in my paper to also include a degree of heterogeneity.

As you can see, ABMs are all about allowing as much heterogeneity as you wish. This is not to say that other methods cannot do heterogeneity (they can), but ABMs major in this dimension, and in practice often keep the behaviour of agents relatively simple compared to a DSGE. (A slight quibble: I would argue that as DSGEs are internally consistent by definition, the orange square representing them should be a slimmer and perhaps taller rectangle.) ABMs (within the bounds of tractability) owe no allegiance to any school of thought: the paper has a nice table of the many different types of consumption function used in a range of ABM studies.

As the macroeconomy is indeed made up of many different types of agents who may be doing different things, and whose interaction may produce unexpected results, it seems like ABMs can only be a good thing. But this additional freedom brings a large cost. Because, and unlike some hard sciences, there is a large amount of uncertainty about how people actually behave, we cannot treat any model as a black box, the output from which has to be accepted without question. No civil servant or central bank economist can go to politicians or governors and simply say it is what the model said.

Exactly the same problem can arise with SEMs, simply because of their complexity or disaggregation. It could also arise from a complex DSGE. The first question any economist asks when seeing an output from any large and complex model is does the result make sense given the smaller theoretical models they carry around in their head. It is why I proposed for SEMs the process I called theoretical deconstruction, where model properties were either reduced to familiar results from simpler models, or show the limitations of those simpler models. Again, as the paper notes, a similar process needs to, and in some cases has, happened with results from ABMs.

How is this all related to Brexit? The results showing how different degrees of Brexit would do the economy damage to different extents that I talked about in my last post were produced by trade theory’s equivalent of ABMs, called computable general equilibrium (CGE) models. These allow for considerable heterogeneity (across sectors and countries) in modelling trade. As Chris Giles recounts in this excellent piece, the model is more complex than anything the Treasury had before Brexit, and was built specifically to help with Brexit.

As Chris writes

“It must have come as a bit of a shock to government economists that the moment some results of this new model were leaked this week, ministers rushed to deny the usefulness of the tools they commissioned. Such models are “always wrong”, declared Steve Baker, a junior Brexit minister, on Tuesday.”

As I note in a postscript to my last post, he went further on Thursday to suggest that civil servants had deliberately cooked the model to sabotage Brexit.

How do we know that this didn’t happen, apart from the implausibility that so many civil servants could concoct such a conspiracy. Precisely because in this case the results from a highly disaggregated model broadly agrees with most other studies, and also common sense: the more difficult you make trade, the less there will be and the more costly that will be for UK output. Chris ends with some words that should be sent to every journalist in the country.

“Ministers now have a choice. They can opt for an honest Brexit in which they argue in public that people should pay an economic price for their policies. Or they can opt for a dishonest Brexit, pretending they have a secret plan for economic nirvana and trashing their own internal economic evidence. Ministers’ initial reaction in disowning the analysis suggests deception is the government’s central Brexit strategy. People talk about a crisis in economics. After this episode, it is the crisis in politics that should really concern us.”

Abusing economic analysis: UK Treasury edition

For US readers, this is about the misuse of dynamic scoring in analysing tax changes

Ask most people if they think a particular tax - like fuel duty - should be reduced, and they will say yes. If you ask people do you think income taxes should be raised to pay for a cut in fuel duty, you will get a rather different response. So just asking people if they would like one particular tax to be cut without saying how it will be paid for is pretty meaningless. Unless of course your aim is to provide ‘evidence’ that taxes are too high, and you are not too worried about the nature of that evidence.

There is a slightly more sophisticated version of this trick, and the UK Treasury have just played it. Each individual tax potentially distorts the pattern of economic activity. If that pattern without any taxes is near some ideal, then we can call taxes ‘distortionary’. If we taxed apples and used this money to subsidise the production of pears, people would eat too many pears and not enough apples. However there is one tax that is not distortionary, because it does not influence incentives and therefore this pattern of economic activity. It is a poll tax - a tax levied on each individual independent of their income, wealth or what they spend their money on. Economists call this a lump sum tax. So cutting any distortionary tax, and paying for this by raising a poll tax, is bound to produce beneficial results in terms of reducing distortions.

There is only one problem with paying for a particular tax cut by raising a lump sum tax - in the UK we do not have a poll tax. We did very briefly - it was not very popular, because people care about fairness as well as the distortionary impact of taxes. For this reason, you should not expect to find a government department like the Treasury modelling the benefits of cutting fuel duty by assuming it was paid for by raising a poll tax. Unfortunately, that is exactly what has been done in a Treasury/HMRC report released this week

George Osborne is not planning to reintroduce a poll tax - veneration of a past Conservative Prime Minister would not go that far. I think the argument the Treasury would use to justify what they have done is simplicity. If you pay for a cut in fuel duty by, say, raising income taxes, you have to model the impact of two taxes on economic behaviour rather than just one. I don’t think that is a very good excuse, but even if we think it has some validity it has a direct implication: an individual study of this kind is meaningless on its own. It can only be used in conjunction with other studies that look at the impact of raising other taxes. Will Treasury officials therefore stop their masters using the numbers from this exercise to justify cuts in fuel duty? No prizes for guessing the answer. (They might if they could but they don’t have that degree of influence.)

So what could have been the beginning of an intelligent discussion of the costs and benefits of particular taxes (as in the Mirrlees review, for example) has been turned into a simple propaganda exercise.

Unfortunately it gets worse. Fuel duty is particularly ‘distortionary’ because its rate is high (see Chart 2.1 of the Treasury paper). There might be a good reason for that. The tax could be high because it is trying to offset damage that is not prevented by the market: road congestion, pollution and of course climate change. In terms of the language of economics it is (at least in part) a Pigouvian tax designed to offset externalities. In that case the tax is not distortionary at all: a world without fuel tax would not be ideal, and imposing a fuel tax gets us nearer that ideal. As Chart 3.1 from the paper indicates, these beneficial impacts of fuel duty are not modelled by the Treasury’s CGE model. (This is why, as John McDermott notes, this kind of partial dynamic modelling tends to be attractive to right wing outfits. Is it significant that the paper does not actually include the words ‘climate change’, and just uses the vaguer term environmental damage?)

So what the Treasury have done is modelled all the benefits of cutting the tax, but ignored all the costs. If this was but one stage in a process that would subsequently look at the cost of these externalities, and would realistically model how these tax cuts were paid for, fine. As a stand alone exercise, I’m afraid the Treasury study is worthless.

As Chris Giles notes in an excellent report, this is really part of a political exercise to build the case for tax cuts. It has two unfortunate side effects. First, it just encourages the suspicion among many that anything coming out of the UK Treasury at the moment is worthless propaganda. Second, it encourages those on the left who think that mainstream economics is inherently biased. But if you saw an opinion poll that asked people if they thought a particular tax was too high, without also asking what tax they would increase to balance the books, you would not say that this shows opinion polls are inherently biased. Instead you would just conclude that the person commissioning the poll had a political agenda. You might also ask whether the polling company should have accepted the commission.