Economics students are taught from an early age that in the short run aggregate demand matters, but in the long run output is determined from the supply side. A better way of putting it is that supply adjusts to demand in the short run, but demand adjusts to supply in the long run. A key part of that conceptualisation is that long run supply is independent of short run movements in demand (booms or recessions). It is a simple conceptualisation that has been extremely useful in the past. Just look at the UK data shown in this post: despite oil crises, monetarism and the ERM recessions, UK output per capita appeared to come back to an underlying 2.25% trend after WWII.
Except not any more: we are currently more than 15% below that trend and since Brexit that gap is growing larger every quarter. Across most advanced countries, it appears that the global financial crisis (GFC) has changed the trend in underlying growth. You will find plenty of stories and papers that try to explain this as a downturn in the growth of supply caused by slower technical progress that both predated the GFC and that is independent of the recession caused by it.
In a previous post I looked at recent empirical evidence that told a different story: that the recession that followed the GFC appears to be having a permanent impact on output. You can tell this story in two ways. The first is that, on this occasion for some reason, supply had adjusted to lower demand. The second is that we are still in a situation where demand is below supply.
The theoretical reasons why supply might adjust to demand are not difficult to find. (They are often described by economists under the jargon word 'hysteresis'.) Supply (in terms of output per capita) depends on labour force participation, the amount of productive capital in the economy, and finally technical progress, which is really just a catch all for how aggregate labour and capital combine to produce output. A long period of deficient demand can discourage workers. It can also hold back investment: a new project may be profitable but if there is no demand it will not get financed.
However the most obvious route to link a recession to longer term supply is through technical progress, which connects to the vast literature under the umbrella of ‘endogenous growth theory’. This can be done through a simple AK model (as Antonio Fatas does here), or using a more elaborate model of technical progress, as Gianluca Benigno and Luca Fornaro do in their paper entitled ‘Stagnation traps’. The basic idea is that in a recession innovation is less profitable, so firms do less of it, which leads to less growth in productivity and hence supply. Narayana Kocherlakota has promoted this idea: see here for example.
The second type of explanation is attractive, in part because the mechanism that is meant to get demand towards supply - monetary policy - has been ‘out of action’ for so long because of the Zero Lower Bound (ZLB). (The ZLB also plays an important role in the Benigno & Fornaro model.) However for some this type of explanation currently seems ruled out by the fact that unemployment is close to pre-crisis levels in the UK and US at least.
There are three quite different problems I have with the view that we no longer have a problem of deficient demand because unemployment is low. The first, and most obvious, is that the natural rate of unemployment might be, for various reasons, considerably lower than it was before the GFC. The second is that workers may have priced themselves into jobs. In particular, low real wages may have encouraged firms to use more labour intensive techniques. If that has happened, it does not mean that the demand deficiency problem has gone away, but just that it is more hidden. (For anyone who has a conceptual problem with that, just think about the simplest New Keynesian model, which assumes a perfectly clearing labour market but still has demand deficiency.)
The third involves the nature of any productivity slowdown caused by lack of innovation. A key question, which the papers noted above do not directly address, is whether we are talking about frontier research, or more the implementation of innovation (for example, copying what frontier firms are doing). There is some empirical evidence to suggest that the productivity slowdown may reflect the absence of the latter. This is very important, because it implies the slowdown is reversible. I have argued that central banks should pay much more attention to what I call the innovation gap (the gap between best practice techniques, and those that firms actually employ) and its link to investment and aggregate demand.
All this shows that there is no absence of ideas about how a great recession and a slow recovery could have lasting effects. If there is a problem, it is more that the simple conceptualisation that I talked about at the beginning of this post has too great a grip on the way many people think. If any of the mechanisms I have talked about are important, then it means that the folly of austerity has had an impact that could last for at least a decade rather than just a few years.