In previous posts I have talked about why I am suspicious of (but not completely opposed to) the idea that the UK (or US) has a serious problem because there is too much personal debt. Too much popular discussion goes as follows: booms and busts are often caused by excess lending and borrowing, household debt to income ratios are currently high compared to a few decades ago, and so we must be on the verge of a new personal debt crisis. The first two points are true, but the third does not follow because of one thing: house prices.
I thought I would illustrate the key point with a graph, based on data from the OECD’s Economic Outlook.
The yellow line is house prices relative to income: the absolute level is arbitrary. The red line is mortgage debt as a ratio to personal income, and the blue line is the total debt to income ratio. The green line is the difference between the blue and red i.e. non-mortgage debt relative to income.
The key point is that most of total household debt is mortgage debt, and this follows house prices. That the two should track each other over the long term is not surprising, but the fact that mortgage debt seemed to fall exactly with house prices is. (If house prices fall, this changes the value of new mortgages, but not the value of existing mortgages.) The reason may be that in the short term the interaction is two way. A fall in the demand for house purchase (and hence mortgages) will impact on price. Non-mortgage debt is now a little lower relative to income than before the crisis.
The basic story is therefore very simple. The main reason people go into debt is to buy a house. The more expensive houses get, the more they have to borrow. If there is a problem, it is not that we have all gone on unaffordable spending sprees. It is that house prices have been rising. Rising house prices increase not only household debt but household wealth, which is a key reason why wealth was also rising rapidly before the financial crisis.
The picture for the US is similar, except that non-mortgage debt has returned to pre-crisis levels.
This suggests no near term risk of any private debt crisis. Indeed for the UK, as Chris Giles reminds us, 2008 itself was not a crisis about personal debt, but a crisis about UK banks overseas lending. As a result, talk about private debt nearing ‘2008 crisis levels’ in the future is highly misleading.
There are two reasons why house prices have been rising in the UK: not enough houses are being built and real interest rates have gradually declined (secular stagnation). As governments have relatively little control over long term real interest rates, you will only reduce mortgage debt by reducing house prices by building more houses. To put it very simply, the aggregate private debt problem in the UK is a reflection of our longstanding inability to build houses.
That is a serious problem, and not just because it prevents a lot of potential first time buyers from being able to afford to buy. It means that, if interest rates were to rise significantly, households with mortgages would be spending much more of their income paying off the mortgage, and they would be more vulnerable to shocks to income as a result. One of the problems with the recent relatively slow growth in nominal wages is that the real burden of a fixed nominal mortgage has not been falling much as the mortgage grows older.
Worse still, if real interest rates did start to recover (secular stagnation proved to be less permanent than many people currently think) this would in itself tend to reduce house prices. That could leave many relatively new home owners with a mortgage larger than their house was worth. In the UK people cannot walk away from this negative equity. Equally lenders could have loans that were no longer covered by the value of an asset. Deflation coupled with rising real interest rates is a toxic mix. But all of these problems reflect the fact that house prices are currently too high. 
I think the simple takeaway is this. Anyone who talks about the growing problem of total private household debt without also talking about what has and what will happen to house prices is missing the elephant in the room.
 It is tempting to write that high levels of private debt are a symptom rather than a cause of these problems. That is too strong: people choose to take out a mortgage to buy a house rather than rent. However as most people only own one house, it has an element of truth. It seems odd to argue that an irresponsible debt fuelled increase in the desire to own houses is pushing up house prices.