Wednesday, 25 January 2017

Understanding Household Spending with Stocks and Flows

Household expenditure is the largest component of GDP, so understanding why and how it varies is crucial to understanding recessions.

Mainstream economics tends to look to the real interest rate and a household rate of time preference as the central explanation for such variations.  Stock flow models tend to place greater emphasis on the levels of financial stocks. 

The idea here is that household expenditure is driven by a desire to influence various ratios between financial stocks and the level of income.  Stock-flow models often describe certain target ratios, sometimes called stock-flow norms.   Household expenditure is equal to income[1] less the net acquisition of net financial assets.  If we can explain the acquisition and disposal of financial assets and liabilities in terms of a desire to move towards stock-flow norms, then we can explain why household expenditure might vary.

Looking at the UK, it is useful to divide household net financial assets into the following:

1. Pensions and life insurance
2. Other financial assets
3. Debt (i.e. liabilities of households)

The graph below shows the level of each of these relative to household disposal income.

I'll say something about each of these in turn, dealing first with pensions, then debt, then other financial assets.


It seems likely that there should be some kind of target ratio between pension assets and income.  On the whole, people want to achieve a balance between what they can spend during their working years and what they can spend in retirement.  This means they have to save a certain amount whilst working to be run down in later life. This process is naturally going to generate a stable stock-flow ratio.

In fact, we don't need to even postulate that people have a given stock-flow ratio in mind.  Much of the time, the amount of pension contributions will be in a fixed relationship to wage levels, and amounts paid as pensions will be related to the stock level of pension funds.  Institutional arrangements including the tax system influence this.  This process will give rise to a stock-flow norm, even if no-one has such a ratio in mind.

If we look at the graph, however, we can see a clear upward trend in the ratio of pension assets to income.  There are various reasons for this.  The most important is rising life expectancy.  This has reduced the amount of pension income that a given stock of pension assets will buy.  Achieving a similar balance between working and retirement consumption therefore means building a greater stock of pension assets during the working lifetime.

The decline in the yields on government securities has a similar effect.  Pension annuities are priced off fixed income yields, so as these have come down, pension funds need to become larger.

The actual size of pension funds is also impacted by the returns on those funds including stock market gains.  So we should expect to see the actual ratio rising when the stock market is doing well.


The ratio of debt to income shows less fluctuations than the two asset classes, because it is not subject to the sort of variations in value due to market movements.

With debt levels, it also seems likely that that stock-flow ratios are important.  In particular, credit constraints will tend to limit the amount that can be borrowed relative to income.  The growth in debt levels between around 1999 and 2008 reflect a general relaxation of credit criteria. 

Maximum acceptable debt levels are often assessed by comparing the interest expense to income.  The upper bound to the debt to income ratio is therefore likely to be a function of interest rates to some extent.  At the current low level of rates, interest expense is at its lowest level of this period (see chart below), which partly explains why the debt ratio remains high despite tightening credit conditions.

Credit constraints may impose a cap on this stock-flow ratio, but that is not the same as saying this is a target ratio.  It could be that households prefer a debt ratio below that dictated by credit conditions.  However, it seems likely that credit conditions provide the binding constraint in most cases for new borrowers, and this element is a key driver of the stock-flow ratio.

Other Assets

Whilst there are good reasons for thinking that stock-flow considerations play an important role in determining accumulation of pensions and debt, it is less clear in the case of other assets.  Typical stock-flow consistent models tend to assume that there is a target ratio for holdings of other assets, but I'm not sure this is necessarily the case. 

With pensions and debt, we should be able to predict what sort of target ratios we expect to see from a knowledge of things like life patterns, the tax structures, regulatory issues and so on.  No such considerations determine the holding of other assets, so it's quite possible that any target ratio could develop over time, including in response to changes in the actual.

However, looking at the first graph, the ratio of other assets has actually changed the least over time.  Apart from an apparent slight upward trend, two factors appear to drive the variations.

1. As with pensions assets, the level of other financial assets is subject to variations in asset valuations such as stock market movements.

2. The periods of strong growth in debt levels seems to be associated with an increase in holdings of other financial assets.  There are reasons why we might expect this to be the case.  The largest part of debt is for house purchase, and an increase in debt reflects a greater volume of house purchase and/or higher house prices.  This means that those households receiving property sales proceeds and not repaying debt (otherwise overall debt levels are not increasing) are receiving greater windfalls.  This type of cash receipt, does not get spend all at once, if it gets spend at all, and it therefore adds to the accumulation  of deposits, which makes up the largest part of this catergory of assets.

Once these elements are taken into account, it does appear that there is some reversion-to-norm taking place in this stock-flow ratio.  When the actual ratio is increased or decreased by one of these factors, accumulation reacts to bring it back to a more "normal" level.  It is remains an open question, however, how stable this "normal" level is over time.

An analysis of household stock-flow ratios provides a useful insight on what drives changes in household spending.  This is particularly so when we apply a certain level of disaggregation, rather than looking at net finacial assets as a single whole.  Often, we can relate the trends to things that we know are going on.  This approach is likely to be more useful than trying to relate spending to variations in the real interest rate.
[1] Household disposable income is usually calculated after certain pension contributions and pension income.  To align more closely with the theoretical issues, we need here to think about household income before these items.


  1. "Maximum acceptable debt levels are often assessed by comparing the interest expense to income."

    They are. That's probably not realistic with household debt levels where the principal amount starts to dominate over the term.

    Even with mortgages the average £250K house is about 50/50 interest and principal on the first payment over 25 years when interest is at 3%.

    1. That's true. It would probably be better to say that maximum debt levels are assessed by comparing monthly payments to income, and that payment levels depend on the interest rate.

      As the interest rate gets lower it has less and less impact on reduced payments. So even if payment cover was the only criteria and if the interest rate were to go to zero, we would still expect to see debt level limits.

      However, I think it is still correct to say that the curent low interest rate plays a role in facilitating debt ratios at the level they are.

  2. Wealthy households, not-for-profits that hold large financial savings, and the less wealthy retirement households will hold financial assets which are based on the liabilities of the typically younger working class households. The price of houses must rise to validate the mortgage debt of working class households. The feedback mechanism between asset prices and debt must be specified and the bailout activities of governments must be linked to asset prices to have a coherent model.

    1. I haven't attempted to give a comprehensive account of all the factors that might be in play here, but certainly the value of non-financial assets, specifically housing, would need to be considered as well.