Posted on by John Sloman

A stock market beauty contest of the machines

Each day many investors anxiously watch the stock market to see if their shares have gone up or down. They may also speculate: buying if they think share prices are likely to go up; selling if they think their shares will fall. But what drives these expectations?

To some extent, people will look at real factors, such as company sales and profits or macroeconomic indicators, such as the rate of economic growth or changes in public-sector borrowing. But to a large extent people are trying to predict what other people will do: how other people will react to changes in various indicators.

John Maynard Keynes observed this phenomenon in Chapter 12 of his General Theory of Employment, Interest and Money of 1936. He likened this process of anticipating what other people will do to a newspaper beauty contest, popular at the time. In fact, behaviour of this kind has become known as a Keynesian beauty contest (see also).

Keynes wrote that:

professional investment may be likened to those newspaper competitions in which the competitors have to pick out the six prettiest faces from a hundred photographs, the prize being awarded to the competitor whose choice most nearly corresponds to the average preferences of the competitors as a whole; so that each competitor has to pick, not those faces which he himself finds prettiest, but those which he thinks likeliest to catch the fancy of the other competitors, all of whom are looking at the problem from the same point of view. It is not a case of choosing those which, to the best of one’s judgement, are really the prettiest, nor even those which average opinion genuinely thinks the prettiest. We have reached the third degree where we devote our intelligences to anticipating what average opinion expects the average opinion to be. And there are some, I believe, who practise the fourth, fifth and higher degrees.

When investors focus on people’s likely reactions, it can make markets very unstable. A relatively minor piece of news can cause people to buy or sell in anticipation that others will do the same and that others will realise this and do the same themselves. Markets can overshoot, until, when prices have got out of line with fundamentals, buying can turn into selling, or vice versa. Prices can then move rapidly in the other direction, again driven by what people think other people will do. Sometimes, markets can react to very trivial news indeed. As the New York Times article below states:

On days without much news, the market is simply reacting to itself. And because anxiety is running high, investors make quick, sometimes impulsive, responses to relatively minor events.

The rise of the machine

In recent years there is a new factor to account for growing stock market volatility. The Keynesian beauty contest is increasingly being played by computers. They are programmed to buy and sell when certain conditions are met. The hundreds of human traders of the past who packed trading floors of stock markets, have been largely replaced by just a few programmers, trained to adjust the algorithms of the computers their finance companies use as trading conditions change.

And these computers react in milliseconds to what other computers are doing, which in turn react to what others are doing. Markets can, as a result, suddenly soar or plummet, until the algorithms kick the market into reverse as computers sell over-priced stock or buy under-priced stock, which triggers other computers to do the same.

Robot trading is here to stay. The articles and podcast consider the implications of the ‘games’ they are playing – for savers, companies and the economy.

Articles

Questions

  1. Give some other examples of human behaviour which is in the form of a Keynesian beauty contest.
  2. Why may playing a Keynesian beauty contest lead to an undesirable Nash equilibrium?
  3. Does robot trading do anything other than simply increase the speed at which markets adjust?
  4. Can destabilising speculation continue indefinitely? Explain.
  5. Explain what is meant by ‘overshooting’? Why is overshooting likely to occur in stock markets and foreign exchange markets?
  6. In what ways does robot trading (a) benefit and (b) damage the interests of savers?