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A portfolio that is invested in multiple instruments whose
returns are
uncorrelated will have an
expected simple return which is the
weighted average of the individual instruments' returns. Its
volatility will be less than
the weighted average of the individual instruments' volatilities. This is
diversification. Diversification is the
"free lunch" of finance. It means that an investor can reduce
market risk simply by investing
in many unrelated instruments. The risk reduction is "free" because
expected returns are not affected. The concept is often explained with the age-old saying
"don't put all your eggs in one basket."
Diversification should not be confused with
hedging, which is the taking of offsetting risks. With
diversification, risks are uncorrelated. With hedging, they have negative
correlations.
A common misperception is the notion that the more uncorrelated risks a
portfolio is exposed to, the lower that portfolio's overall market risk
will be. This is not true. If a portfolio is
leveraged in order to take
new risks, the net result is likely to be an increase in risk.
Let's consider a common example:
A salesman for a foreign exchange trading firm approaches
the trustees of a pension plan and proposes that they add a currency
overlay strategy to their existing portfolio of domestic
stocks and
bonds. The strategy will consist of an actively traded portfolio of
currency forwards. Because forwards represent
long/short positions, they
require little or no up-front investment. Accordingly, the
strategy could be implemented without changing any of the plan's existing
investments. That is why it is called an "overlay" strategy.
In addition to possibly generating positive returns, the salesman argues that the added exposure to currencies
will have a diversifying effect on their portfolio—decreasing the
portfolio's total market risk.
Is the salesman right? Will the overlay
strategy reduce the portfolio's market risk? At first blush, it is
difficult to say. Fluctuations in the value of the overlay portfolio
should have little or no correlation with returns on the existing
portfolio. On the other hand, the overlay strategy introduces a new risk
in addition to the portfolio's existing risks.
In fact, the salesman is wrong. Far from reducing market
risk, the overlay strategy will increase total market risk. The overlay strategy
does diversify the portfolio's risks, but it also leverages them. The
diversification effect will reduce market risk, but this will be more than
offset by the leveraging effect.
Let's look at the situation in terms of eggs and baskets.
Suppose you are carrying a basket of 12 eggs. To diversify your risk, you
might obtain a second basket and place six of the eggs in it. Now,
carrying one basket in each hand, you will have reduced risk. Suppose
instead, you act under a misperception that risk is reduced by simply
carrying more baskets of eggs. Instead of dividing your 12 eggs between
two baskets, you instead offer to carry your friends basket of 12 egg as
well as your own. Now you are carrying two baskets of 12 eggs each. In financial
terminology, you have leveraged your position. The net result is an
increase in risk. In effect, this is what the salesman's overlay strategy
will do to the pension portfolio.
For diversification to work, it is not sufficient to add
risks to a portfolio. Instead, where there are concentrations of risk,
these need to be reduced while other, unrelated risks are taken on.
The issue of how investors can use
diversification to optimize their portfolios is a central concern of
portfolio theory.
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