Diversification - A Useful Tool, Until You Need It!
Essentially, Markowitz showed that selecting assets that have a positive expected return but exhibit low or (preferably) negative correlation to one another produces a combined portfolio that retains the positive expected return properties, but with lowered risk (as defined by variance).
Theoretically, this result arises due to the presence of at least two major sources of risk: nonsystematic (or unique) risk and systematic (or market) risk. While it is very difficult to eliminate market risk, it is possible to reduce the risks associated with unique investment assets. By combining investment assets that are subject to certain specific, unique risks with other investment assets that are subject to other unique risks, it may be possible to reduce the overall risk of the combined portfolio.
For the past several decades, this has been the mantra to which all investment managers adhered. Unfortunately, recent experiences in the capital markets have led both academics and professional investment practitioners to rethink portfolio construction. With the increasing interconnectedness of global markets and investment pools, we have seen that correlation structures among various investment assets are not always stable.
In fact, assets that typically exhibit low correlation with one another can dramatically change direction and begin exhibiting increased correlation during periods of market distress. The increased correlation leads to a reduction in the power of diversification and thus to increased risk in the overall portfolio. Unfortunately, this upward shift in correlation happens at exactly the time when an investor needs correlation the most: market distress.
