I respectfully differ with your suggestion that time periods in Modern Portfolio Theory should be separated by decades. One of the criticisms of Modern Portfolio Theory is that it sometimes produces ill advised allocation results too heavily skewed toward one or two asset classes in violation of the very tenet of broad diversification designed to avoid portfolio volatility resulting from being too heavily allocated in too few investment types. One of the reasons this occurs is that a base of data is used which is too short and absent of the litany of market conditions that occur over long periods of time. The time period selected in the article’s analysis, nearly 87 years, includes the breadth of types of markets that occur over time, bull markets, bear markets, flat markets, recession, depression, war, peace, etc. It is only by consideration of all these in a single analysis that Modern Portfolio Theory can properly advise the investor so that they can achieve their long-term goals while experiencing these different markets.

Modern Portfolio Theory does not care why changes occur from decade to decade and I would argue that it is not necessary for the passive investor to care either. The results of the strategy incorporate all the investor needs to know to maintain as straight a line as is possible toward their ultimate financial goal.

The mutual funds and index tracking stocks the article suggests as preferable for a young investor with a long time horizon are indeed the small cap stocks in the more volatile stock market with their higher yields obtained over time. Agreed that a more diversified allocation should be transitioned into over time as the investor inches closer to retirement.

Thanks for taking the time to post your thoughts!