The problems with Modern Portfolio Theory (MPT)
“..I would like to spend a moment on MPT, as I believe it is important to understand the shortcomings of the prevailing approach to investment and risk management. [..] Let’s take a closer look at three of the most important assumptions behind MPT (there are many more assumptions behind Modern Portfolio Theory. Wikipedia is a good place to start should you wish to read more about it):
1. Risk-free investments exist and every rational investor invests at least some of his savings in such assets, which pay a risk-free rate of return.
2. Returns are independently and identically-distributed random variables (returns are trendless and follow a normal distribution, in plain English).
3. Investors can establish objective and accurate forecasts of future returns by observing historical return patterns. Strictly speaking, this assumption was relaxed by Fischer Black in 1972 when he demonstrated that MPT doesn’t require the presence of a risk-free asset; an asset with a beta of zero to the market would suffice.
Well, if these assumptions are meant to stand the test of time, then good old Markowitz (the father of MPT) is in trouble. Truth be told, none of the three stand up to closer scrutiny. The concept of risk-free investing no longer exists, post 2008. Banks are giant hedge funds which cannot be trusted and even government bonds look dicey in today’s world. Secondly, returns are clearly not random. If you have any doubts, just look at how the trend-following managed futures funds make their money. Thirdly, from 26 years of investment experience, I can testify to the fact that historical returns provide little or no guidance as to the direction of future returns.
A new approach is required.
So what does all of this mean? First of all it means that universities and business schools all over the world should clear up their acts. Two generations of so-called financial experts have been indoctrinated to believe that MPT is how you should approach the management of investments and risk whereas, in reality, nothing could be further from the truth. It also means that investors should kick some old habits and re-think how they do their portfolio construction. Specifically, it means that: [..]
i. the notion of the “market portfolio” being an appropriate performance benchmark should be discarded;
ii. there is in reality no meaningful distinction between strategic and tactical asset allocation – the difference is illusory;
iii. investors should once and for all reject the notion that there is an optimal portfolio for each investor from which he or she should only deviate “tactically” in the shorter run;
iv. market timing deserves more credit than it is given;
v. MPT is a straitjacket preventing investors from rotating between different classes of risky assets (with vastly different risk/return profiles) as market conditions change.
Please note that this does not imply that asset allocation is irrelevant. Far from it. However, it does mean that a bespoke approach to asset allocation, where individual circumstances drive portfolio construction, is likely to be superior to a more generic approach based on a strategic core and a tactical overlay.
A solution to the problem
Here is what I would do in terms of applying his thinking into a modern day investment approach:
1. Do what you do best. Some investors are made for short-term trading. Others are much more suited for long-term investing (like me). Don’t be shy to utilize whatever edge you may have. MPT suggests that markets are efficient. Nothing could be further from the truth. If you have spent your entire career in the medical device industry, the chances are that you understand this industry better than most. Use it when managing your own assets. Insider trading is illegal; utilizing a life time of experience is not.
2. Take advantage of mean reversion. Mean reversion is one of the most powerful mechanisms in the world of investments. At the highest of levels, wealth has a long term ‘equilibrium’ value of about 3.5 times GDP. As recently as 2007, wealth was well above the long term equilibrium value and signaled overvaluation in many asset classes. But be careful with the timing aspect of mean reversion. The fact that an asset class is over - or undervalued relative to its long term average tells you nothing in terms of when the trend will reverse. A good rule of thumb is to buy into asset classes when they are at least a couple of standard deviations below their mean value.
3. Be cognizant of herding. We are all guilty of keeping at least one eye on other investors, and we are certainly guilty of letting it influence our own investment decisions. This is how investment trends become investment bubbles and fortunes are wiped out. Herding is relatively easy to spot despite the fact that former Fed chairman Alan Greenspan argued otherwise – probably because it was a convenient argument at the time. But herding is also subject to the greater fool theory. You can make a lot of money investing in fundamentally unsound assets, as long as you can find a greater fool to whom you can sell it at a higher price. It works fine but only to a point.
4. Think outside-the-box. All those millions of baby boomers all over the western world who will retire in the next 10-15 years have been told by the MPT-trained financial advisers that they need to lighten up on equities and fill their portfolios with bonds, because they need the income to live on in old age. STOP! Who says that bonds can’t be riskier investments than equities? When circumstances change, you should change your investment approach accordingly and not rely on historical norms. Given the state of fiscal affairs in Europe and North America, it does not seem unreasonable to suggest that circumstances have indeed changed.
5. Bring non-correlated asset classes into the frame. One should consider having a core allocation to non-correlated assets. Traditionally, many non-correlated asset classes have not met the liquidity terms required by the majority of investors [..], but there are exceptions, the most obvious one being managed futures. The asset class proved its worth in 2008 with managed futures funds typically up in the range of 20-30% that year.
6. Take advantage of investor constraints and biases. The classic, but by no means only, example is the outsized impact a downgrade to below investment grade (i.e. a credit rating below BBB) may have on corporate bonds, as some institutional investors are not permitted to own high yield bonds and are thus forced to sell regardless of price when the downgrade takes place.”
Source: Birthdays and Investment Risk.