Portfolio Management

Investment Philosophy, Creating a Portfolio and Portfolio Management

We republish this work with permission of  the author Aswath Damodaran, a Professor of Finance at the Stern School of Business at New York University.

We all dream of beating the market and being super investors. So, we are easy prey for the magic bullets and the secret formulae offered by eager salespeople pushing their wares. In spite of our best efforts, most of us fail in our attempts to be more than average investors. Nonetheless, we keep trying, hoping that we can be more like the investing legends – another Warren Buffett or Peter Lynch.

In our search, though, we are whipsawed by contradictions and anomalies. In one corner of the investment town square, stands one advisor, yelling to us to buy businesses with solid cash flows and liquid assets because that’s what worked for Buffett. In another corner, another investment expert cautions us that this approach worked only in the old world, and that in the new world of technology, we have to bet on companies with solid growth prospects. In yet another corner, stands a silver tongued salesperson with vivid charts and presents you with evidence of his capacity to get you in and out of markets at exactly the right times. It is not surprising that facing this cacophony of claims and counterclaims that we end up more confused than ever.

In this introduction, we present the argument that to be successful with any investment strategy, you have to begin with an investment philosophy that is consistent at its core and which matches not only the markets you choose to invest in but your individual characteristics.

In other words, the key to success in investing may lie not in knowing what makes Peter Lynch successful but in finding out more about yourself.

What is an investment philosophy?

An investment philosophy is a coherent way of thinking about markets, how they work (and sometimes do not) and the types of mistakes that you believe consistently underlie investor behaviour. Why do we need to make assumptions about investor mistakes? As we will argue, most investment strategies are designed to take advantage of errors made by some or all investors in pricing stocks. Those mistakes themselves are driven by far more basic assumptions about human behaviour. To provide an illustration, the rational or irrational tendency of human beings to join crowds can result in price momentum – stocks that have gone up the most in the recent past are more likely to go up in the near future. Let us consider, therefore, the ingredients of an investment philosophy.

1.     Human Frailty

Underlying all investment philosophies is a view about human behaviour. In fact, one weakness of conventional finance and valuation has been the short shrift given to human behaviour. It is not that we (in conventional finance) assume that all investors are rational, but that we assume that irrationalities are random and cancel out. Thus, for every investor who tends to follow the crowd too much (a momentum investor), we assume an investor who goes in the opposite direction (a contrarian), and that their push and pull in prices will ultimately result in a rational price. While this may, in fact, be a reasonable assumption for the very long term, it may not be a realistic one for the short term.

Academics and practitioners in finance who have long viewed the rational investor assumption with scepticism have developed a new branch of finance called behavioural finance which draws on psychology, sociology and finance to try to explain both why investors behave the way they do and the consequences for investment strategies. As we examine different investment philosophies, we will try at the outset of each philosophy to explore the assumptions about human behaviour that represent its base. 

2.     Market Efficiency

A closely related second ingredient of an investment philosophy is the view of market efficiency or its absence that you need for the philosophy to be a successful one. While all active investment philosophies make the assumption that markets are inefficient, they differ in their views on what parts of the market the inefficiencies are most likely to show up and how long they will last.

Some investment philosophies assume that markets are correct most of the time but that they overreact when new and large pieces of information are released about individual firms – they go up too much on good news and down too much on bad news. Other investment strategies are founded on the belief that markets can make mistakes in the aggregate – the entire market can be under or overvalued – and that some investors (mutual fund managers, for example) are more likely to make these mistakes than others.  Still other investment strategies may be based on the assumption that while markets do a good job of pricing stocks where there is a substantial amount of information – financial statements, analyst reports and financial press coverage –they systematically misprice stocks on which such information is not available.

3.     Tactics and Strategies

Once you have an investment philosophy in place, you develop investment strategies that build on the core philosophy. Consider, for instance, the views on market efficiency in the last section.

The first investor, who believes that markets over react to news, may develop a strategy of buying stocks after large negative earnings surprises (where the announced earnings come in well below expectations) and selling stocks after positive earnings surprises.

The second investor who believes that markets make mistakes in the aggregate may look at technical indicators to find out whether the market is over bought or over sold and take a contrary position.

The third investor who believes that market mistakes are more likely when information is absent may look for stocks that are not followed by analysts or owned by institutional investors.        

It is worth noting that the same investment philosophy can spawn multiple investment strategies. Thus, a belief that investors consistently overestimate the value of growth and under estimate the value of existing assets can manifest itself in a number of different strategies ranging from a passive one of buying low PE ratio stocks to a more active one of buying such companies and attempting to liquidate them for their assets. In other words, the number of investment strategies will vastly outnumber the number of investment philosophies.

Why do you need an investment philosophy?

Most investors have no investment philosophy, and the same can be said about many money managers and professional investment advisors. They adopt investment strategies that seem to work (for other investors) and abandon them when they do not. Why, if this is possible, you might ask, do you need an investment philosophy? The answer is simple.

In the absence of an investment philosophy, you will tend to shift from strategy to strategy simply based upon a strong sales pitch from a proponent or perceived recent success

There are three negative consequences for your portfolio:

  1. Lacking a rudder or a core set of beliefs, you will be easy prey for charlatans and pretenders, with each one claiming to have found the magic strategy that beats the market.
  2. As you switch from strategy to strategy, you will have to change your portfolio, resulting in high transactions costs and you will pay more in taxes.
  3. While there may be strategies that do work for some investors, they may not be appropriate for you, given your objectives, risk aversion and personal characteristics. In addition to having a portfolio that under performs the market, you are likely to find yourself with an ulcer or worse.


With a strong sense of core beliefs, you will have far more control over your destiny

Not only will you be able to reject strategies that do not fit your core beliefs about markets but also to tailor investment strategies to your needs.  In addition, you will be able to get much more of a big picture view of what it is that is truly different across strategies and what they have in common.

The Big Picture of Investing – Creating an Investment Portfolio

To see where the different investment philosophies fit into investing, let us begin by looking at the process of creating an investment portfolio. Note that this is a process that we all follow – amateur as well as professional investors - though it may be simpler for an individual constructing his or her own portfolio than it is for a pension fund manager with a varied and demanding clientele.

Step 1: Understanding the Client

The process always starts with the investor and understanding his or her needs and preferences. For a portfolio manager, the investor is a client, and the first and often most significant part of the investment process is understanding the client’s needs, the client’s tax status and most importantly, his or her risk preferences. For an individual investor constructing his or her own portfolio, this may seem simpler, but understanding one’s own needs and preferences is just as important a first step as it is for the portfolio manager.

Step 2: Portfolio Construction

The next part of the process is the actual construction of the portfolio, which we divide into three sub-parts.

1.     The first of these is the decision on how to allocate the portfolio across different asset classes defined broadly as equities, fixed income securities and real assets (such as real estate, commodities and other assets).  This asset allocation decision can also be framed in terms of investments in domestic assets versus foreign assets, and the factors driving this decision.

2.     The second component is the asset selection decision, where individual assets are picked within each asset class to make up the portfolio. In practical terms, this is the step where the stocks that make up the equity component, the bonds that make up the fixed income component and the real assets that make up the real asset component are selected.

3.     The final component is execution, where the portfolio is actually put together. Here investors must weigh the costs of trading against their perceived needs to trade quickly. While the importance of execution will vary across investment strategies, there are many investors who fail at this stage in the process.

Step 3: Evaluate portfolio performance

The final part of the process, and often the most painful one for professional money managers, is performance evaluation. Investing is after all focused on one objective and one objective alone, which is to make the most money you can, given your particular risk preferences. Investors are not forgiving of failure and unwilling to accept even the best of excuses, and loyalty to money managers is not a commonly found trait. By the same token, performance evaluation is just as important to the individual investor who constructs his or her own portfolio, since the feedback from it should largely determine how that investor approaches investing in the future.

These parts of the process are summarised in Figure 1.1, and we will return to this figure to emphasise the steps in the process as we consider different investment philosophies. As you will see, while all investment philosophies may have the same end objective of beating the market, each philosophy will emphasize a different component of the overall process and require different skills for success. 

 Figure 1.1 The Investment Process

Categorising Investment Philosophies

We will present the range of investment philosophies in this section, using the investment process to illustrate each philosophy. While we will leave much of the detail for later, we will attempt to present at least the core of each philosophy here.

Market Timing versus Asset Selection

The broadest categorisation of investment philosophies is on whether they are based upon timing overall markets or finding individual assets that are mispriced. The first set of philosophies can be categorized as market timing philosophies, while the second can be viewed as security selection philosophies.

Within each, though, are numerous strands that take very different views about markets. Consider market timing first. While most of us consider market timing only in the context of the stock market, there are investors who consider market timing to include a much broader range of markets – currency markets, bond markets and real estate come to mind.  The range of choices among security selection philosophies is even wider and can span charting and technical indicators, fundamentals (earnings, cashflows or growth) and information (earnings reports, acquisition announcements).

While market timing has allure to all of us (because it pays off so well when you are right), it is difficult to succeed at for exactly that reason. There are all too often too many investors attempting to time markets, and succeeding consistently is very difficult to do. If you decide to pick stocks, how do you choose whether you pick them based upon charts, fundamentals or growth potential? The answer, as we will see, in the next section will depend not only on your views of the market and empirical evidence but also on your personal characteristics.

Activist versus Passive Investing

 At the broadest level, investment philosophies can also be categorised as active or passive strategies. In a passive strategy, you invest in a stock or company and wait for your investment to pay off. Assuming that your strategy is successful, this will come from the market recognizing and correcting a misvaluation. Thus, a portfolio manager who buys stocks with low price earnings ratios and stable earnings is following a passive strategy. So is an index fund manager, who essentially buys all stocks in the index.  In an activist strategy, you invest in a company and then try to change the way the company is run to make it more valuable. Venture capitalists can be categorised as activist investors since they not only take positions in promising companies but they also provide significant inputs into how these firms are run. We should hasten to draw a contrast between activist investing and active investing. Any investor who tries to beat the market by picking stocks is viewed as an active investor. Thus, active investors can adopt passive strategies or activist strategies.

Time Horizon

Different investment philosophies require different time horizons. A philosophy based upon the assumption that markets overreact to new information may generate short term strategies. For instance, you may buy stocks right after a bad earnings announcement, hold a few weeks and sell (hopefully at a higher price, as the market corrects its over reaction). In contrast, a philosophy of buying neglected companies (stocks that are not followed by analysts or held by institutional investors) may require much longer time horizons.

One factor that will determine the time horizon of an investment philosophy is the nature of the adjustment that has to occur for you to reap the rewards of a successful strategy. Passive value investors who buy stocks in companies that they believe are under valued may have to wait years for the market correction to occur, even if they are right. Investors who trade ahead or after earnings reports, because they believe that markets do not respond correctly to such reports, may hold the stock for only a few days. At the extreme, investors who see the same (or very similar) assets being priced differently in two markets may buy the cheaper one and sell the more expensive one, locking in arbitrage profits in a few minutes.

Coexistence of Contradictory Strategies

One of the most fascinating aspects of investment philosophy is the coexistence of investment philosophies based upon contradictory views of the markets. Thus, you can have market timers who trade on price momentum (suggesting that investors are slow to learn from information) and market timers who are contrarians (which is based on the belief that markets over react). Among security selectors who use fundamentals, you can have value investors who buy value stocks, because they believe markets overprice growth, and growth investors who buy growth stocks using exactly the opposite justification. The coexistence of these contradictory impulses for investing may strike some as irrational, but it is healthy and may actually be responsible for keeping the market in balance.  In addition, you can have investors with contradictory philosophies co-existing in the market because of their different time horizons, views on risk and tax status. For instance, tax exempt investors may find stocks that pay large dividends a bargain, while taxable investors may reject these same stocks because dividends are taxed at the ordinary tax rate.

Investment Philosophies in Context

We can consider the differences between investment philosophies in the context of the investment process, described in figure 1.1. Market timing strategies primarily affect the asset allocation decision. Thus, investors who believe that stocks are under valued will invest more of their portfolios in stocks than would be justified given their risk preferences. Security selection strategies in all their forms – technical analysis, fundamentals or private information – all centre on the security selection component of the portfolio management process. You could argue that strategies that are not based upon grand visions of market efficiency but are designed to take advantage of momentary mispricing of assets in markets (such as arbitrage) revolve around the execution segment of portfolio management. It is not surprising that the success of such opportunistic strategies depend upon trading quickly to take advantage of pricing errors, and keeping transactions costs low.  Figure 1.2 presents the different investment philosophies.

Figure 1.2 Investment Philosophies

Developing an Investment Philosophy: The Steps

If every investor needs an investment philosophy, what is the process that you go through to come up with such a philosophy? While portfolio management is about the process, we can lay out the three steps involved in this section.

Step 1: Understand the fundamentals of risk and valuation

Before you embark on the journey of finding an investment philosophy, you need to get your financial toolkit ready. At the minimum, you should understand

  • How to measure the risk in an investment and relate it to expected returns
  • How to value an asset, whether it be a bond, stock or a business
  • The ingredients of trading costs, and the trade off between the speed of trading and the cost of trading

We would hasten to add that you do not need to be a mathematical wizard to do any of these and it is easy to acquire these basic tools.

Step 2: Develop a point of view about how markets work and where they might break down

Every investment philosophy is grounded in a point of view about human behaviour (and irrationality). While personal experience often determines how we view our fellow human beings, we should expand this to consider broader evidence from markets on how investors act before we make our final judgments.

Over the last few decades, it has become easy to test different investment strategies as data becomes more accessible. There now exists a substantial body of research on the investment strategies that have beaten the market over time. For instance, researchers have found convincing evidence that stocks with low price to book value ratios have earned significantly higher returns than stocks of equivalent risk but higher price to book value ratios. It would be foolhardy not to review this evidence in the process of developing your investment philosophy.  At the same time, though, you should keep in mind three caveats about this research:

  1. Since they are based upon the past, they represent a look in the rearview mirror. Strategies that earned substantial returns in the 1990s may no longer be viable strategies now. In fact, as successful strategies get publicized either directly (in books and articles) or indirectly (by portfolio managers trading on them), you should expect to see them become less effective.
  2. Much of the research is based upon constructing hypothetical portfolios, where you buy and sell stocks at historical prices and little or no attention is paid to transactions costs. To the extent that trading can cause prices to move, the actual returns on strategies can be very different from the returns on the hypothetical portfolio.
  3. A test of an investment strategy is almost always a joint test of both the strategy and a model for risk. To see why, consider the evidence that stocks with low price to book value ratios earn higher returns than stocks with high price to book value ratios, with similar risk (at least as measured by the models we use). To the extent that we mismeasure risk or ignore a key component of risk, it is entirely possible that the higher returns are just a reward for the greater risk associated with low price to book value stocks.

Since understanding whether a strategy beats the market is such a critical component of investing, we will consider the approaches that are used to test a strategy, some basic rules that need to be followed in doing these tests and common errors that are made (unintentionally or intentionally) when running such tests. As we look at each investment philosophy, we will review the evidence that is available on strategies that emerge from that philosophy.

Step 3: Find the philosophy that provides the best fit for you

Once you understand the basics of investing, form your views on human foibles and behaviour and review the evidence accumulated on each of the different investment philosophies, you are ready to make your choice. In our view, there is potential for success with almost every investment philosophy  but the prerequisites for success can vary. In particular, success may rest on:

  • Your risk aversion: Some strategies are inherently riskier than others. For instance, venture capital or private equity investing, where you invest your funds in small, private businesses that show promise is inherently more risky than buying value stocks – equity in large, stable, publicly traded companies. The returns are also likely to be higher. However, more risk averse investors should avoid the first strategy and focus on the second. Picking an investment philosophy (and strategy) that requires you to take on more risk than you feel comfortable taking can be hazardous to your health and your portfolio.
  • The size of your portfolio: Some strategies require larger portfolios for success whereas others work only on a smaller scale. For instance, it is very difficult to be an activist value investor if you have only $100,000 in your portfolio, since firms are unlikely to listen to your complaints. On the other hand, a portfolio manager with $100 billion to invest may not be able to adopt a strategy that requires buying small, neglected companies. With such a large portfolio, she would very quickly end up becoming the dominant stockholder in each of the companies and affecting the price every time she trades.
  • Your time horizon: Some investment philosophies are predicated on a long time horizon, whereas others require much shorter time horizons. If you are investing your own funds, your time horizon is determined by your personal characteristics – some of us are more patient than others – and your needs for cash – the greater the need for liquidity, the shorter your time horizon has to be. If you are a professional (an investment adviser or portfolio manager), managing the funds of others, it is your clients’ time horizon and cash needs that will drive your choice of investment philosophies and strategies.
  • Your tax status: Since such a significant portion of your money ends up going to the tax collectors, they have a strong influence on your investment strategies and perhaps even the investment philosophy you adopt. In some cases, you may have to abandon strategies that you find attractive on a pre-tax basis because of the tax bite that they expose you to.

Thus, the right investment philosophy for you will reflect your particular strengths and weaknesses. It should come as no surprise, then, that investment philosophies that work for some investors do not work for others. Consequently, there can be no one investment philosophy that can be labelled ’best’ for all investors. 


An investment philosophy represents a set of core beliefs about how investors behave and markets work.

To be a successful investor, you not only have to consider the evidence from markets but you also have to examine your own strengths and weaknesses to come up with an investment philosophy that best fits you.  

Investors without core beliefs tend to wander from strategy to strategy, drawn by the anecdotal evidence or recent success, creating transactions costs and incurring losses as a consequence. Investors with clearly defined investment philosophies tend to be more consistent and disciplined in their investment choices.