Why don't market indexes use aggregate market capitalization?
"They do but you're missing some calculations needed to gain an understanding. Intro To Stock Index Weighting Methods notes in part: Market cap is the most common weighting method used by an index.   Market cap or market capitalization is the standard way to measure the   size of the company. You might have heard of large, mid, or small cap   stocks? Large cap stocks carry a higher weighting in this index. And   most of the major indices, like the S&P 500, use the market cap   weighting method. Stocks are weighted by the proportion of their market cap to the total   market cap of all the stocks in the index. As a stock’s price and   market cap rises, it gains a bigger weighting in the index. In turn   the opposite, lower stock price and market cap, pushes its weighting   down in the index.  Pros Proponents argue that large companies have a bigger effect on the   economy and are more widely owned. So they should have a bigger   representation when measuring the performance of the market. Which is   true. Cons It doesn’t make sense as an investment strategy. According to a market   cap weighted index, investors would buy more of a stock as its price   rises and sell the stock as the price falls. This is the exact   opposite of the buy low, sell high mentality investors should use. Eventually, you would have more money in overpriced stocks and less in   underpriced stocks. Yet most index funds follow this weighting method. Thus, there was likely a point in time where the S & P 500's initial sum was equated to a specific value though this is the part you may be missing here. Also, how do you handle when constituents change over time? For example, suppose in the S & P 500 that a $100,000,000 company is taken out and replaced with a $10,000,000,000 company that shouldn't suddenly make the index jump by a bunch of points because the underlying security was swapped or would you be cool with there being jumps when companies change or shares outstanding are rebalanced? Consider carefully how you answer that question. In terms of histories, Dow Jones Industrial Average and S & P 500 Index would be covered on Wikipedia where from the latter link: The ""Composite Index"",[13] as the S&P 500 was first called when it   introduced its first stock index in 1923, began tracking a small   number of stocks. Three years later in 1926, the Composite Index   expanded to 90 stocks and then in 1957 it expanded to its current   500.[13] Standard & Poor's, a company that doles out financial information and analysis, was founded in 1860 by Henry Varnum Poor. In   1941 Poor's Publishing (Henry Varnum Poor's original company) merged   with Standard Statistics (founded in 1906 as the Standard Statistics   Bureau) and therein assumed the name Standard and Poor's Corporation.   The S&P 500 index in its present form began on March 4, 1957.   Technology has allowed the index to be calculated and disseminated in   real time. The S&P 500 is widely used as a measure of the general   level of stock prices, as it includes both growth stocks and value   stocks. In September 1962, Ultronic Systems Corp. entered into an agreement   with Standard and Poor's. Under the terms of this agreement, Ultronics   computed the S&P 500 Stock Composite Index, the 425 Stock Industrial   Index, the 50 Stock Utility Index, and the 25 Stock Rail Index.   Throughout the market day these statistics were furnished to Standard   & Poor's. In addition, Ultronics also computed and reported the 94 S&P   sub-indexes.[14] There are also articles like Business Insider that have this graphic that may be interesting: S & P changes over the years The makeup of the S&P 500 is constantly changing notes in part: ""In most years 25 to 30 stocks in the S&P 500 are replaced,"" said   David Blitzer, S&P's Chairman of the Index Committee. And while there   are strict guidelines for what companies are added, the final decision   and timing of that decision depends on what's going through the heads   of a handful of people employed by Dow Jones." "would constantly fluctuate and provide an indication of how well the market is doing. The index is there to tell if you made profit or loss by investing in the market.  Using a pure total market cap will only tell you ""Did IPO activity exceed bankruptcy and privatization activity""."
"With regards to ""the stock market,"" there are actually two markets involved here: PRIMARY MARKET Value is created in the primary market where capital is exchanged for a residual interest in an opportunity.  As a theoretical example, if a person operating solo (or with a small team) were to discover or create a breakthrough product, such as an retro-aging pill, that person likely wouldn't have the financial means to fully capitalize on his new-found idea.  Others with more capital may also soon discover his idea or improve upon it and exploit it before he has a chance to. For a real life example, a person studying at a California university during the 1990s discovered a method to index internet webpages and was approached by some students after a talk on the subject.  He returned to his native southern Europe country seeking funds to develop the web-indexing business and failed to do so.  Two of the students that approached him found capital readily available from investors in their campus sphere; their business is today one of the biggest in the world.  They had exchanged part of their residual interest for capital to develop their business. The primary market of the stock market works mostly same in creating value.  It is also dependent upon the secondary market. SECONDARY MARKET The secondary market indicates the day-to-day value of an enterprise.  That market allows shareholders to manage their risk appetites and the enterprise's operators to execute their shareholders' interest for gains.  In most cases, a secondary market reference will be used for pricing a primary market issuance.  Without that reference, capital would be allocated less efficiently creating additional costs for all involved, issuers and investors. Consider what would happen if you sought to purchase a house and the mortgage lenders had no indication what the property was worth.  This would make capital very expensive or possibly deny you access to credit.  By having an indication, all involved are better off.  That is value creating. There are some large developed economies' equity markets, such as that in Germany, where many large enterprises stay privately held and credit financing, mostly from banks, is used.  The approach has proven successful as well. So why do some nations' financial markets still rely on capricious stock markets when private credit financing may do just fine in many cases?  It's largely a matter of national culture.  Countries such as the Netherlands, the UK and the US have long had active equity markets in continuous use that investors have trusted for centuries. CONCLUSION When leaders of an enterprise wish to grow the business to a large size with investment from the stock market, they aren't limited by the size of their banks' capital.  Those leaders and their prospective investors will rely on the secondary market to determine values.  In addition, if the leaders raise equity instead of debt capital, they are usually accorded more flexibility to take risks since shareholders usually have their own flexibility to transfer those risks to other investors if for any number of reasons they choose to do so. Stock markets create value in many other ways.  The above are the main ways." There are two main ways you can make money through shares: through dividends and through capital gains. If the company is performing well and increasing profits year after year, its Net Worth will increase, and if the company continues to beat expectations, then over the long term the share price will follow and increase as well. On the other hand, if the company performs poorly, has a lot of debt and is losing money, it may well stop paying dividends. There will be more demand for stocks that perform well than those that perform badly, thus driving the share price of these stocks up even if they don't pay out dividends. There are many market participants that will use different information to make their decisions to buy or sell a particular stock. Some will be long term buy and hold, others will be day traders, and there is everything in between. Some will use fundamentals to make their decisions, others will use charts and technicals, some will use a combination, and others will use completely different information and methods. These different market participants will create demand at various times, thus driving the share price of good companies up over time. The annual returns from dividends are often between 1% and 6%, and, in some cases, up to 10%. However, annual returns from capital gains can be 20%, 50%, 100% or more. That is the main reason why people still buy stocks that pay no dividends. It is my reason for buying them too. "Index funds are well-known to give the best long-term investment. Not exactly. Indexes give the best long term performance when compared to actively managing investments directly in the underlying stocks. That is, if you compare an S&P500 index to trying to pick stocks that are part of it, you're more likely to succeed with blindly following the index than trying to actively beat it. That said, no-one promises that investing in S&P500 is better than investing in DJIA, for example. These are two different indexes tracking different stocks and areas. So when advisers say ""diversify"" they don't mean it that you should diversify between different stocks that build up the S&P500 index. They mean that you should diversify your investments in different areas. Some in S&P500, some in DJIA, some in international indexes, some in bond indexes, etc. Still, investing in various indexes will likely yield better results than actively managing the investments trying to beat those indexes, but you should not invest in only one, and that is the meaning of diversification. In the comments you asked ""why diversify at all?"", and that is entirely a different question from your original ""what diversification is?"". You diversify to reduce the risk of loss from one side, and widen the net for gains from another. The thing is that any single investment can eventually fail, regardless of how it performed before. You can see that the S&P500 index lost 50% of its value twice within ten years, whereas before it was doubling itself every several years. Many people who were only invested in that index (or what's underlying to it) lost a lot of money. But consider you've diversified, and in the last 20 years you've invested in a blend of indexes that include the S&P500, but also other investments like S&P BSE SENSEX mentioned by Victor below. You would reduce your risk of loss on the American market by increasing your gains on the Indian market. Add to the mix soaring Chinese Real Estate market during the time of the collapse of the US real-estate, gains on the dollar losing its value by investing in other currencies (Canadian dollar, for example), etc. There are many risks, and by diversifying you mitigate them, and also have a chance to create other potential gains. Now, another question is why invest in indexes. That has been answered before on this site. It is my opinion that some methods of investing are just gambling by trying to catch the wave and they will almost always fail, and rarely will individual stock picking beat the market. Of course, after the fact its easy to be smart and pick the winning stocks. But the problem is to be able to predict those charts ahead of time." "I read the book, and I'm willing to believe you'd have a good chance of beating the market with this strategy - it is a reasonable, rational, and mechanical investment discipline. I doubt it's overplayed and overused to the point that it won't ever work again. But only IF you stick to it, and doing so would be very hard (behaviorally). Which is probably why it isn't overplayed and overused already. This strategy makes you place trades in companies you often won't have heard of, with volatile prices. The best way to use the strategy would be to try to get it automated somehow and avoid looking at the individual stocks, I bet, to take your behavior out of it. There may well be some risk factors in this strategy that you don't have in an S&P 500 fund, and those could explain some of the higher returns; for example, a basket of sketchier companies could be more vulnerable to economic events. The strategy won't beat the market every year, either, so that can test your behavior. Strategies tend to work and then stop working (as the book even mentions). This is related to  whether other investors are piling in to the strategy and pushing up prices, in part.  But also, outside events can just happen to line up poorly for a given strategy; for example a bunch of the ""fundamental index"" ETFs that looked at dividend yield launched right before all the high-dividend financials cratered. Investing in high-dividend stocks probably is and was a reasonable strategy in general, but it wasn't a great strategy for a couple years there. Anytime you don't buy the whole market, you risk both positive and negative deviations from it. Here's maybe a bigger-picture point, though. I happen to think ""beating the market"" is a big old distraction for individual investors; what you really want is predictable, adequate returns, who cares if the market returns 20% as long as your returns are adequate, and who cares if you beat the market by 5% if the market cratered 40%. So I'm not a huge fan of investment books that are structured around the topic of beating the market. Whether it's index fund advocates saying ""you can't beat the market so buy the index"" or Greenblatt saying ""here's how to beat the market with this strategy,"" it's still all about beating the market. And to me, beating the market is just irrelevant. Nobody ever bought their food in retirement because they did or did not beat the market. To me, beating the market is a game for the kind of actively-managed mutual fund that has a 90%-plus R-squared correlation with the index; often called an ""index hugger,"" these funds are just trying to eke out a little bit better result than the market, and often get a little bit worse result, and overall are a lot of effort with no purpose. Just get the index fund rather than these. If you're getting active management involved, I'd rather see a big deviation from the index, and I'd like that deviation to be related to risk control: hedging, or pulling back to cash when valuations get rich, or avoiding companies without a ""moat"" and margin of safety, or whatever kind of risk control, but something. In a fund like this, you aren't trying to beat the market, you're trying to increase the chances of adequate returns - you're optimizing for predictability. I'm not sure the magic formula is the best way to do that, focused as it is on beating the market rather than on risk control. Sorry for the extra digression but I hope I answered the question a bit, too. ;-)" "Market Capitalization is the value the market attributes to the company shares calculated by multiplying the current trading price of these shares by the total amount of shares outstanding. So a company with 100 shares trading at $10 has a market cap of $1000. It is technically not the same as the value of a company (in the sense of how much someone would need to pay to acquire the company), Enterprise value is what you want to determine the net value of a company which is calculated as the market capitalization + company debt (as the acquirer has to take on this debt) - company cash (as the acquirer can pocket this for itself). The exact boundary for when a company belongs to a certain ""cap"" is up for debate. For a ""large cap""  a market capitalization of $10 billion+ is usually considered the cutoff (with $100+ billion behemoths being called ""mega caps""). Anything between $10 billion and ~$1 billion is considered ""mid cap"", from ~$1billion to ~$200 million it's called a ""small cap"" and below $200 million is ""nano cap"".  Worth noting that these boundaries change quite dramatically over time as the overall average market capitalization increases as companies grow, for example in the 80s a company with a market cap of $1 billion would be considered ""large cap"". The market ""determines"" what the market cap of company should be based (usually but certainly not always!) on the historical and expected profit a company makes, for a simple example let's say that our $1000 market cap company makes $100 a year,  this means that this company's earnings per share is $1. If the company grows to make $200 a year you can reasonably expect the share price to rise from $10 to ~$20 with the corresponding increase in market cap. (this is all extremely simplified of course)." "This is an example from another field, real estate. Suppose you buy a $100,000 house with a 20 percent down payment, or $20,000, and borrow the other $80,000. In this example, your ""equity"" or ""market cap"" is $20,000. But the total value, or ""enterprise value"" of the house, is actually $100,000, counting the $80,000 mortgage.  ""Enterprise value"" is what a buyer would have to pay to own the company or the house ""free and clear,"" counting the debt." I think you're missing Simon Moore's point. His point is that, due to low inflation, the returns on almost all asset classes should be less than they have been historically, so we shouldn't rebalance our portfolio or withdraw from the market and hold cash based on the assumption that stocks (or any other asset) seem to be underperforming relative to historical trends.  His last paragraph is written in case someone might misunderstand him, he is not advocating to hold cash, just that investors should not expect as good returns as has happened historically, since those happened in higher inflation environments. To explain: If the inflation rate historically has been 5% and now it's 2%, and the risk-free-market return should be about 2%, then historically the return on a risk-free asset would be 7% (2%+5%), and now it should be expected to be 4% (2%+2%). So, if you have had a portfolio over some time you might be concerned that the rate of return is worsening, but Simon's point is that before you sell off your stocks / switch investment brokers, you should try to figure out if inflation is the cause of the performance loss. On the subject  of cash: cash always loses value over time from inflation, since inflation is a measure of the increase in prices over time-- it's a part of the definition of what inflation is.  That said, cash holdings lose value more slowly when inflation is lower, so they are relatively less worse than before. The future value of cash doesn't go up in low inflation (you'd need deflation for that), it just decreases at a lower rate, that is, it becomes less expensive to hold- but there still is a price. As an addendum, unless a completely new economic paradigm is adopted by world leaders, we will always see cash holdings decrease in value over time, since modern economics holds that deflation is one of the worst things that can happen to an economy. "With regards to ""the stock market,"" there are actually two markets involved here: PRIMARY MARKET Value is created in the primary market where capital is exchanged for a residual interest in an opportunity.  As a theoretical example, if a person operating solo (or with a small team) were to discover or create a breakthrough product, such as an retro-aging pill, that person likely wouldn't have the financial means to fully capitalize on his new-found idea.  Others with more capital may also soon discover his idea or improve upon it and exploit it before he has a chance to. For a real life example, a person studying at a California university during the 1990s discovered a method to index internet webpages and was approached by some students after a talk on the subject.  He returned to his native southern Europe country seeking funds to develop the web-indexing business and failed to do so.  Two of the students that approached him found capital readily available from investors in their campus sphere; their business is today one of the biggest in the world.  They had exchanged part of their residual interest for capital to develop their business. The primary market of the stock market works mostly same in creating value.  It is also dependent upon the secondary market. SECONDARY MARKET The secondary market indicates the day-to-day value of an enterprise.  That market allows shareholders to manage their risk appetites and the enterprise's operators to execute their shareholders' interest for gains.  In most cases, a secondary market reference will be used for pricing a primary market issuance.  Without that reference, capital would be allocated less efficiently creating additional costs for all involved, issuers and investors. Consider what would happen if you sought to purchase a house and the mortgage lenders had no indication what the property was worth.  This would make capital very expensive or possibly deny you access to credit.  By having an indication, all involved are better off.  That is value creating. There are some large developed economies' equity markets, such as that in Germany, where many large enterprises stay privately held and credit financing, mostly from banks, is used.  The approach has proven successful as well. So why do some nations' financial markets still rely on capricious stock markets when private credit financing may do just fine in many cases?  It's largely a matter of national culture.  Countries such as the Netherlands, the UK and the US have long had active equity markets in continuous use that investors have trusted for centuries. CONCLUSION When leaders of an enterprise wish to grow the business to a large size with investment from the stock market, they aren't limited by the size of their banks' capital.  Those leaders and their prospective investors will rely on the secondary market to determine values.  In addition, if the leaders raise equity instead of debt capital, they are usually accorded more flexibility to take risks since shareholders usually have their own flexibility to transfer those risks to other investors if for any number of reasons they choose to do so. Stock markets create value in many other ways.  The above are the main ways."