6 ideas
Because all investors together own the entire market, they collectively earn exactly the market's gross return before costs. Every dollar one active investor gains above that return must be matched by a dollar another loses below it. So before costs, active management as a whole must break even against the index.
Investors as a group earn the market return minus the costs of financial intermediation: advisory fees, trading costs, sales loads and taxes. Once costs are subtracted, the zero-sum game becomes a loser's game, and the average active investor must trail a low-cost index by roughly the amount of those costs. Performance gaps between funds are therefore best predicted by their expense ratios, not by manager skill.
A seemingly small annual fee is not charged once. Judge fees by their cumulative share of lifetime ending wealth, not as a percentage of assets in a given year.
Dollar-weighted returns, the returns investors actually earn, fall well below the time-weighted returns that funds report. This happens because investors pour money into funds after strong performance and pull it out after declines, buying high and selling low. A buy-and-hold index investor avoids this timing penalty along with the cost penalty.
Top-quartile active funds in one period rarely remain top-quartile in the next, and results tend to revert toward and below the market mean. Star managers and hot sectors attract assets just as their edge fades. Choosing funds by past returns is therefore close to random selection, with higher costs attached.
Rather than searching for the needle in the haystack, buy the haystack: a broad-market index fund holding every stock in proportion to its value. This guarantees the market return minus minimal costs, removes manager risk, and generates little turnover and therefore few taxable gains. Hold it indefinitely and let business earnings and dividends compound without interference.