Survivorship Bias — The Track Records That Disappear
Poorly performing funds tend to quietly disappear from the record — flattering the industry's own reported average.
A specific, often underappreciated distortion in how mutual fund performance gets reported: funds that perform poorly for long enough are disproportionately likely to be closed, merged into another fund, or otherwise quietly removed from the very databases used to calculate "average" fund performance — meaning the historical record available today is biased toward the funds that happened to survive.
This means a simple average of "all funds that exist today, going back 20 years" systematically overstates how a real investor, invested in the full original population of funds (including the ones that later disappeared), would actually have fared — the failures have been selectively edited out of the very data being used to judge the industry.
Bogle treats this as one of the more important, and more commonly overlooked, reasons that reported industry-wide fund performance looks better in retrospect than what investors, as a whole, actually experienced in real time.
Suppose 100 similar funds launch in the same year. A decade later, 30 of them have underperformed badly enough to be closed or merged away, and only the 70 survivors remain in the industry's performance databases.
Calculating the "average 10-year return" using only those 70 survivors necessarily overstates what an investor who'd spread money evenly across the original 100 funds would actually have earned, since the 30 worst outcomes have been removed from the average entirely.
Survivorship bias is a much smaller issue for a fund that simply holds the whole market and doesn't get closed for underperforming its own benchmark, since there's no comparable culling of "bad" broad-market index funds relative to the market itself — the distortion specifically affects the reported track record of the actively-managed fund industry as a whole.
- Poorly performing funds are disproportionately closed or merged away, removing their results from the databases used to calculate industry-wide average performance.
- This systematically flatters reported historical fund performance relative to what a real investor, spread across the full original population of funds, actually experienced.
- The effect is far smaller for broad index funds, which aren't culled the same way actively managed funds are.
- Reported "average fund performance" figures should be read with this bias in mind, not taken at face value.