Where Portfolio Performance Comes From
One of the first things I do when onboarding clients is write an Investment Policy Statement.
Investment policy is the long-term mix of assets across accounts, the target you commit to aim for and stick with.
This targeted mix is called the asset allocation. It is how your money is divided across broad categories: stocks, bonds, cash, and other assets.
This paper compares investment policy results against two things active managers do to try to beat the market: market timing, which means shifting the mix to chase short-term moves, and security selection, which means picking individual stocks or bonds.
The methods and conclusions are detailed in the 1986 study by Gary Brinson, Randolph Hood, and Gilbert Beebower called "Determinants of Portfolio Performance," published in the Financial Analysts Journal.
They examined the quarterly returns of 91 large US pension funds from 1974 through 1983 and asked a simple question: how much of a fund's return swings over time can be traced to its policy asset mix versus everything the managers actively did.
Their answer, an average of 93.6% of portfolio returns can be traced to its policy asset mix.
Their findings became one of the most cited numbers in investing (CFA Institute).
A 1991 follow-up on updated data landed at 91.5 percent (SACRS Trustee Handbook).
That idea helped fuel a decades-long shift toward low-cost index investing, which began when Jack Bogle launched the first index fund for individual investors, Vanguard's First Index Investment Trust, on August 31, 1976 (Vanguard).
The scale today is hard to overstate. Passive US fund assets overtook active for the first time around September 2019 (CNBC).
By mid-2026 indexed US mutual funds and ETFs held 21.82 trillion dollars against 18.75 trillion for active (Investment Company Institute).
