The famous 90 percent is three different answers.
Brinson and his co-authors ran the numbers in 1986. Ibbotson and Kaplan cleaned up the misreading in 2000. Same finding, three separate questions.
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The question it answers
90pct
How much do my returns bounce around over time?
The mix explains about ninety percent of the variability of one portfolio's returns across the years. This is the number everyone quotes.
40pct
Why did my fund do differently from yours?
Across funds, the allocation difference explains about forty percent of the spread. The rest is the picking and the timing.
100pct
Where did my long-run return level come from?
On average across everyone, the policy mix delivers roughly the whole return, because active picking is close to a zero-sum game before costs.
It does not mean ninety percent of your return comes from the split. It means the split explains most of how your ride feels over time. The brochure shortens that into a slogan. The slogan is wrong.