Wealthy PotThe Twin Factor
When two funds look alike, the question is not "which is better" but "do I need both?" The Twin Factor answers that in three parts, because the parts often disagree.
Figures verified 2026-09-21. 11 pairs scored so far.
The three axes
Only one of them is live today. We would rather say that plainly than publish a number we cannot reproduce.
1. Holdings Twin not yet live
The weighted overlap of the two portfolios: Σ min(wA,i, wB,i). This is the complement of Active Share, the measure introduced by Cremers and Petajisto in How Active Is Your Fund Manager? (Review of Financial Studies, 2009): Active Share = ½ Σ |wA,i − wB,i|. It needs each fund's full holdings, which we do not yet ingest.
2. Outcome Twin not yet live
How similarly the two have actually behaved, measured as the dispersion of their monthly return differences. Deliberately not correlation: almost every broad equity pair scores above 0.98, so correlation discriminates nothing. This needs licensed price history.
3. Practical Twin live
How interchangeable the two are for you. Starts at 100 and subtracts a published penalty for each axis on which they differ:
| Axis | Max | When it applies |
|---|---|---|
| Market scope | 30 | One excludes a region, a market tier or a size band the other holds |
| Portability | 15 | One cannot be transferred in kind to another broker |
| Wrapper | 15 | ETF against mutual fund: trading, minimums and tax efficiency all differ |
| Fee | 15 | Banded by ratio, not absolute size: up to 1.25× costs nothing, 1.25–2× costs 8, over 2× costs the full 15 |
| Tax treatment | 10 | State-tax exemption differs, or one wrapper is materially more tax-efficient |
| Index family | 10 | Different index provider, including an in-house proprietary index |
| Minimum | 5 | One has an investment minimum the other does not |
The fee, wrapper, portability, index-family and minimum penalties are computed from each fund's published figures. Market scope and tax treatment are editorial judgement, because no rule can settle them from a data sheet. Those weights are our view of what changes a decision, not an empirical derivation. We publish them so you can disagree.
Why the diagrams are not to scale
The circles show the structural relationship between two funds' universes, taken from each issuer's stated index. They are not drawn to scale, because scale requires weighted holdings we have not computed. That distinction matters: we can verify that every S&P 500 company sits inside a total-market fund, but only weights can tell you whether that makes the two funds substitutes. A diagram that looked measured when it was not would be the exact failure this page exists to prevent.
On wash sales
The Twin Factor is not a wash-sale determination and must not be used as one. The IRS has never defined "substantially identical" for funds, and there is no ruling on whether two ETFs from different issuers tracking the same index qualify. Practitioners generally treat same-index pairs as risky and different-index pairs as the conventional swap, but that is convention, not law. No number on this page, ours or anyone else's, makes a swap safe. Talk to a tax professional about your own situation.
Every pair we have scored
Lowest Practical Twin first — the pairs where the funds look most alike but behave least alike for you.