Index funds now cast roughly a quarter of the shareholder votes at America’s largest companies. Yet the investors whose money they manage have little say in how those votes are cast, and the funds themselves would often rather not cast the votes at all. That has turned index-fund voting power into a target of both the left and the right, and Washington is fighting over what to do about it.

In a new paper, we show that, historically, the least disruptive answer by far would have also been one of the simplest: If the funds had mirrored how every other shareholder voted. Over the past 20 years, that single change would have altered the outcome of a shareholder vote just 12 times out of nearly 645,000.

The fight is no longer hypothetical. In November 2025, the Wall Street Journal reported that the White House was drafting an executive order to curb proxy advisers’ influence over corporate elections and to require large index-fund managers to “mirror” their clients’ votes—to cast the fund’s shares in the same proportions as the investors who choose to vote them. (The order issued the following month as Executive Order 14366.) Three weeks later, Congress reintroduced the INDEX Act, which would go further still: The largest fund families would have to pass their votes through to the investors who submit instructions and, for the shares whose owners stay silent, cast them in proportion to how the rest of the market votes—mirror voting, in the sense we use here. That same year, the SEC granted no-action relief for ExxonMobil’s Retail Voting Program, BlackRock expanded its Voting Choice program to retail investors, and a coalition of state attorneys general opened an investigation into Big Three voting practices.

The debate over these proposals has been almost entirely theoretical. Supporters and critics alike argue from intuition about what each reform would do—who would gain power, whose votes would carry more weight, which outcomes would change. In our paper, we bring data to a debate that has had almost none. We ask a simple counterfactual question: If these reforms had already been in place, what would actually have happened to historical shareholder votes?

We group the live proposals as follows. Mirror voting reassigns the index-fund block of shares to track the vote of some other group. Full abstention removes the index-fund votes entirely. Pass-through or voting choice (the Big Three’s voluntary programs) returns the vote to the beneficial owner. Standing voting instructions let owners pre-commit to a policy, such as voting with a designated investor; ExxonMobil’s program implements a single-option version that defaults to the board. Market mirroring reassigns the index vote to the non-indexed shareholders in proportion: If 60 percent of active shareholders vote in favor of a proposal, the index block records 60 percent as in favor.

Each version hands the indexed shares to a different decision-maker. That makes them comparable on the historical record and lets us ask which assignment changes the fewest outcomes and imposes the least bias.

We use 20 years of ISS Voting Analytics data, covering more than 600,000 agenda items at 9,451 firms from 2005 through 2025, and re-tabulate each vote as it would have come out under each reform.

Market mirroring barely changes anything. Had index funds mirrored the non-indexed vote across all 644,954 items in our sample, the outcome would have changed just 12 times—about one vote in 50,000. By comparison, full abstention would have flipped 1,996 outcomes, roughly 166 times as many, and an Exxon-style rule that defaults retail shares to management would have flipped 1,439. Rules that hand the index vote to management or require adherence to a proxy adviser’s recommendation move two orders of magnitude more outcomes than market mirroring.

The 12 flips share a common signature. Not one is a director election or a proxy-fight item; all 12 occur on contentious items where a proxy adviser disagreed with management, precisely where the informed, non-indexed vote carries the most signal. Ten of the 12 are pro-management, but that skew reflects how non-indexed shareholders voted on those items, not anything built into the rule. Among the four reforms, market mirroring is the only one with no thumb on the scale by design.

The same picture holds for vote margins, not just flipped outcomes. Mirroring occasionally shifts a margin by several points—the largest single shift across all 644,954 items is eight percentage points—but those movements almost never come near changing a result. Only 22 items move by more than five points, and the eight-point outlier is not one of the 12 that flip.

Not every reform is so benign. Full abstention does not just change outcomes; it can make meetings fail. When index-fund shares disappear from the count, roughly one shareholder meeting in 10 in our sample would have fallen below its quorum threshold. Market mirroring, by construction, casts a ballot on every eligible item, so it never raises that problem.

Critics of mirror voting have raised several theoretical concerns, and we apply each to the data. First, the worry that low non-indexed turnout would let an unrepresentative minority dominate (minority amplification) shows up barely at all: the flip count is flat across turnout deciles, and even the lowest-turnout votes move margins only marginally. Second, the worry that a small activist stake would swing the mirrored block (activist amplification) finds no support: Zero director elections flip, and margin shifts in contested settings stay a fraction of a point. Third, the worry that supermajority bylaws would distort mirrored votes (threshold distortion) touches just five of more than 7,000 supermajority items, all comfortably far from their thresholds. In each case, the mechanism critics feared is real in theory but tiny in the historical record.

To be sure, our estimates are static. We re-tabulate the votes that were cast; we do not model how investors might change their behavior if mirror voting became the rule, an equilibrium question we leave for future research. We measure two things: how much each reform would have disrupted historical outcomes, and how much directional bias each carries. On both, market mirroring is the least intrusive of the four.

That is not an argument that mirror voting is the best reform, and we do not make one. Which proposal to adopt also turns on selection bias, capture, and drafting risk, considerations we do not weigh here. For two decades this debate has run on intuition; we supply the measured magnitudes it has lacked.

Edwin Hu is an associate professor at the University of Virginia School of Law. Robert E. Bishop is an associate professor at Duke University School of Law. Frank Partnoy is the Adrian A. Kragen Professor of Law at the University of California, Berkeley, School of Law and a research member of the European Corporate Governance Institute (ECGI). This post is based on their recent paper, “Mirror Voting,” available here.