The idea that senators beat the market is one of the most durable beliefs in retail investing. It powers tracker accounts, dashboards, two exchange-traded funds and a steady stream of headlines. It also rests on a real academic finding. The problem is that the finding is from data collected three decades ago, and the research since then points the other way.
This article walks through the actual studies, in order, and then looks at what the disagreement means for anyone using congressional trading data today.
The study that started it: Ziobrowski (2004)
The claim has a specific origin. Ziobrowski, Cheng, Boyd and Ziobrowski published "Abnormal Returns from the Common Stock Investments of the U.S. Senate" in the Journal of Financial and Quantitative Analysis in 2004. They built portfolios from senators' disclosed trades between 1993 and 1998 and measured performance against the market.
The result was striking. A portfolio mimicking senators' purchases beat the market by roughly 85 basis points per month. Stocks senators sold went on to lag. The authors interpreted the pattern as consistent with an information advantage: senators are close to government decisions that move markets, and their trades looked like they reflected that.
A follow-up study by the same authors, published in Business and Politics in 2011, examined House members' trades from 1985 to 2001 and found abnormal returns that were smaller than the Senate result but still positive.
These two papers are the academic foundation of the copy-Congress idea. They were widely covered, they featured in the 2011 political fight over congressional trading, and they are still the citations behind most "senators beat the market" claims. It is worth being precise about what they cover: hand-collected disclosure data from the 1980s and 1990s, before electronic filing, before the STOCK Act, and before anyone was watching.
The first counterevidence: Eggers and Hainmueller (2013)
Andrew Eggers and Jens Hainmueller published "Capitol Losses: The Mediocre Performance of Congressional Stock Portfolios" in the Journal of Politics in 2013. They studied congressional stock portfolios from 2004 to 2008 and also reexamined the earlier period.
Their findings were the opposite of Ziobrowski's. They found no evidence of informed trading or above-market returns for Congress as a whole or for any subset of members they examined. The average congressional investor in their sample underperformed the market by 2 to 3 percent per year. Their conclusion was blunt: most members would have been better off in index funds, and widespread congressional insider trading looked more like myth than reality in their data.
Two careful researchers looking at adjacent periods reached opposite conclusions. That alone should adjust anyone's confidence in the simple version of the story.
The modern verdict: post-STOCK Act studies
The STOCK Act of 2012 (Pub. L. 112-105) created the modern disclosure regime: transactions over $1,000 reported within 45 days, published by the House Clerk and the Senate eFD system. That produced far better data, and researchers used it.
The largest study of the disclosure era is Belmont, Sacerdote, Sehgal and Van Hoek, "Do senators and house members beat the stock market? Evidence from the STOCK Act," published in the Journal of Public Economics in 2022. Using trades from January 2012 through December 2020, they found no evidence of superior investment performance, in aggregate or among senators specifically accused of informed trading. Stocks bought by House members underperformed on average by 26 basis points over a six-month horizon. Even the best-performing trades in their sample were consistent with random stock picking rather than skill. An earlier version circulated as an NBER working paper in 2020 under a title that summarizes the finding: "Relief Rally: Senators As Feckless As the Rest of Us at Stock Picking."
The modern evidence, on the best data available, does not show senators beating the market.
Why the studies disagree
Several explanations are plausible, and they matter for how you use the data.
The world changed. The 1993 to 1998 Senate was trading in an environment with paper disclosures, little media attention and no rapid reporting requirement. After 2012, every trade becomes public within 45 days and tracker accounts amplify it within minutes of filing. If an edge existed, scrutiny is exactly the kind of thing that shrinks it. Research on Senate behavior after the 2011 60 Minutes broadcast and the STOCK Act suggests trading activity itself changed under the spotlight.
Averages hide variation. Finding no edge on average does not mean no individual trade was ever informed. It means informed trades, if they exist, are rare enough not to move the aggregate. Individual enforcement questions and portfolio-level statistics are different subjects.
Old data was thin. The early studies relied on hand-collected records with unavoidable gaps. Modern studies work from a far more complete electronic record. When better data reverses a finding, the newer result usually deserves more weight.
The real-world test: the copy-Congress ETFs
Since February 2023 the question has had a live experiment. Two ETFs, NANC and KRUZ, build portfolios from the disclosed trades of Democratic and Republican members and their spouses respectively. Both funds necessarily buy after disclosure, which means after the up-to-45-day lag, exactly like any copier.
Their ongoing performance is public fund data anyone can check against a benchmark. Whatever the numbers show in any given period, the structural point stands: a disclosure-based portfolio holds what members held weeks ago, concentrated in whatever those members happened to own, which for the Democratic fund has meant a heavy tilt toward large-cap technology. Separating "congressional information" from "held big tech in a big tech bull market" is exactly the kind of question the academic studies are designed to answer, and their answer so far is that the information effect is not there.
What this means for using congressional trading data
The honest conclusion is not that the data is useless. It is that the data answers different questions than the popular narrative assumes.
As a mechanical trading signal, congressional trades have weak support. The disclosure lag runs up to 45 days, amounts are ranges rather than exact figures, and the best modern study finds no outperformance to copy in the first place.
As transparency and research data, the filings are excellent and getting better. They show which members hold positions in sectors they oversee. They surface consensus, when multiple members buy the same ticker in the same window, which is a research lead regardless of whether it predicts returns. They give journalists the raw material that has repeatedly produced accountability reporting. And they provide context: knowing that Congress is accumulating a sector tells you something about Washington's attention even if it tells you nothing tradable.
That is the frame worth keeping. Senators are, on the current evidence, roughly as good at picking stocks as everyone else. Their trades are still worth reading, for what they reveal rather than for what they promise.
The Congress Stock Trades Report turns these filings into one scored, ranked document. Get the free preview.
DataSignals Lab publishes data and research. This is not investment advice.
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