Do Congress Stock Trades Beat the Market? What the Research Actually Says

"Congress beats the market" is one of those claims that gets repeated until it sounds settled. It isn't. The academic evidence points in more than one direction, and the studies measure different things. Understanding what each one actually tested is the difference between a useful research habit and a story you've talked yourself into.

This piece summarizes the main studies, explains why they disagree, and covers the part that matters most if you're thinking of acting on disclosed trades: the delay between a trade and its disclosure.

The Senate Study: Purchases That Beat the Market

The best-known result comes from a 2004 paper by Alan Ziobrowski and coauthors in the Journal of Financial and Quantitative Analysis. They examined senators' common stock transactions from 1993 to 1998. Their finding: a portfolio mimicking senators' purchases outperformed the market by about 85 basis points per month, while a portfolio mimicking their sales trailed the market by about 12 basis points per month. The gap between what senators bought and what they sold was close to a full percentage point a month.

An 85 basis point monthly edge is large if it holds up. This is the figure behind most of the "Congress beats the market" claims you've seen.

The House Study: Smaller, But Still Positive

The same group followed up in 2011 with a study of the House of Representatives, published in Business and Politics. They looked at more than 16,000 stock transactions by roughly 300 House members between 1985 and 2001, using the same methods. House members' purchases also earned positive abnormal returns, but a considerably smaller amount: about 55 basis points per month, which the authors put at roughly 6% a year.

So the first wave of research said both chambers showed some edge, with the Senate's larger than the House's.

The Study That Found the Opposite

In 2013, Andrew Eggers and Jens Hainmueller published "Capitol Losses" in the Journal of Politics. They took a different approach: instead of only looking at trades, they analyzed members' actual portfolio holdings, covering the 422 members who reported owning U.S. stocks between 2004 and 2008.

Their conclusion was close to the reverse. They found no evidence that Congress, or any identifiable subset of it, earned above-market returns or traded on privileged information. The average member would have done better in a passive index fund, with underperformance of roughly 2 to 3 percent a year. They also reinterpreted the earlier studies' 1985–2001 data and argued it didn't show an information advantage either.

Why the Studies Disagree

None of these researchers are obviously wrong. Several differences between them explain most of the gap:

• Different periods. The Senate data end in 1998 and the House data in 2001, before the STOCK Act. Eggers and Hainmueller studied 2004–2008. Disclosure rules, market conditions, and member behavior all changed.
• Trades versus holdings. The Ziobrowski studies measure the returns on specific purchases and sales in a window after each transaction. A holdings study measures how a whole portfolio performed over time. A member can make some well-timed trades and still own a portfolio that lags an index.
• Data quality. Before 2012, disclosures were annual and reported in broad ranges, so researchers had to estimate transaction dates and sizes. That makes precision difficult in every study of this era.
• Small samples and big outliers. A handful of very active traders can swing an average.

A 2012 Senate committee report that cited this body of work noted that the studies reached different results and did not give a definitive answer on how members invest. That remains a fair summary: the question is genuinely contested.

The Part That Matters for Copying Trades: Time

Suppose the 85 basis point figure is entirely real. It still wouldn't tell you what you'd earn by following disclosed trades today, because the studies measure returns from the date of the transaction. You don't get to trade on the transaction date. You see it when the filing appears.

Under the STOCK Act, members have up to 45 days after a transaction to report it. A trade that happened on the first of the month can first appear on a filing near mid-month, or later. If a stock jumps in the weeks after a member buys it, much of that move may be over before anyone outside the household can see the purchase. By then, the information in the disclosure is old.

So "do members beat the market?" and "can you beat the market by copying them?" are two separate questions, and the second is harder. For a walk through how the two dates on each filing work, see our guide to reading a congressional trade disclosure.

What You Can Reasonably Take From This

• Treat "Congress beats the market" as unproven. The strongest evidence for it is decades old, and a later study contradicts it.
• Don't confuse a member's trades with a member's information. A large share of reported activity belongs to spouses and managed accounts, as we explain in which members of Congress trade stocks the most.
• Use disclosures as a research input, not a signal. A disclosed purchase can be a reason to look at a company. It isn't a reason to buy it.
• Be skeptical of any backtest that ignores the filing delay. If a strategy buys on the transaction date, it's using information nobody had.

None of this means disclosures are useless. They're a primary-source record of what elected officials hold, and that has value for transparency and for spotting patterns. Just don't let the strongest historical result stand in for current evidence.

Where to Look Instead of a Headline

Check the underlying filings. The congressional trading tracker lists disclosed trades member by member and links to each source filing, so you can see the transaction date, the filing date, and the owner of every row before you form a view. The companion insider trading tracker does the same for corporate executives, where the research case for following purchases is better developed, as we explain in insider buying versus insider selling.

Past research does not predict future results, and disclosed trades can lose money. This article is general information, not investment advice. Never trade on a single filing, and consider speaking with a licensed financial professional about your own situation.

We track House and Senate trade disclosures as they're filed — the primary record, not a headline. Free. Drop your email below.

Sources: Ziobrowski, Cheng, Boyd and Ziobrowski, Abnormal Returns from the Common Stock Investments of the U.S. Senate (JFQA, 2004) · Ziobrowski, Boyd, Cheng and Ziobrowski, Abnormal Returns from the Common Stock Investments of Members of the U.S. House of Representatives (Business and Politics, 2011) · Eggers and Hainmueller, Capitol Losses: The Mediocre Performance of Congressional Stock Portfolios (Journal of Politics, 2013) · Congressional Research Service, Taking Stock of the STOCK Act