Here are my two best guesses. Either:
(A) Congressmen are just smarter than most people and more finacially savvy,
(B) Having inside information on pending legislation gives you an advantage over ordinary investors.
Or maybe there's another explanation for why House members's investments beat the market by 6% annually, as reported in a study in the Journal of Business & Politics (how do I not yet subscribe to that?).
The Center for Responsive Politics blogs, "Congressmen are allowed to trade common stocks without special restrictions, and they do not have to recuse themselves from voting on legislation that could affect the values of their stock holdings."
And it turns out the benefit mostly accrues to Democrats. The Washington Times reports:
Despite the GOP’s reputation as the party of the rich, House Republicans fared worse than their Democratic colleagues when it comes to investing, according to the study. The Democratic subsample of lawmakers beat the market by 73 basis points per month, or 9 percent annually, versus 18 basis points per month, or 2 percent annually, for the Republican sample.