A new study out notes that:

Four university researchers examined 16,000 common stock transactions made by approximately 300 House representatives from 1985 to 2001, and found what they call "significant positive abnormal returns," with portfolios based on congressional trades beating the market by about 6 percent annually.

...

A study of senators by the same team of researchers five years ago found members of the higher chamber even better at beating the market -- outperforming it by about 10 percent, an amount the academics said was "both economically large and statistically significant."