"We find weak evidence of informed trading for the pre-Congress period, suggesting that informed traders are not being selected into office. When combined with our finding that the portfolios of members serving on powerful committees outperform the market during their second term in office, this provides additional evidence that serving on influential committees is the mechanism by which members of Congress earn abnormal returns."

http://busecon.wvu.edu/phd_economics/pdf/16-25.pdf