For those of us who watch Congress and steep ourselves in its history, there are a handful of theories purporting to explain how and why Congress changes. Political parties behave like cartels gathering power at another's expense. Election-minded members shape Congress to ease the passage of pork-barrel bills and to trade votes. Congress often seems designed to encourage legislators to become policy experts, and their expertise is protected by deference to committees that fairly closely represent the interests of the whole House or Senate. For at least the last 15 years, and in the name of New Institutionalism, full-throated fans of various theories have been arguing over which one is “right.”