American Congress members' social circles are more important in how they vote than their liberal or conservative beliefs or constituents' opinions, according to a new model of voting behavior created by Dartmouth College researchers.

The study appears in the journal Research & Politics.

The standard model of voting behavior basically assumes there is only one factor that matters: where a legislator lives on the liberal-conservative axis. That position, derived from their roll call votes, serves as an ideological marker that presumably summarizes the various forces that can influence the legislators' votes, including personal preferences, party preferences and constituent opinion.

But the Dartmouth researchers take a different approach to the problem of identifying ideology and sorting legislators into ideological categories. They developed a new model called "social identity voting" based on social identity theory, which says our identity is partially created and reinforced by the various circles within which we move and the various ideologies with which we identify. In other words, it's not just friends and friends of friends, but also potentially something more subtle -- you can identify with a movement without necessarily being part of an explicit "social circle."

The researchers analyzed the roll call votes of the 35th through 112th Congresses. They then reconstructed a voting record as essentially a weighted combination of "ideal voters" per politician's circles, which were discovered from the voting records that make up each Congressional session. They found that their new model is a much more accurate representation of the voting behavior by Senators and Representatives. The model was especially accurate in predicting Congressional voting in two periods -- between 1876 and 1883, and in the 1970s and 1980s.

The new model also enables a more nuanced look at the composition of each Congress. For example social identity voting shows that the Tea Party in the 112th House split essentially into two subgroups that have issue-specific ideological differences, demonstrating that the Tea Party is not an ideologically distinct faction of the Republican Party.

"Legislators derive usefulness from voting with the groups to which they belong and from voting against those to which they do not belong," says lead author Scott Pauls, a professor of mathematics. "As a consequence, legislators will select groups with ideologies consistent with their own. Subsequently, as group members, they face pressure to conform in ideology to the majority ideology of that group. Moreover, in this model, groups benefit from larger memberships because the outcome of votes generates political power for the group and detracts from other groups' political power. The model also gives us the ability to drill down on the record and discover more interesting influences working within the parties. The example of the Tea Party caucus in 112th Congress is a good one, where we find that its members are best distinguished by differing opinions on foreign policy and defense appropriations."

Says co-author Dan Rockmore, a professor of mathematics and computer science: "By focusing on group rather than individual preference, social identity voting gives a new way to look at and even uncover the forces driving Congress's voting behavior. Our approach sees political identity as derived from a range of group memberships, which are then expressed in the voting record."