Shortly after the 2006 election, in which the Democrats recaptured control of the U.S. Congress, the spoof newspaper The Onion ran a story in which Nancy Pelosi, the new Speaker of the House, reprimanded her partisan colleagues for supporting her legislative agenda without necessarily meaning it. Referring to a fictitious bill, The Onion had Pelosi admonishing Democrats not to “just pass it because I want it, but because you want it, too,” and went on to describe Pelosi's “concern that her relationship to the House was based completely on voting” (The Onion, 42 [49], December 4, 2006).

Legislative decisions are about votes, and voting behavior is organized by parties. If we want to understand legislatures and the representation they provide, it makes sense to look at partisan voting. To The Onion, the joke was that Pelosi might care about anything beyond that bottom line.

It never got big laughs, but I had a similar idea in mind around a decade ago, when I started the project that became this book. At the time, the study of voting in the U.S. Congress was a bustling cottage industry, but there was almost no information about legislative voting outside the United States. The reason, it seemed to me, had to be the lack of available data on votes. So, to begin, I set out to collect data on votes in a number of legislatures, mostly in Latin America where I had some experience, but also in other assemblies where I could establish research connections.