  Cooperation in the form of vote trading, also known as logrolling, is central
for law-making processes, shaping the development of democratic societies.
Empirical evidence of logrolling is scarce and limited to highly specific
situations because existing methods are not easily applicable to broader
contexts. We have developed a general and scalable methodology for revealing a
network of vote traders, allowing us to measure logrolling on a large scale.
Analysis on more than 9 million votes spanning 40 years in the U.S. Congress
reveals a higher logrolling prevalence in the Senate and an overall decreasing
trend over recent congresses, coincidental with high levels of political
polarization. Our method is applicable in multiple contexts, shedding light on
many aspects of logrolling and opening new doors in the study of hidden
cooperation.
