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Co-sponsorship analysis of party politics in the 20th National Assembly of Republic of Korea

We investigate co-sponsorship among lawmakers by applying the principal-component analysis to the bills introduced in the 20th National Assembly of Korea. The most relevant factor for co-sponsorship is their party membership, and we clearly observe a signal of a third-party system in action. To identify other factors than the party influence, we analyze how lawmakers are clustered inside each party, and the result shows significant similarity between their committee membership and co-sponsorship in case of the ruling party. In addition, by monitoring each lawmaker's similarity to the average behavior of his or her party, we have found that it begins to decrease approximately one month before the lawmaker actually changes the party membership.