  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.
