From OpenCongress Wiki
Co-sponsorship is the voluntary insertion of a congressperson’s name to a bill that is introduced in the U.S. Congress. It is a very common act in both houses of Congress today. While some members co-sponsor hundreds of bills in a given legislative session, others co-sponsor less than ten.  The purpose of co-sponsoring legislation is to indicate one’s support for a bill. Scholarly research, discussed in more detail below, has also indicated that legislation which carries the names of several members has a better chance of passage than that which does not. House Rule 12 of the U.S. House of Representatives sets the following procedures for co-sponsoring legislation:
- The primary sponsor of a public bill or public resolution may name cosponsors. The name of a cosponsor added after the initial printing of a bill or resolution shall appear in the next printing of the bill or resolution on the written request of the primary sponsor. Such a request may be submitted to the Speaker at any time until the last committee authorized to consider and report the bill or resolution reports it to the House or is discharged from its consideration.
- The name of a cosponsor of a bill or resolution may be deleted by unanimous consent. The Speaker may entertain such a request only by the Member, Delegate, or Resident Commissioner whose name is to be deleted or by the primary sponsor of the bill or resolution, and only until the last committee authorized to consider and report the bill or resolution reports it to the House or is discharged from its consideration. The Speaker may not entertain a request to delete the name of the primary sponsor of a bill or resolution. A deletion shall be indicated by date in the next printing of the bill or resolution.
- The addition or deletion of the name of a cosponsor of a bill or resolution shall be entered on the Journal and printed in the Congressional Record of that day.
- A bill or resolution shall be reprinted on the written request of the primary sponsor. Such a request may be submitted to the Speaker only when 20 or more cosponsors have been added since the last printing of the bill or resolution.
- When a bill or resolution is introduced “by request,” those words shall be entered on the Journal and printed in the Congressional Record.
- Any legislator may request that their name be added to a bill, starting with the time of a bill’s initial introduction to the Congress and ending with the time that the bill is passed from a committee onto the House floor.
In the U.S. House of Representatives, co-sponsorship of legislation began in 1967. In 1978, House rules were altered again to allow unlimited co-sponsorship. Prior to these dates, House sessions recognized similar tactics that acted as co-sponsorships. "Multiple introduction," or the submission of identical bills with different resolution numbers and different principal sponsors, for instance, was prevalent for many years. 
Political scientists have found that co-sponsorship has a consistent legislative impact. In their 1997 study, Wilson and Young found that the number of co-sponsors for bills passing the House over a substantial period of time was greater than the number of co-sponsors for bills that were voted down. In addition, the number of co-sponsorships of a bill was positively and significantly associated with the probability that a bill gained some consideration by a committee rather than simply being "killed," or taken out of consideration. (Wilson and Young, 1997)
Articles and Resources
- David Mayhew, Congress: The Electoral Connection, New Haven: Yale University Press, 1974.
- James Campbell, "Cosponsoring Legislation in the U.S. Congress." Legislative Studies Quarterly, 3: 415-422, 1982.
- Keith Krehbiel, "Cosponsors and Wafflers from A to Z," American Journal of Political Science, 39: 906-923, 1995.
- Rick K. Wilson and Cheryl D. Young, "Cosponsorship in the U.S. Congress," Legislative Studies Quarterly, 12: 25-43, 1997.