“…This important property has been studied under different names ["orthogonality" in Guesnerie and Oddou (1988), "games without externalities" in Bloch (1996), or "games without spillovers" in Konishi, Le Breton and Weber (1997e)]. 4 It obviously rules out some important group formation problems, such as cartel or association agreements in industrial organization (Bloch, 1995;Belleflamme, 2000), custom unions (Yi, 1997) or environmental agreements (Chandler and Tulkens, 1997;Ray and Vohra, 2001). Note that we use the term "group formation" here rather than "coalition formation", which covers strategic settings where independent players have the opportunity to form coalitions and make bidding agreements.…”