Research joint ventures (RJVs) have been widely acclaimed for their alleged ability to restore private incentives to undertake R&D. Economists have, however, also sounded the alarm concerning the opportunities RJVs may create for collusion between partners. The danger of anti-competitive behavior increases significantly when repeated R&D collaboration occurs between firms that also "meet" in many product markets. This phenomenon is shown to be present in a large set of U.S.-based RJVs. The question is about the incentive trade-off: Are the alleged advantages of RJVs in terms of enhancing incentives for R&D sufficient to overcome the potential disadvantages in terms of decreasing incentives for R&D due to simultaneous multiproject and multimarket contact? Significant foreign participation, high technological and market uncertainties, and the set up of "porous' RJVs may operate as a check to anti-competitive behavior.