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We systematically investigate the relationship between the number of firms in a market and tacit collusion by means of a meta-analysis of the literature on oligopoly experiments as well as two of our own experiments with a total of 368 participants. We show that the degree of tacit collusion decreases strictly with the number of competitors in industries with two, three and four firms. Although previous literature could not affirm that triopolies are more collusive than quadropolies, we provide evidence for this fact for symmetric and asymmetric firms under Bertrand and Cournot competition. † Authors' affiliations:

Social logins, such as "Log in with Facebook," improve a website's user experience and therefore enjoy great popularity among content providers (CPs) and users alike. Moreover, they also enable the social network and the CPs to share data, which individually improves their ability to place targeted advertising. On the basis of a game-theoretic model that offers a microfoundation for CPs' competition for advertisements, on the one hand, and CPs' competition for users, on the other hand, we demonstrate the strategic effects of social logins in the online advertising ecosystem. We fully characterize the market conditions under which social logins are offered and adopted, and when the adoption is actually profitable for the CPs. In particular, we show across several model extensions that the voluntary adoption of the social login may yield a prisoner's dilemma outcome for the CPs.

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