A broader analysis of multi-bi aid across all institutions has only become possible recently with the new multi-bi aid dataset (see Eichenauer and Reinsberg, 2015<sup>2</sup> ). On this basis, Reinsberg, Michaelowa, and Eichenauer (2015) track the evolution of multi-bi aid after the Cold War, discussing its underlying motives and testing some widespread hypotheses on its implications for aid effectiveness (Reinsberg, Michaelowa, and Eichenauer 2015). From a theoretical perspective, Eichenauer and Hug (2015) study how the combination of donor preferences, discretion granted to the multilateral organization, and voting rules influence allocation decisions among bilateral aid, unearmarked voluntary contributions, and earmarked funding. Michaelowa, Reinsberg, and Schneider (2014) examine the choices of a specific donor, namely the European Union institutions. Further ongoing work on multi-bi aid studies the country allocations of trust funds in comparison to core-funded operations (Eichenauer and Knack 2015) and the implications of multi-bi financing on international development organizations in terms of distorting program priorities, rivalry with core resources, and funding sustainability (Reinsberg 2015). These studies illustrate that trust funds are used for various purposes and that the effectiveness of trust funds hinges upon their funding purposes and underlying motivations. None of these studies, however, explicitly examine the variety of choices within multi-bi aid.

These academic studies are complemented by a large body of agency reports (for a review, see Reinsberg, Michaelowa, and Eichenauer 2015). These reports suggest a variety of different donor motives behind multi-bi aid, and their outcomes show the need for more systematic analysis of the tradeoffs across various types of multi-bi aid. Many studies argue that multi-bi aid is popular with donors because it combines the "best of two worlds". On the one hand, it gives an individual donor nearly as much control as with bilateral aid, but without requiring the donor to sustain a full-fledged aid bureaucracy, and still benefiting from the expertise and professionalism of multilaterals (e.g., Carlsson 2007: 63; IEG 2011: 6; OECD 2011: 28; Tortora and Steensen 2014: 15). On the other hand, bilateral donors hope that by pooling their resources multi-bi aid can achieve key principles of the Paris Declaration<sup>3</sup> , including recipient-country ownership, aid harmonization, and mutual accountability (e.g., Barakat 2009; Guder 2009; OECD 2011), although success is sometimes difficult to achieve (e.g., Barakat, Rzeszut, and Martin 2012).<sup>4</sup> The variety of choices between different trust funds is not addressed by these studies, however, and most treatments of multi-bi aid pertain primarily to larger multi-donor trust funds. To analyze the choice among different types of trust funds, we draw on three related, more general strands of the literature on international organization and adapt their insights to our research questions.

The first strand concerns the rational design of international institutions that can be used to explain the associated institutional choices from a donor perspective. In general, the ra-

> 2 For the codebook, see Eichenauer and Reinsberg (2014).

> 3 For details on the Paris Declaration and the related Accra Agenda for Action, see http://www.oecd.org/dac/ effectiveness/parisdeclarationandaccraagendaforaction.htm (accessed February 1, 2015).

> 4 This ambivalence on the potential purpose of trust funds – "bilateralization of multilateral aid" and "multilateralization of bilateral aid" – also features in other policy studies (e.g., Browne and Weiss 2012; Mahn 2012; Thalwitz 2015).

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