Susumu Shikano and Eric Linhart, "Coalition-Formation as a Result of Policy and Office Motivations in the German Federal States: An Empirical Estimate of the Weighting Parameters of Both Motivations," Party Politics, 16 (January, 2010), 111-130. [Available at http://ppq.sagepub.com/content/vol16/issue1/.]

First paragraph:
Theories of coalition-formation can be categorized within two groups: models with office motivations and models with policy motivations. The former assume that only offices motivate parties in the coalition-formation process (e.g. Riker, 1962; Schofield and Laver, 1985). According to the latter, in contrast, coalition-formation depends on the programmatic proximity among parties in the policy space (see Laver and Schofield, 1990, for an overview). Besides these lines of research, there are also models that combine both kinds of motivation. The theory of minimal connected coalitions suggested by Axelrod (1970) is the earliest concept among them. While this model can be seen as a variation of the office-oriented minimal winning coalition, Axelrod reduced the number of predicted coalitions by integrating the connectedness on the unidimensional policy scale. Austen-Smith and Banks (1988), by contrast, introduced a model in which both kinds of motivation are considered equal. A similar model suggested by Crombez (1996) and Baron and Diermeier (2001) assumes that a utility function contains both an office-motivated and a policy-motivated part. Sened (1995, 1996) extended this model, allowing the weight of these two motivations to vary among parties (see also Schofield and Sened, 2006). His model can be seen as more general, since it includes the purely policy-oriented and purely office-oriented models as special cases.

Figures and Tables:

Table 1. Predictive performance of the purely policy-oriented and office-oriented models

Table 2. Predicted and actually formed coalitions

Table 3. Estimated results of the model considering both motivations (median and 90% confidence interval)

Figure 1. Posterior distribution of £] (generic for all parties)

Figure 2. Posterior distribution of £] (party-specific)

Appendix Table. Categorizing functions for the categories in the CMP data

Last Paragraph:
(First paragraph of discussion) In this article, we have estimated the weighting parameters for both kinds of motivation systematically through using empirical data from German state-level coalition-formations. The results show that it is not sufficient to consider either policy or office motivations of political parties on their own. One can achieve a significant improvement in predictive power by integrating both motivations as separable additive components of a combined utility function. The article shows a systematic way in which this can be done rather than using ad hoc explanations that switch between both kinds of motivation.