Modelling community structure in the Italian Parliament: a penalized inference approach

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• Mirko Signorelli (Università degli Studi di Padova ; Rijksuniversiteit Groningen)
• Thursday 25 August 2016, 12:10-12:30
• Seminar Room 1, Newton Institute.

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SNAW02 - Network science and its applications

In many parliamentary systems, bills can be proposed by a single parliamentarian, or cosponsored by a group of parliamentarians. In the latter case, bill cosponsorship defines a symmetric relation that can be taken as a measure of ideological agreement between parliamentarians.  
Political scientists have often analysed bill cosponsorship networks in the US Congress, assessing its community structure and the behaviour of minorities therein. In this talk, I will consider data on bill cosponsorship in the Italian Chamber of Deputies over the last 15 years. If compared to the US Congress, a distinguishing feature of the Italian Chamber is the presence of a large number of political groups: the primary purpose of the analysis is thus to infer the pattern of collaborations between these groups.  

We consider a stochastic blockmodel for edge-valued graphs that views bill cosponsorship as the result of a Poisson process, which explicitly depends on membership of parliamentary groups. As the number of model parameters increases quickly with the number of groups, we pursue a penalized likelihood approach to model estimation that enables us to infer a sparse reduced graph, which summarizes relations between parliamentary groups.  

Besides showing the effects of gender and geographic proximity on bill cosponsorship, the analysis points out the evolution from a highly polarized political arena, in which Deputies base collaborations on their identification with left or right-wing values, towards an increasingly fragmented Parliament, where a rigid separation of political groups into coalitions does not seem to hold any more, and collaborations beyond the perimeter of coalitions become possible.

Joint work with Ernst Wit.

Related links: https://arxiv.org/abs/1607.08743 (arXiv preprint)

This talk is part of the Isaac Newton Institute Seminar Series series.

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