Poli tical scientists have often analysed bill cosponso rship networks in the US Congress\, assessing its community structure and the behaviour of minoritie s therein. In this talk\, I will consider data on bill cosponsorship in the Italian Chamber of Deput ies 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 poli tical groups: the primary purpose of the analysis is thus to infer the pattern of collaborations bet ween these groups. \;

We consider a stochastic blockmodel for edge-valued graphs that views bill cosponsorship as the result of a Poiss on process\, which explicitly depends on membershi p of parliamentary groups. As the number of model parameters increases quickly with the number of gr oups\, we pursue a penalized likelihood approach t o model estimation that enables us to infer a spar se reduced graph\, which summarizes relations betw een parliamentary groups. \;

Besides showing the effects of gender and geograph ic proximity on bill cosponsorship\, the analysis points out t he evolution from a highly polarized political are na\, in which Deputies base collaborations on thei r identification with left or right-wing values\, towards an increasingly fragmented Parliament\, wh ere a rigid separation of political groups into co alitions does not seem to hold any more\, and coll aborations beyond the perimeter of coalitions beco me possible.

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