Although legislatures have been thoroughly analysed, only recently did political scientists start to pay systematic attention to mega-seats (Carroll et al., 2006), the internal bodies that grant much of the de facto power within the legislature. Mega-seats consist of committee chairs and vice-chairs, and leadership bodies. To know who controls these institutional channels is crucial to understand who holds the key to policy decisions. This paper offers a new empirical and theoretical approach to the distribution of mega-seats. The argument starts from a 'proportionality perspective', borrowed from findings on portfolios allocation by coalition scholarly. Thus, it assumes that mega-seats allocation follows a linear proportionality, with the number of seats in plenary acting as reference category. Using an original data set for 350 parties, in 12 European parliamentary democracies, I propose an index to measure the degree of disproportionality in the allocation of legislative mega-seats. The empirical findings convey a counterintuitive result: conversely to the established 'iron law' on portfolio allocation, mega-seats allocation is significantly disproportional. Moreover there is considerable variation. The second part of the paper is devoted to model an explain for variation in mega-seats allocations. This paper advances an explanation based on two central arguments: first, party power (an index measuring party size, presence in the executive, holding the premiership) explains the degree of mega-seats disproportionality. Second, the intrinsic value of mega-seats (measured as institutional power) provides different incentives for parties to seek control of these institutional bodies. Empirical findings show a strong support for these two main hypotheses.
