According to a conventional wisdom, politics is nowadays more personalized than before. The proposition of the personalization of politics is relevant for three specific areas: institutions, media and voting behavior. This dissertation deals with the latter, since it focuses on the influence of party leaders on vote choice. So far the empirical scholarship tested whether “leader effects” have increased overtime (longitudinal studies) or investigated which conditions can enhance or discourage the electoral influence of party leaders (conditionality literature). We argue that both approaches have their limits, the former being based on the customary assumption of linearity, the latter investigating the role of (micro, meso and macro) “characteristics” rather than overarching and dynamic condition(s). This dissertation puts the cursor on a specific overarching condition: the (in)stability of parties and party systems (supply-side complexity). Actually, we argue that the magnitude of leader effects on the vote is conditional to the quality of the electoral environment. We therefore attribute to the “leader variable” a heuristic value: the leader appeal is conceived as an electoral shortcut more likely to be activated in complex electoral environment than in clear and stable contexts. There are two ways of testing the link between (variations in) and supply-side complexity and (variations in) leader effect on the vote. The first requires the adoption of a synchronic perspective, which implies a cross-national and cross-partisan empirical posture. This perspective considers space (i.e. “horizontal”) variations of stability, by assuming that certain parties and party systems are stable, while others are less or not stable. Are leader effects on the vote greater in the latter than in the former? The second one focuses on time (i.e. “longitudinal”) variations of stability. Indeed, electoral contexts vary across time – and not only across spaces - thus modifying the perceptions that voters may have of their electoral environment. When parties and party systems get convulsed, voters lose their frame of references. As such, they become cue-takers and rely more easily on leader appeals. On the contrary, when the political environment becomes clarified, voters will be less encouraged to rely on leader heuristic.This dissertation gives voice to both dimensions (space variation vs time variation). In a first place, we dig into the synchronic perspective. The relationship between supply-side complexity and leader effect on the vote is tested on a sample composed by 20 countries and 125 parties, included within the Comparative Studies of Electoral Systems. In a second place, we chart the magnitude of the leader variable according to longitudinal variations of supply-side complexity. Actually, our analytical effort will henceforth be centered on two distinguished case studies: Poland (1997-2011) and Italy (1996-2013), which have both experienced important variations in the stability of their electoral environment.We demonstrated that de facto differences (i.e. cross national and cross-partisan variations) in terms of stability do not automatically determine the magnitude of leader effects. On the contrary, we detect a link between the leader variable and longitudinal variations of the electoral environment. However, the quality of this link proves to be different in Poland and Italy. In the young democracy of Poland, leader effects and stability seem to be convergent rather than antagonist forces, while the Italian case properly fits our expectations: the convolution of the electoral environment makes voters more sensitive to the leader heuristic. / Doctorat en Sciences politiques et sociales / info:eu-repo/semantics/nonPublished