This thesis is a quantitative study based on survey data aiming to record to what degree voters in Swedish national elections (1998, 2006, 2010, 2018) have voted according to preferences in party leader, party politics or both. The purpose is to measure whether or not electoral presidentialization has affected voter behaviour towards what is called (in scholarly literature) the leader effect. The findings clearly show evidence of the leader effect - that a minority of voters put their vote on a party whose leader they like but whose politics they do not agree with and on a party they do not feel convinced by. A secondary, and in the thesis fulfilled, purpose is to create a methodology designed to measure the leader effect while isolating the definition of persons affected by the leader effect to persons who only like the leader. This is in contrast to methodologies in previous research which include persons that can also like the party, as long as they like the leader more, as affected by the leader effect.