Cumulative Party System Innovation: sum of the vote share received by non-founder parties in each election. A party is considered as a founder if it has received at least 1% of the national vote share in at least one of the first two post-WWII elections (or, in the case of Greece, Portugal and Spain, the first two democratic elections). Otherwise, the party is counted as a non-founder. The rationale behind this choice is that we look at the first two post-WWII or post-authoritarian elections and make a dichotomous distinction between relevant parties that formed the system (those who received more than 1% of the votes) and parties that emerged later or were only marginal actors (those below 1%) at that time.