[Update: I added in a comment a list of cases with streaks of four or more elections with largest party having over half the vote.]
Given the news that a proposed merger of two opposition parties in South Africa was called off, I was wondering just how unusual is South Africa in having had a string of elections in which one party won a substantial majority of the vote.
Querying a dataset of over 1000 elections in countries that can be considered democracies (including many developing countries and young democratic regimes), the answer is: quite unusual.
The mean vote share of the largest party in these 1,148 elections is .42. In South Africa’s four democratic elections to date, the ANC has averaged .64. Such a high figure is just beyond the 95th percentile (.63) of the worldwide distribution.
Of course, the failure of opposition parties to offer a clear alternative does not help the cause of “normalizing” South Africa relative to other democracies. On the other hand, the extreme PR system also is an impediment. While normally, the more extreme the PR, the less likely is a party with over half the votes, if your starting point is a hegemonic party, then extreme PR presumably inhibits the development of an opposition that could challenge the leader. That is, numerous very small parties can all win their share without unifying, and the benefits from unifying are less clear for parties than they would be in a country with either a less proportional system or direct presidential elections (or both).
Speaking of direct presidential elections, the distribution of the worldwide data does not change much if we focus only on parliamentary systems, although there is small (and totally insignificant) tendency for bigger shares for the largest party under presidential systems (mean .43, although 95th percentile .61).