Many years (decades, half-centuries) ago I read a book which reckoned that there is correlation between the complexity of the voting system and the number of spoilt ballot papers, ie. Switzerland with one of the world's most complicated voting systems had a negligible number of spoilt ballot papers, while the UK with a supposedly simple voting system had high numbers of spoilt ballot papers.

Maybe, of course, it's just that we have to have ballot papers simple enough for our politicians to understand.