I just discovered several papers on the determinants of ballot invalidity (informal voting) in Australia. Two recent conversations prompted me to look for these. The overarching issue is whether an invalid ballot reflects confusion or protest voting. You can find the papers in my reference list. The answer seems to be: both but more of the latter, especially among young people dissatisfied with the party system.
Australia is a crucial case because national House elections require a voter to rank all candidates. Locally known as “full preferential voting,” this rule is meant to offset voters’ tendency to rank one (or two) and be done with it. Another way to interpret the rule is that it forces majority winners and possibly certain kinds of outcomes. One such outcome is a flow of votes from independent and minor-party candidates to major-party candidates. No such rules exist (anymore) in the U.S., and I have written a bit on this blog (and elsewhere) about the potential for coordination failure as a result.
I am not advocating compulsory ranking. Some of the papers I found have interesting things to say about that. For example, the decision to ‘spoil’ a ballot by truncating rankings shows that the solution is not perfect!