Compulsory voting
, 22 countries were recorded as having compulsory voting. Of these, only 10 countries (additionally one Swiss canton and one Indian state) enforce it. Of the 30 member states of the Organisation for Economic Co-operation and Development, 10 had forms of compulsory voting.
Donkey vote
In systems where voting is not compulsory, it seems counter-intuitive that many who attend the polls would be apathetic. However, there may be countervailing factors that produce a "donkey vote" even with voluntary turnout. In many US elections, a voter may well be intensely interested in (e.g.) the Presidential contest but not in other, less prominent races on the same "long ballot".