I develop four related measures of the “ethnicization” of electoral behavior. Each measure increases as ethnic identity becomes more central to vote choice, but the measures differ along two theoretical dimensions. The first dimension contrasts a group-based perspective (which focuses on cohesion in the voting patterns of group members) with a party-based perspective (which focuses on the composition of groups supporting political parties). The second dimension contrasts a fractionalization perspective (which assumes that more groups or parties cause more problems) with a polarization perspective (which assumes that problems are greatest when there are two equal-sized groups or parties). Using survey data to implement the measures in 43 countries, the article shows that proportional electoral laws are associated with lower levels of ethnicization—the opposite of what is widely assumed. I argue that the lower levels of ethnicization in PR systems should be unsurprising.