In fact, the German electoral system is the best-known example of mixed electoral system, with voters voting simultaneously for a local candidate and for a national party list. Overall (surplus mandates apart), the allocated seats are exactly proportional to the parties’ vote share.
Almost the same electoral system has ended up in chaos in Albania, Italy, Lesotho and Venezuela. As my article shows, political parties can easily organise in a special pattern of strategic voting, which puts the electoral system completely out of order. More precisely, large parties can achieve over-representation by encouraging their voters to split their votes. They vote with the candidate vote (Germany: “Erststimme”) for their favourite party, but cast their list vote (“Zweitstimme”) for a different party, which is allied to their favourite party. If many voters follow this recommendation, then they outsmart the electoral system mechanism which is designed to lead to proportional results. Instead, the parties applying such a strategy will be massively over-represented.
I have simulated the consequences of such electoral strategies, and showed that there is no solution that would prevent them. Interesting enough, however, in 60 years of application of this electoral system in Germany, there is no known instance where such a strategy would have been used at large-scale. My tests (not published) for several recent elections show no sign that vote-splitting between CDU and FDP or between SPD and Greens has anything to do with such a strategy. In contrast, in young democracies, voters and parties have quickly learned how to abuse the electoral system.
I do not think that politicians and voters will ever be honest (except for Germans). Therefore, I am wondering whether German-style mixed electoral systems should be on the catwalk, or rather in the trash bin.