Political scientists who study mixed-member systems often refer to a concept of “contamination”.* One can find somewhat different meanings of the term in different works, but the most common conceptualization is the entry of candidates in nominal-tier districts where they can’t realistically win, but where their presence may help boost the list. The notion of “contamination” assumes that we should see a reduction towards two significant parties in the single-seat district, but this reduction may be retarded because of smaller parties’ vote-seeking for their party lists. The reasoning goes: the party may fail to mobilize supporters for its list if it is seen to have abandoned the district by not running a candidate.
Is the concept realistic? Scholarly literature is mixed (so to speak). But sometimes politicians act as though they believed political scientists. The Green Party in New Zealand seems to believe in contamination. (That was a fun sentence to write!)
First-term Green MP Holly Walker, elected form the list (like all Green MPs), is withdrawing her candidacy, according to the NZ Herald. Her list candidacy, that is. The article further notes,
Ms Walker, who will still contest the Hutt South seat in order to help the Greens’ party vote, said she was announcing the decision with “real sadness”.
So, even though she won’t be returning to parliament, her electorate campaign remains active, in order to attract votes to the list.
The NZ Green Party is an especially tough test for the contamination thesis, because Green voters in recent elections have shown a very strong tendency to split their tickets, strategically voting for the local Labour candidate. Ticket-splitting runs against the grain of the contamination thesis, which several authors have said is valid only to the extent that voters are generally reluctant to split their tickets.
However, the basic notion of contamination, as I articulated it above, is that the party can’t be seen to have abandoned the district. To do so would reduce their ability to get list votes. The logic does not actually require that voters vote for the district candidate. It only requires that the party’s putting a human face on the list by nominating a candidate in the district helps the party’s list vote. That seems to be precisely what the Greens and Walker are counting on.
* Other authors call it “spillover”. A short bibliography on the topic (and I should note that not all these authors claim to find such an effect, and some specifically argue against it):
Cox, Karen E., & Schoppa, Leonard J. (2002). “Interaction effects in mixed-member electoral systems: Theory and evidence from Germany, Japan, and Italy.” Comparative Political Studies, 35, 1027-1053.
Crisp, Brian F., Joshua D. Potter, and John J. W. Lee. 2012. “Entry and Coordination in Mixed-Member Systems: A Controlled Comparison Testing the Contamination Hypothesis.” The Journal of Politics 74 (02): 571–583.
Ferrara, Federico, & Herron, Erik S. (2005). “Going it alone? Strategic entry under mixed electoral rules.” American Journal of Political Science, 49, 16-31.
Herron, Erik S., and Misa Nishikawa. 2001. “Contamination Effects and the Number of Parties in Mixed-Superposition Electoral Systems.” Electoral Studies 20(1): 63– 86.
Karp, Jeffrey A. 2009. “Candidate effects and spill-over in mixed systems: Evidence from New Zealand.” Electoral Studies 28(1): 41–50.
Ellis S. Krauss, Kuniaki Nemoto and Robert Pekkanen, “Reverse Contamination: Burning and Building Bridges in Mixed Member Systems,” Comparative Political Studies, Vol. 45, No. 6 (June 2012), pp. 747-77.
Moser, Robert G., and Ethan Scheiner. Electoral Systems and Political Context. Cambridge, 2012.