The recent decision by Leona Alleslev to change political parties two years after being elected has caused some people to question whether this change is legitimate, that is to say whether voters elect an individual or vote for a party. Those who are loyal to their political party in particular are convinced that everyone votes for the party, not the person. They cite studies saying that only a small percentage vote for the person.
We could go through the methodology of each of those studies and explain what it is really saying, given the high correlation between the voter’s opinion of the person and their opinion of the party, but there is a much more conclusive test: when the MPs who change parties run again under the new party banner, do they get re-elected or do the voters punish disloyalty to the party they voted for, and vote for the same party again?
The majority of floor crossers get re-elected, between 50 and 60% win again under the new party. In general, incumbents who stay with the same party have a 75% chance of being re-elected. Incumbents have something like a 10% greater chance of winning than non-incumbents. How much of that incumbency advantage is from the individual and how much from the party, that is to say if a different person from the party of the incumbent runs, how much advantage do they have? The data say none of the advantage transfers to the new candidate.
Like it or not, the evidence is clear that the candidate as an individual is very important to voters. Some voters vote for the individual and some for the party, and probably most consider both factors and even others such as who do they wish not to represent them. The logic of most electoral reforms breaks down unless you believe that each voter votes for a party and consents to have their vote transferred to a different individual of the same party.
One of the advantages of single-member systems like preferential systems and to some degree FPTP is that they allow voters to vote for individuals or for parties or for any other criteria they wish. Electoral reform that presumes that votes are for parties takes away the franchise of a large part of voters. On the other hand, reform like preferential voting, which allows them to express their second choices, which they never had the opportunity to express in the past, increases their franchise.