Based on 'issue voting'; some voters are not committed to one party and will switch based on key issues.
Rational choice voters view some issues as more salient than others.
In the 2015 General Election, 30% of voters made their decision based on Asylum and Immigration policies.
Altruistic voters will vote for a Party that is good for society as a whole; for instance, Richard Branson supported New Labour in 1997 even though he is a millionaire.
Instrumental voters will vote selfishly; voters on benefits will usually vote for Labour.