That an individual vote changes nothing in the real world almost goes without saying - you can trivially compare the hypothetical world in which that person doe

That an individual vote changes nothing in the real world almost goes without saying - you can trivially compare the hypothetical world in which that person does vote to the one in which she doesn't. If and only if the outcome changes, marginal votes can be said to have "made a difference". The way it makes the voter less rational is that once you have voted for somebody you become emotionally invested in that candidate. If they lose, you now get to feel bad about their losing. if they win, you can either feel bad about their inevitable advocacy of bad policies and abandonment of election promises...or you can reconfigure your opinions so as to convince yourself that what they actually do is what ought to be done. Being personally invested in candidates makes people dumber. The fact that somebody voted for Obama makes it harder for them to see what a empty suit Obama has been as President; ditto for people who voted for Bush when he was in office. And neither the Bush voters nor the Obama voters can see how SIMILAR the two are. If you want to be able to assess politicians impartially, it's easier to do that if you didn't vote for them. When I cast a vote, I know for a fact that it won't change the outcome of the election but it will change ME. I should therefore only cast a vote if I want the change in myself that I expect that vote to cause.