Most people drink less, weigh less, exercise more, and watch less television when they're answering questions asked by a chap with a clipboard than they do in real life. They have more frequent intercourse, attend worship more regularly, read more, and listen to far more classical music and jazz in the alternate reality of the poll than they do in the reality in which we actually live. Most good polls have this factored into their results.
Similarly, when the poll is political, many people are a few steps to the left when they're being polled than they are when they actually step into the voting booth.
- When the pollster is at the front door (or on the blower), many people will identify with those candidates, parties, and causes which correspond with their deepest ideals, values, and hopes. They identify with what Lincoln called "the better angels of our nature". They may thus indicate a preference for a centre-left candidate that reflects these ideals.
- However, when the same people enter the actual voting booth on the day of the election, some of the same people will actually vote for a more right-of-centre (or even far-right) candidate who expresses their darkest fears and their unspoken prejudices.
(Note at the end of 2016: Given the fact that the polls got the results of both the Brexit referendum and the US presidential election so frighteningly wrong, the polls may need to change the statistical formula by which they allow for those who try to put their best foot forward when speaking to a pollster, while embracing "the dark side" in the polling booth.)