How we change what others think, feel, believe and do
After we have made a decision, we will feel dissonance regarding the possibility of it being wrong. We will often change our perceptions to reduce this dissonance and make the decision seem more attractive.
This is the basis of the foot-in-the-door technique where people who are asked to make a small commitment (such as signing a petition) will later change their views to align with the action and consequently be more amenable to a more significant request. It is also the basis of brainwashing.
Brehm (1956) asked shoppers to rate the attractiveness of household appliances. They were then allowed to choose, as a gift, between two appliances they had rated equally attractive. Twenty minutes later, they were asked to re-evaluate the appliances. Guess what? They now rated their gift somewhat more highly.
Knox and Inkster (1968) found that after placing a $2 bet, race-goers increased their estimation as to the likelihood of their horse winning the race.
How often have you bought something then worried about whether you have got a bargain or otherwise. Seeing the item for more money in another shop is always comforting (and the reverse, of course, is also true).
When you want people to do something of which they do not approve, start small. Get them to do something similar in a very small way, downplaying it. Then let them know of what they have really done.
Beware of people asking you for small favors in areas where you do not particularly agree. Stick to your principles, no matter how small the request.