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Benjamin Franklin was possibly the Greatest Diplomat Ever. He convinced the French to help the fledgling American colonies fight the British - just 13 years after the colonies had fought against the French!
What did Ben have to say about his tactics? "A man convinced against his will is of the same opinion still." In other words, external pressure does not convince someone. In fact, it often does the opposite. If you truly want someone to be convinced, you must let him convince himself.
But it is not easy to engineer a mechanism to let someone convince himself. Leonardo DiCaprio almost died in his attempt.
There is, however, a way it can be done. Social scientists have determined that we accept inner responsibility for a behavior when we think we have chosen to perform it in the absence of strong outside pressures. Looking at it a different way, once someone has taken some action that shifts his self-image, he is likely to continue to act in accordance with that self-image. This occurs because we value consistency. Nobody wants to be viewed as a flake.
So what we need is a catalyst. That catalyst causes someone to take some action or make some decision. Then out of a desire for consistency (or simply to feel good about his decision), he will create his own internal reasons for taking that action or making that decision. Then he will continue down that path, even if the catalyst is removed.
And here's the data. An experiment was done in Iowa to test this theory in the domain of energy conservation. An interviewer gave residents some energy-saving tips and asked them to try to conserve energy in the future. Nobody did.
For another group, the interviewer said that residents who agreed to conserve would have their names publicized in the newspaper as public-spirited, fuel-conserving citizens. A month later, they had reduced their energy usage by 12%.
Here comes the rub. After one month, each family that had been promised their name in the paper got a letter saying that that would no longer be possible. Did they stop conserving? For the remaining winter months, they reduced usage by 15% - more than they had during the month they were promised their name in the paper!
The newspaper promise was a catalyst. Of course, nobody wants to admit (not even to themselves) that they conserve energy simply because their name would be in the paper. So they start making up other reasons: to save the Earth, to save money, to reduce America's dependence on foreign fuel. Those reasons make them feel good... and they came up with them on their own. So even when the catalyst reason was removed, they continued in that course.
Disclaimer: I'm not smart enough to have figured this out on my own. This is discussed thoroughly in Robert Cialdini's book, Influence.