In 1983, Phillips Davison wrote a paper called The Third Person Effect in Communication. Here is his hypothesis:

…individuals who are members of an audience that is exposed to a persuasive communication (whether or not this communication is intended to be persuasive) will expect the communication to have a greater effect on others than on themselves. And whether or not these individuals are among the ostensible audience for the message, the impact that they expect this communication to have on others may lead them to take some action.

Any effect that the communication achieves may thus be due not to the reaction of the ostensible audience but rather to the behaviour of those who anticipate, or think they perceive, some reaction on the part of others.

Here’s an example he provides:

It was hypothesized that the brutal persecution of European Jews by the Nazis and the strong support of the Allied cause by Jews everywhere might have caused Americans generally to look with greater favour on their Jewish fellow citizens.

Two separate surveys by the Opinion Research Corporation in 1945 showed that this was not the case. Nearly four out of five respondents in both surveys said that the mass killings of Jews in Europe had caused no change in their attitudes toward Jews in the United States.

When asked in one of the surveys, however, over half of the respondents said that they expected other people’s attitudes to change, in either a favourable or an unfavourable direction.

Davison concluded:

Why are exaggerated expectations about the effects of communications on others so common? Do they occur in response to all categories of persuasive communications, or only certain categories? Or, is it possible that we do not overestimate effects on others so much as we underestimate effects on ourselves?

You can find it all at The Public Opinion Quarterly, Vol. 47, No. 1. (Spring, 1983), pp. 1-15.