This new paper (pdf) by Baris Yoruk sheds light upon this. He studied the “Give Five” campaign which ran in the US in the early 90s, which was intended to encourage people to give 5% of their income to charity and do five hours a week of voluntary work.
He found that people who had heard of this campaign through the media were more likely to give to charity and do voluntary work. But this is not necessarily because of the power of the media. It could be because people who wanted to donate to charity were more likely to seek out adverts and information about charities.
Mr Yoruk controlled for this effect. And when he did so, he found that there was no link between awareness of the campaign and charitable giving. In this sense, the media had no impact upon what people did.
However, there was an effect upon doing volunteer work.
Why the difference? It could be, says Mr Yoruk, that the campaign changed the behaviour (a little) of people for whom there is a low opportunity cost of time. People who watch lots of TV, such as the retired or unemployed, can afford to give a little time for voluntary work.
Where the opportunity cost is higher - for example where money is at stake - the media have no impact.
How can we reconcile this with other evidence, which suggests that local newspapers can (with exceptions) enhance local democracy, and with evidence that TV news can affect voting behaviour?
Maybe easily. Voting is “cheap talk” - its opportunity cost is low. And a preference that’s cheap is one that’s easily swayed. When our own money is at stake, we are less susceptible to media pressure*. Perhaps what's going on here is a variant of Sayre's law - the media matter only when the stakes are low.
For me, this strengthens my prior support for demand-revealing referenda, because if people had to risk money when they voted, they’d be less swayed by the trash press.
* Yes, of course, our money is at stake in general elections, but only collectively so. What I mean is that there’s no individual link between how I vote and how much I lose individually. And this is what matters in shaping incentives.