The subjects, who were paid for participating and received bonuses based on their performance, were given the official instructions and told to complete income and spending reports for a small group with a shoestring budget that supported an initiative raising the minimum wage. Although the scenario was straightforward, not a single subject properly complied with the relevant law -- their average score was 41 percent.

"In the real world," Milyo noted, "all 255 participants could be subject to legal penalties, including fines and litigation." Nine out of 10 subjects agreed that "red tape and the specter of legal penalties would deter citizens from engaging in political activity."

Before the experiment, the vast majority of Milyo's subjects did not even realize they had to register and file reports if they wanted to join with a few like-minded neighbors and take a stand on a ballot initiative. Neither did the residents of Park North, Colo., who in 2006 opposed their neighborhood's annexation by a nearby town. They found out after supporters of annexation sued them for neglecting to register as an "issue committee" and failing to report the money they had spent on lawn signs and flyers. A Colorado think tank, the Independence Institute, was hit by a similar lawsuit after it ran radio ads that criticized two 2005 tax referenda.

The benefits of regulations aimed at exposing astroturf are uncertain. After all, an argument against a particular public policy proposal, no matter who pays for it or what their motivation might be, is successful only if it persuades people. But there is a clear downside to mandatory disclosure: Because reporting requirements are burdensome and intimidating, efforts to make political speech more "transparent" can also make it inaudible.