Chaining yourself to bulldozers and throwing paint over company executives is more likely to influence environmental policy than schmoozing on Capitol Hill. So says an analysis of the impact of the green movement in the US between 1960 and 1994.

The study compares the number of bills passed by Congress with tactics employed by green groups in the same year. Jon Agnone, a sociologist at the University of Washington, Seattle, found that sit-ins, rallies and boycotts were highly effective at forcing new environmental laws.

Each protest raised the number of pro-environment bills passed by 2.2 per cent.

Neither effort spent schmoozing politicians nor the state of public opinion made any difference.

But conventional politics does play a part. Environmental legislation is 75 per cent more likely to pass when Democrats control both houses of Congress.

And it gets a 200 per cent boost in congressional election years, presumably because politicians see it as a vote winner.

Agnone, who presented his results on 17 August at the American Sociological Association's meeting in San Francisco, says protest groups lose their edge when they become part of the system. Their most effective weapon is disruption.

If you make a big enough disturbance then people have to recognise what you are doing, said Agnone.