There are many "green fakers" who only pretend to be eco-friendly, claims a psychologist who has been studying what is revealed by body language.
Geoff Beattie, based at the University of Manchester, has published research showing how people's green opinions can be contradicted by their gestures.
People "may care a good deal less" than the views they express, he says.
While words can be controlled, he says "gestures are difficult, if not impossible, to edit".
This research, by the psychologist who advises the Big Brother television show, will confirm the suspicions of those doubting the sincerity of the green credentials of some individuals and businesses.
Do hotel chains really want to save the planet - or do they want to save money by washing fewer towels?
Professor Beattie has been examining the gap between the public support for green values - and the different way people behave in practice.
Researchers, at the university's Sustainable Consumption Institute, made video recordings of people talking about issues such as global warming - looking at how their words matched their body language, such as hand gestures and expressions.
The study found that while people could control their speech to express green opinions, their unconscious gestures suggested their "true thoughts and feelings" lay elsewhere.
This could help to explain the difference between what people said and how they made personal decisions, such as what they bought in the supermarket.
"This research shows there are 'green fakers' out there, who say one thing but believe another," says Professor Beattie.
"This material shows for the first time a behaviour clash between what people espouse openly and explicitly on green attitudes and what they hold unconsciously and implicitly.
"Explicitly, people may want to save the planet and appear green, but implicitly they may care a good deal less.
"Given it is these implicit attitudes that direct and control much of our behaviour in supermarkets and elsewhere, these are the attitudes that we have to pursue and understand and change."
The research is to be published as a book called Why Aren't We Saving The Planet? A Psychologist's Perspective.