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I didn't read the actual study but based on this article it seems horribly flawed.

The participants who were tracked were imo totaly biased towards this kind of "eat less meat" viewpoint, so to then be suprised that they ate less meat (knowing they would be tracked) totaly destroys any credibility.

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You're onto something important, but I'd offer a different interpretation. All 'effects' -- the sense of being watched, the activation of prior viewpoints -- are downstream of treatment. Theoretically, the kids who were randomly assigned to the control group had the same biases (in expectation), so all observed effects can be attributed to watching the documentary.

Yes, the bias created by being informed is exactly the point of the study.

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