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As the authors put it with coy understatement: “Attitude change dissipated somewhat with time…”

They surveyed more than 500 students in two cohorts: one that had just read the book, and another that read it a year previously.

The subjects, all students in a psychology class, answered survey questions about, for example, whether they are reluctant to eat meat, inclined to buy and eat organic foods, trust major corporations, or oppose government corn subsidies. They were also polled on whether they think the quality of the American food supply is declining, and whether they support the environmental movement. These were used to build a composite score to reflect their attitudes to Mr. Pollan’s thesis, vegetarianism in general, and political action on food supply.

Habits are difficult to break, and therefore individuals may revert back to their prior beliefs

As it turned out, the attitude change that was so evident in recent readers was barely perceptible in the others.

Partly this is the result of impressionable young people being attracted to idealistic, fundamentalist, or extremist moral arguments, which they eventually reject or moderate, according to co-author Julia Hormes, a psychologist at the State University of New York at Albany.

It is hard to be certain, though, she said.

“The only way to control for that would be, obviously, to repeat that study in a group of 40-, 50-, 60-year-olds, and see if we find the same thing, and it’s very possible that we wouldn’t,” she said. Young students “probably are amongst the people who would be most open to that sort of thing, looking for a message and a cause.”