You know how penniless students save money by buying used textbooks? And you know how those textbooks are often filled with tons of highlighting — relics of the previous owners?

Researchers have known for a while that highlighting has a strong cognitive effect on readers. People tend to pay attention to highlighting — even when it’s not their own. That’s why pre-owned textbooks can have a certain pedagogical appeal. I remember friends of mine who preferred to buy pre-highlighted textbooks because it would “save them time”; the previous student had already done all the work of identifying the relevant passages, right?

Except — what if the previous owner was a moron?

In that case, reading the textbook turns you into a moron too. According to a study by the academics Vicki Silver and David Kreiner, students who were given textbooks with “inappropriate highlighting” wound up scoring worse on tests than students who were allowed to do the highlighting themselves. (The study isn’t online, but an abstract of it is here, about three-quarters of the way down the page.)

Silver and Kreiner won an Ig Nobel Prize this year for their study. I was at the ceremony, and in her acceptance speech, Silver summed up the results of their research neatly: “Don’t buy textbooks from dumb people.”