The amazing thing about all the fuss is that until now there hasn’t been any really big data on how computing in the classroom affects learning. There were some small, short-term experiments. AND A study from 2003 they found that laptops made it difficult for students to remember what they had just learned in class. AND A study from 2014 showed that students were less likely to understand complex ideas when they were forced to take notes on a computer instead of by hand. But these were all fictional situations involving momentary recall. It is less clear how laptop use affects students during the semester.

Now there is an answer, thanks to a a big, new experiment from economists at West Point, who last year randomly banned computers from parts of a popular economics course at the military academy. One third of the sections could use laptops or tablets to take notes during lectures; one third could use tablets, but only to view teaching materials; and one third were prohibited from using any technology.

Not surprisingly, students who were allowed to use laptops — and 80 percent of them did — scored worse on the final exam. It is interesting that the smartest students seem to be the most damaged.

Among students with high ACT scores, those in the laptop-friendly sections performed significantly worse than their counterparts in the technology-free sections. In contrast, there wasn’t much of a difference between students with low ACT scores — those who were allowed to use laptops did just as well as those who weren’t. (The same pattern held when the researchers looked at students with high and low GPAs.)

These results are a little strange. We might have expected the brightest students to use their laptops judiciously. Instead, they have become the biggest victims of technology. Perhaps hubris played a role. Smarter students may have overestimated their ability to multitask. Or the best students could get the most attention in class.

The disadvantage of the size of the laptop was modest. The average score on the final was 72 out of 100, and students in sections with laptops scored about 1.7 points lower. But it’s hard to understand what 1.7 points means without knowing how clustered or far apart people’s test scores are. (For those familiar with statistics, the effect size was about 0.2 standard deviations.)

So here’s another way to think about it. The average score on the math section of the SAT last year was 511 out of 800. The difference between test scores on the laptop-friendly sections and test scores on the non-laptop sections is the same as the difference between scoring a 511 and scoring a 491 on the SAT math section . (This is roughly the same incentive for a high school student could expect from hiring SAT teachers.)

The researchers – Susan Payne Carter, Kyle Greenberg and Michael Walker – were also surprised to find that the tablet-only parts fared just as poorly as the laptop parts. Although students were not allowed to check email or play games on their tablets, the technology seemed to interfere with their studies.

However, these results are probably optimistic. At West Point, sections are limited to 18 students, so instructors could easily call out people who were clearly screwing up on their laptops. The problem of computer distraction is probably much more serious at other faculties, where lectures can be held by hundreds of students.

The West Point study has lessons even for those whose graduation days are far behind them. This is another proof that multitasking does not work. Beware of people who bring laptops to meetings — even “just to take notes.” They probably aren’t listening to you.