US: Students doing less study
Academic work included such activities as homework, revision and attending lectures.
Marks says the decline has occurred among all kinds of students and at all types of universities.
It is not clear why students are studying less. Students do more paid work than in the past, she says, but that does not explain the drop in time spent on study.
The research also found that students spend more time on leisure activities than they used to.
But Marks says her peers are not surprised by the findings. "When I tell people, if anything they're shocked there was a time that students used to put in the required amount of time."
She says it is possible that modern students are more productive thanks to technology such as computers, but most of the decline in study hours happened in the 1970s - before computers were generally available.
Her sense is that the main reason for the decline is that lecturers are not demanding so much work from their students. And that has been driven by two factors: first, professors want more time for research so they set less work; second, students give better course evaluations to easier courses.
"There's really very little incentive in the current system for anybody to teach a demanding class. You're going to get worse course evaluations, the students aren't going to like it, it takes time away from other things that you could do," Marks says.
"Other than your sheer love of the material and the topic and your desire to spread that love to your students, that's it."
She says increased stratification of US universities might also have played a part. The reputation of a university sends more of a signal to potential employers than its graduates' grades.
There is less incentive for students to work hard in order to differentiate themselves from their peers.
The research indicates that students are doing less work than is required for their degrees but there is no evidence that the quality of graduates has decreased, Marks says.
It also indicates the opportunity cost of doing a degree has decreased since the 1960s. She says that is particularly interesting given the benefit in terms of increased earnings has not decreased.
As for her own classes, Marks tells students at the start of the year they are expected to do two hours of work for every hour of contact time. She says she will hold them to that and it works - they do the required hours of study.
Marks suspects that what is required to increase student study hours is a combined effort by all lecturers to lift expectations. But that will not necessarily be easy.
She notes that some universities use their leisure activities to promote themselves - for example, sending frisbees to new students. And a lecturer was recently removed for running a course that was too demanding.
The research is forthcoming in the Review of Economics and Statistics .
* Mindy Marks is an assistant professor of economics at the University of California, Riverside, and Philip Babcock is an assistant professor of economics at the University of California, Santa Barbara.