I didn't read it in detail, so forgive me if this was accounted for, but I feel like this would be hard to control as people who are not working at all or working extremely long hours may not be as capable as those working a normal or slightly lower than normal numbers of hours to begin with.
For example, if I can get a job and make enough money in a shorter number of hours, I'm probably smarter than the person who can't get or a job or the person who has to work 65+ hours a week, even if accounting for degree or similar.
>if I can get a job and make enough money in a shorter number of hours, I'm probably smarter than the person who can't get or a job or the person who has to work 65+ hours a week
This may or my not be true. I'm certainly not convinced; the smartest people I know work the longest hours.
I'm not convinced by this line of reasoning. Some of the highest paid jobs - Doctor, lawyer - have the longest hours. On the other hand, part time jobs are often minimum wage and low paying jobs (Walmart etc).
It's mentioned in the abstract: 'We deal with the potential endogeneity of the decision of how many hours to work by using the instrumental variable estimation technique.'
Essentially they use 'instruments' which are just other variables to control for this issue. Page 7 has the variables that they're using.
Sure, 'smarter' was a poor choice of words, but the point is I suspect there may be a correlation between general cognitive performance and job attainment.
An economics paper would usually try to exploit a natural experiment changing work hours for people in a way independent of their intelligence and jobs. For example, new labor legislation banning 40-hour work weeks, causing a drop in worked hours: you should be able to see cognitive scores of the same individual increase after the legislation takes effect.
Measure productivity of the same people working different numbers of hours per week? (And the study needs to run longer than one week at each number of hours. Working overtime boosts your productivity - the first week.)
How do you measure the productivity of cognitive work over an entire week? It's kind of subjective, and having the subjects self-report their level of satisfaction would probably be unreliable.
EDIT: I guess the trick might be to not use a survey, but to give the participants all the same project to work on (very expensive), then adjust the results based on their previous performance. I think an ideal group for this would be students because they're all already working on the same projects. Tell one (random) group that they can have some money if they promise to work on their class project for no more than one hour a day. Could this work?
For example, if I can get a job and make enough money in a shorter number of hours, I'm probably smarter than the person who can't get or a job or the person who has to work 65+ hours a week, even if accounting for degree or similar.