It's certainly reasonable for a studies across fields with hourly pay or, e.g., including both part-time and full-time (such as the BLS survey cited by the Forbes article), but I'm not sure we can make that assumption for software development - programmer productivity in a given time period is, on its own, somewhat mythically considered to vary by orders of magnitude.
You don't think people who are willing to work longer hours end up getting paid more? Are people who work more than forty hours each week doing it for nothing?
