It’s impossible to tell for sure, but it looks like approximately half of open source contributions take place during working hours, and workdays have twice as many contributions as weekends, so I would say that yes, it is at least one main mode of operation.

You mention Linus Torvalds in another comment, but did you know he has been paid to work on Linux since around 1997?

https://www.techrepublic.com/article/for-50-percent-of-devel...

Such stats are notoriously noisy. Ohloh only analyzes the most popular free software projects so there is a long, long tail of projects that is unaccounted for in the statistics. Furthermore, even if most commits are submitted during office hours (in what timezone?) how can we know that they are paid specifically for contributing? E.g. could be some bored developers commit to their own personal project.

Regardless, the amount of companies involved in free software could be evidence of the superiority of voluntarism over "workism" for complex human systems.

I agree it’s noisy, just offering it as some evidence.

I think the statistic that half as many commits are on weekend days is particularly interesting, because people who are really doing it for free would likely contribute MORE outside their working hours.

Do you have anything more concrete?