voluntary
And that’s exactly the problem. It’s impossible to be voluntary if you have a power imbalance between employer and employee. “Hey, I need you to work this weekend” “But is Jan 1st, I can’t.” …. “Hey, sorry I have to fire you, you’re not a team player” People need money, they will subject themselves to this kind of thing.

Interesting question. I knew two twin sisters and they didn’t have the same boob type.