Actually, I think another issue is in the question "do you like your job". I guess, most doctors, engineers, scientists, etc. really like what they do, so, they can easily work 40 hours a week (or more) without considering it to be hard labour. But they did sacrifice a lot (4 years or partying in high school, 4 years of partying at College, 2-6 years of no income in grad school, 2-6 years of low income as interns). Most electricians, plumbers, etc. are OK with their job: they prefer not to, but they are getting good money, so,they work. Walmart cashiers, Tim Horton employees hate their jobs: they make little money but have to work full-time to support themselves. Miners and other hard labour jobs: they realise that their job is hard but they want to do it anyway since they have families to support, so, they do work full time although working part-time can be sufficient to earn enough money to live on. IMHO, SPs are like miners: their job is hard if you do it full-time, but the pay is good too. However, they have different life priorities and, as a result, value their leisure more and chose to work less. And, just to be clear, what SPs job is: to pretend to be happy (not being happy).