@boomerang,
I think it's too simple to just look at the source of the money in those studies. You also need to consider what the child will be responsible for in terms of purchasing.
By Mo's age, the hamburgers had stopped paying for my clothing - other than winter coats and boots. I was responsible for saving my allowance and other income to buy the rest of my clothing. I've told the story several times - the summer I turned 10, hamburgboy gave me my allowance for the upcoming quarter and I ended up spending 3 months dressed entirely in pink - even after I complained mightily.
Financial intelligence isn't just about income - it's also about how to use the money wisely.
There are a number of studies which look at that side of things as well.
An earlier poster has commented on it - asking how Mo's money was to be used. There are some interesting studies about children's financial learning approaches out there. I was always interested in one that required the child to pay 5 or 10% of their allowance back to the parents (representing government) as tax - required some money to be put aside for gift-giving to others - some money was to go into an education fund - it had 5 or 6 savings groupings. The kids in those plans seemed to be very wise about income as well as use of money when they were quite young.
A friend used a homegrown variant with her kids starting at age 3 - in their late teens/early twenties her kids are incredibly money savvy - her 23 year-old daughter still has over $22,000 in savings after paying for university while living away from home.
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On the original question - if you're paying Mo, he's not volunteering and there's no upside to calling it volunteer work. He's working and you're paying him. He doesn't show up, he doesn't get paid ... and if you've set it up right, his expenses will continue so he'll need to think about the effects of not showing up.