Only 47% of working age Americans have full time jobs
theautomaticearth.blogspot.com
theautomaticearth.blogspot.com
I would assume those people would fall into that category, but I might be missing something.
For example, I know people who would go stir crazy taking care of kids or elderly parents at home if that was all they did, or people who have interests that they're passionate about but don't pay the rent (starving artists, startup founder, etc.)
When I was starting my business, I asked for and got reduced hours at my "day job", and it helped me get over the hump to profitability (still doing consulting for them).
Underemployment is a problem - usually caused a structural deficiency in the way that non-pay benefits are allocated in this country. Plenty of employers wouldn't care if you worked less than full time if they could get the work done and they didn't have to pay extra per employee for health care and other benefits.