Here's what I found:
"In a negative income tax system, people earning a certain income level would owe no taxes; those earning more than that would pay a proportion of their income above that level; and those below that level would receive a payment of a proportion of their shortfall, which is the amount their income falls below that level."
So a specific income level is picked, and unless that income level is exceeded the person has no incentive to work.
If we pick $7K/yr as the guaranteed level, there's absolutely no reason for my H.S. Senior (now turned 18) to work his usual $3,500/yr fast-food job (true example) - he can stay home & study harder or goof off and still receive the same $7K.
Individuals will forgo any traceable income or job, unless it pays reasonably well in excess of $7K. See the problem here? It *is* income tested in actuality. And has the same disincentives as other means-tested systems.
Unless, I'm misunderstanding something? (I may have).