Another factor is people withdrawing from the labor force to pursue more education. Stanford's Eric Hanushek, evaluating the non-labor force effects of the experiments, found that "for youth the reduction in labor supply brought about by the negative income tax is almost perfectly offset by increased school attendance."
That's not the only positive education finding. One study looking at the New Jersey experiment found that a negative income tax of mid-range generosity increased odds of completing high school by 25 to 30 percent; a similar analysis of the Seattle-Denver experiments put the number at 11 percent. While the evidence on academic performance was more limited, there was some evidence that children in NIT households did better at standardized tests in lower grades.