NIT is just moving the progressive tax system into negative values, which means you subsidize the poor but once you make enough money you won't get subsidized.
UBI is identical to NIT. The names are different framings of the concept, but the policy is identical and widely recognized as such, which is why the experiments frequently referred to tests of UBI are also the ones characterized as tests of NIT.
> NIT is just moving the progressive tax system into negative values
“Progressive” refers to marginal rates, which remain non-negative in NIT; NIT just has a flat personal refundable credit (which is equivalent to a flag annual payment) included. Which is exactly what a UBI is, except the payment in some forms of UBI is outside of the tax system, but that implementation detail is irrelevant: whether it's called a tax credit or a non-tax payment is still the same thing.
But I'd like to elaborate a bit here. UBI is not identical to NIT because the difference is marketing, not math. Most of the electorate's basis for evaluating a given policy is not to perform a mathematical comparison of two systems and checking whether they yield the same results. How something is done is as important in politics as what gets done.
Which is why, as I've pointed out, BI isn't like NIT at all. The difference is how the idea is going to get sold to the public, not what each taxpayer is going to end up contributing to the system as a function of their income.
But this is explicitly what the proponents of BI want. It's as much about the optics as who pays for it. I.e. the idea is to create a program that draws its political support from everyone getting paid a check, similar to how Roosevelt enacted social security to include all citizens, even those who were millionaires and had no need for it.
Which is what sets it apart from Negative Income Tax.
