And what do said Americans do with the refund check? If they just blow it anyway, it doesn’t really count as an argument in favor of overwithholding as an interest-free forced savings program.

Not my fault if people are generally morons.

Or the Roth 401(k), which was not mentioned at all. If you’re lucky enough to work for an employer which offers it.

Most bonuses are subject to Federal supplemental income withholding of 25%, except when the total is $1mm or more, in which case the maximum Federal rate applies (currently 39.6%).