A key part of the Senate tax bill is repeal of the individual health insurance mandate. The budget scoring relies on this repeal reducing Federal deficits by $318 billion — and the bulk of these spending cuts would hit lower-income families. Republicans argue, however, that these families won’t really be hurt, because they’ll be making a voluntary choice not to be covered and collect government subsidies.

This argument might make sense in a world of hyper-rational individuals. In the world we actually live in, however, it’s a very bad argument. In fact, the very budget savings Republicans are counting on depend on people making bad choices.

For if you look at CBO’s estimates of the savings from mandate repeal, more than half come from a reduction in the number of people on Medicaid. Why wouldn’t someone eligible for Medicaid sign up for free insurance? The answer, surely, is that he or she isn’t aware of the option, or simply fails to act in his or her self-interest.

If you think such things don’t happen, consider that one of the major triumphs of behavioral economics involves the demonstration that many people fail to take advantage of retirement plans that cost little or nothing — unless they’re automatically enrolled. If they are automatically enrolled, but with the option of dropping the plan, enrollment is much higher than if they’re offered the same plan, but have to opt in. Sorry, but financial decisions like whether to get health insurance are not made well, even by the well-educated and affluent, let alone the poorer, stressed people who are the main targets of GOP cuts.

Or consider the “woodwork effect” of the ACA: Medicaid enrollment increased even in states that didn’t accept Medicaid expansion, because greater publicity led some people to look into their options and discover benefits they should have been collecting all along. (Not woodwork effect exactly, but I know people in New Jersey who tried to sign up for the exchanges and discovered that they had long been eligible for Medicaid.)

So one main effect of the individual mandate is, in effect, to make Medicaid opt-out (at a cost) rather than opt-in; a lot of people who should have been getting the benefit won’t unless something like this happens, and their failure to get the benefit is a true cost, not the result of a well-informed choice.

And of course it doesn’t stop with Medicaid: many of those collecting subsidies on the exchanges wouldn’t have done something that turns out to be a big advantage without the mandate in effect forcing them to take a better look at their own self-interest.

Oh, and if you don’t believe any of this will happen, or it won’t happen on a large scale, then you can’t simultaneously believe in those $318 billion in savings.

So are reduced outlays on lower-income families a true cost to those families? Yes. Maybe not 100 cents on the dollar, but a lot closer to that than to zero.