Constitutionality of Joint Assessment

It has to be admitted that the point of making tax credits only partly transferrable (instead of fully transferrable) was, explicitly, to incentivise women to enter the workforce at a greater rate than they were doing at the time. As has been pointed out it's an incentive, not an obligation, and there's a difference between being better off/more tax-efficient as a result of entering the workforce, and being "obliged by economic necessity" to enter the workforce. I doubt that a constitutional challenge on this ground would succeed.

Still, there's an important issue of social policy here. On the one hand, you've got more and more couples where both work. On the other, you have an ever-growing population of elderly and frail people. The high rate of workforce participation undercuts the informal support mechanism that used to operate for older people, at a time when there is more and more need for support. The result must be either (a) more and more older people living in poverty/in danger, or (b) a substantial increase in formal support mechanism, largely paid for by the state, or (c) a bit of both. And I don't think we have got to grips with this yet.

Obviously, you can't say that it's women's duty to look after the frail elderly for no pay. But the point is that giving a woman a fully transferrable tax credit maximises her freedom in this regard. Making it only partly transferrable skews her choice towards formal work, and we need to accept the implications of that for the unpaid but social valuable work that she might otherwise do.