Spain’s Popular Party has proposed that women receive tax breaks for… being women! Since parties to its left are likely to pile on board, Spain will soon have a tax system that discriminates on the basis of gender. There’s a radical feminist argument for this, but there’s also a utilitarian argument based on the Ramsey taxation principle: If the demand or supply of something is more elastic, tax it less. Since women’s labor has greater elasticity than men’s, it should be taxed less. This seems to reflect a profound tension between utilitarianism and equality.
Now, it turns out that if I am maximising any welfare criterion, I can always do better by discriminating than by not discriminating. This is because non-discrimination is a special case of discrimination, where all groups are treated equally. If different groups have different economic behaviour, then to maximise my welfare function I need to discriminate as much as possible, and I will treat each group differently. So we should have different taxes depending on sex, age, race, marital status, city of residence, state of health, and so on….
It remains true that there is a case for taxing marginal hours at the household level at a lower rate than infra-marginal ones. But this can be achieved by a gender-neutral reduction in the tax rate on the secondary earner’s hours (which could well also apply to the primary earner’s marginal hours such as overtime). And since such a scheme can make the household better-off by supplying more hours without reducing the total taxes they pay to the government, one can actually leave the household free to choose between that and a more traditional tax schedule.
A progressive tax schedule like that in the United States and in most European countries has the effect of taxing the secondary earner more heavily.