IN AUGUST I blogged about Alberto Alesina and Andrea Ichino's proposal to cut taxes for women and raise them for men. Their argument was that because women's labour supply is more elastic than men's, a cut would send many women into the labour force, improving gender equality, while a small increase for men would have little effect on labour supply and tax revenues. I roundly criticized the authors for cavalierly flouting the ideal of equality under the law. I am therefore delighted to see Gilles Saint-Paul of the Toulouse School of Economics doing the same. Along the way he makes some excellent observations:

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. At the "optimum", some of these taxes would sound "right" to those with a "progressive" mind-like the one favouring women proposed by Alesina et al. Others would sound horribly wrong to these same people; in fact the very gender-tax proposal could be implemented as a reduction in transfers to women at the bottom of the distribution of income, since their greater labour supply elasticity implies these transfers are more distortionary than for men.

This suggests an extremely important point. The rarefied theory of optimal taxation abstracts from grimy real-world politics and instead assumes a far-seeing technocrat planner who simply calculates the revenue-maximising tax and dispassionately applies it. So neat, so clean, so rational. But real politics, and real fiscal policy, is a blood and guts fight for opportunistic exemptions and special subsidies. What's more, liberal societies are marked by rampant disagreement over values. Puritans and health nuts want to use taxes to discourage smoking. Population patriots think tax policy should discourage vacant wombs, while eco-warriors want to reward Prius owners.

For their part, Messers Alesina and Ichino are interested in gender equality in labour market participation. So it is convenient for them to find a rationale in good, hard economic science. Enemies of the demon rum I'm sure are delighted to remind the public that high taxes on booze, for which demand is relatively inelastic, will not much hurt sales. But they say so in the ardent hope of hurting sales. The view that efficiency-minded discrimination in taxes is legitimate creates a pretext for opportunism and moralism and encourages the costly political conflict that necessarily comes as opportunists and moralists clash for control.

Mr Saint-Paul submits that

if "society" (up to now) wrote down constitutions saying citizens are equal before the law, while our utilitarian social welfare functions say that some should be more equal than others, it is unlikely that these welfare functions represent the social preferences implicit in those constitutions.

Yes, but perhaps the stronger point is simply that the economist's idealised utilitarian social planner does not take seriously the costs incurred by the conflicts that will flare up when some are made "more equal than others". Perhaps liberal constitutions prominently feature equality before the law for a good reason: official non-discrimination works as a truce, a way of keeping the social peace.