But such a move has happened before. In Britain right up until 1971, wives weren’t usually taxed on their income; their husbands were. A wife’s income was deemed to be “stated and accounted for by her husband”. It wasn’t until 1950 that wives ceased to be classified for tax purposes as incapacitated along with “infants, lunatics, idiots and the insane”. South Australia broke ranks early, in 1884, taxing married women as individuals and giving them the right to own property. By the time the Commonwealth introduced national income tax in 1915, all the states had fallen into line. Treasurer Scott Morrison. Credit:AAP What possible modern-day reason could there be for taxing women differently to men, as mentioned by Melbourne University tax expert Miranda Stewart in evidence to the Senate last week?

Morrison himself provided a clue while ridiculing the idea. He said the Tax Act was designed “to treat people’s income the same, and so you pay tax according to what you earn”. But we don’t. Someone who earns $1000 from wages pays twice as much as someone who earns $1000 by making a capital gain selling an asset. Income from capital gains is taxed more lightly in accordance with what’s known as optimal taxation theory. It suggests taxing heavily things that tax is unlikely to stop, such as work, and taxing more lightly things that tax is more likely to stop, such as the movement of capital. It’s the basis of the argument for a lower company tax rate as well as a lower capital gains tax rate. The concession isn’t “fair”, but it’s efficient. Miranda Stewart suggested taxing women differently. Credit:Daniel Munoz As would be the logical extension, which is to tax female wages more lightly than male wages. Male work turns out to barely react to after-tax pay. Most men will continue to work full-time regardless of what happens to what they take home, regardless of how much they grumble.

And there’s another argument for discriminating on the basis of gender. It’s that, for most of us, gender is innate. We won’t change it. Tax theorists say that, ideally, we should be taxed on our underlying ability to earn an income rather than the income itself. Otherwise some of us with ability will avoid tax by avoiding earning an income. Although the ability to earn is hard to measure, markers for it are easy to measure, such as height. In a half tongue-in-cheek paper entitled The Optimal Taxation of Height, Harvard economists Gregory Mankiw and Matthew Weinzierl note that someone who is 183 centimetres tall can expect to earn $US5500 ($7300) more per year than someone 165 centimetres tall. They say tall people should pay several thousands more in tax than short people on the same income. It’s a way of getting at their earning capacity, as would be a higher tax on the earnings of men. And there’s yet another practical reason to tax women more lightly. The withdrawal of family benefits and the imposition of childcare costs as mothers return to work mean some face extraordinarily high “effective” marginal tax rates of up to 90 per cent. If ever there were people who ought to be affected by high tax rates, it’s returning mothers. Loading But here’s what’s odd. Australian mothers are hardy. When the Productivity Commission recommended a new, simpler and more generous formula for childcare support along the lines of the one introduced in this year’s budget, it found it would boost employment by just 15,000 full-time worker equivalents in a workforce at present growing by hundreds of thousands per year. More than mothers in the United States, Germany and Britain, Australian mothers seem undaunted by tax rates. The case for treating them gently is strong in theory, weak in practice.

Peter Martin is economics editor of The Age. Follow Peter Martin on Twitter and Facebook