It is not often that a government amends a law that the Supreme Court has just upheld. Indeed, not only did the court rule against Susan Thibaudeau in her challenge to the current tax treatment of child-support payments, but it was clear that senior ministers in the government agreed with the ruling. For Ms. Thibaudeau, had she won, would have turned the child-support system on its head, and with it every known principle of tax fairness.

Under current law, income transferred under court order from one divorced spouse to another for the purpose of child support is taxable on receipt, not on payment. That is, the payment is counted as part of the custodial parent's taxable income, rather than the non-custodial parent's. This is exactly as it should be.

In many cases, the non-custodial parent - the man, most of the time - never even sees the money; it is deducted from his paycheque, just like a tax. For all practical purposes, it might as well have been earned by the custodial parent - the woman. Where he has neither access to nor control over that money, she has both. Logically, she should be the one to pay the tax.

It is perhaps understandable that certain feminist groups would pass over this elementary principle of taxation, in favour of an even simpler one: Make the men pay. The government, however, can make no such defence. It knew there was nothing wrong with the law as it stands, and knows it to this day. But what chance has well-established principle, however sound, against well-entrenched prejudices, however muddled? As far as the government was concerned, all that mattered was that the public, or certain aggrieved sections of the public, perceived the tax as unfair. And so, as of last week's federal budget, the onus of paying the tax has been reversed.

The day before the budget, Liberal MP David Walker, the Finance Minister's parliamentary secretary, who in the past has supported retaining the current system, was propped up on The National to explain why the government had capitulated to divorced mothers who complained about paying tax on their support payments. "People didn't come at it from the rational point of view," he acknowledged. And so, of course, neither will the government.

Naturally, the budget tries to dress up this cravenness in the language of principle - or, if not principle, at least popular opinion. "Few Canadians think it is right," it reports, "to tax child support as if it is the custodial parent's own income, or to provide a special tax break to the support-paying parent for performing the ordinary obligations of a parent." Elsewhere it notes, "Child support is not discretionary."

Very true. But if that is an argument for exempting the custodial parent, why is it an argument for taxing the non-custodial parent? If child support should not be taxed "as if it is the custodial parent's income," how on Earth can it be considered part of the non-custodial parent's income? It's hardly a "special tax break" to offer tax relief on income used to defray the costs of child-rearing. The support- receiving parent, after all, gets the child tax credit in recognition of her own contributions to the cause. With the changes in the budget, she gets his tax break, too.

It is the disposition of the income, not its source, that makes it non- discretionary, and hence eligible for exemption from tax. It doesn't matter one whit that the income she receives from her ex-husband is called "child support," while what she earns herself is earmarked "general expenses." It all goes into the same pot. If she didn't get the money from him to pay for the kids, she'd have to pay for them out of her own wages. The more she gets from him for child support, the more she has left to spend on other things.

The payments from her ex-husband for child support add just as surely to her total income, and so to her ability to pay her fair share of tax, as the wage she receives from her employer. Or in the arcane language of tax theory, a buck is a buck is a buck. That's what's really at stake here. It's not just a matter of taxing all income equally. It is a matter of taxing people equally, where they are in similar circumstances.

Forget the support-paying fathers. Think of other single mothers without such support: widowed, or never married. Under the new law, a divorced mother with, say, $40,000 in total income - $30,000 in salary, $10,000 in child support - will pay tax on only the $30,000. Yet a widowed mom with the same total income, but all of it earned, pays tax on the whole shot. Is this fair? Is this rational? Is this what we really want?