How greatly a woman's retirement income will be influenced by last week's Supreme Court decision on equal-pension payments is hard to guess. "It's an easy rule to evade," says Lloyd Kaye, of the William M. Mercer consulting firm.

The court handed women a win on the important principle of equal incomes in retirement. But it's a long way from the principle to the money. Some plans that do not now pay unisex benefits will make the switch; others will find ways of avoiding it.

First, some background on where women stand with pensions:

* More than 90 percent of pension plans already pay women, on an individual basis, the same monthly benefits as men. If you're in such a plan, the new Supreme Court decision may have no effect on you.

* Many company thrift plans and profit-sharing plans pay employes their benefits in a single lump sum or in fixed payments over five or 10 years. Men and women receive equal payments, so these plans don't appear to be affected.

* Your plan will change, however, if the company lets you take your benefit in the form of a lifetime income, paid through an insurance company. Insurers stretch lump-sum retirement benefits over the expected life of the customer. Since women, on average, live longer than men, their money has to last longer--so each monthly payment will be a little smaller. Many teacher and public-employe plans provide these kinds of annuities, as do many thrift and profit-sharing plans.

The Supreme Court says that the plans will have to stop discriminating. But what will they do? The hoped-for solution is that they'll switch to equal-payment annuities, and many probably will. As an alternative solution, however, they can quit offering lifetime annuities altogether. Instead, all retirees--men and women--will be offered either a lump sum or a fixed pay-out over five to 10 years. Neither form of payment discriminates, so neither is illegal.

The state of Arizona--whose discriminatory pension plan prompted Nathalie Norris' lawsuit and the new Supreme Court decision--has already dropped its lifetime annuity. Many other plans are now expected to follow suit, which is why the ruling is so easy to evade.

Employes in these plans will not be denied access to a lifetime annuity, if that's what they want. But instead of having the pension plan arrange it for them, they'll have to arrange it themselves.

What will the employes do? They'll go to an insurance company--the only source of guaranteed lifetime annuities. And what will the insurance company do? It will sell women an annuity that pays them smaller monthly benefits than it pays to men.

The Supreme Court said only that employers cannot offer discriminatory pension plans, which are prohibited by the civil-rights act. But nothing in the court decision, or the law, stops insurance companies from selling discriminatory annuities outside of pension plans.

So from a practical point of view, women in some discriminatory plans may find their situation unchanged. If they want a lifetime annuity, they'll have to buy it themselves from an insurance company--and they'll still get the same reduced income as they got before. "They won't get equal payments unless Congress forbids insurance companies from charging men and women differently for annuities," Kaye says.

As it happens, Congress is considering just such a law, and the insurance industry is vigorously opposing it. If the law fails to pass, the right to an equal retirement income, for which Nathalie Norris fought so--dare I say it?--manfully, may come to nothing.

Well, almost nothing. One little-known fact about pension plans is that some of their payment options--like the joint-and-survivor option that covers spouses--give slightly higher payments to female workers, which is also illegal. To stop discriminating, companies might offer a little less to the women who choose these options, and a little more to men.

So the irony is this: The Supreme Court may force pension plans to raise certain payments to men. But the Congress may fail to pass the law that would raise annuity payments to women. If such is the outcome, the real winners from the Supreme Court's worthy blow for equal retirement incomes will be--you guessed it--men.