America’s political landscape is infested with many zombie ideas — beliefs about policy that have been repeatedly refuted with evidence and analysis but refuse to die. The most prominent zombie is the insistence that low taxes on rich people are the key to prosperity. But there are others.

And right now the most dangerous zombie is probably the claim that rising life expectancy justifies a rise in both the Social Security retirement age and the age of eligibility for Medicare. Even some Democrats — including, according to reports, the president — have seemed susceptible to this argument. But it’s a cruel, foolish idea — cruel in the case of Social Security, foolish in the case of Medicare — and we shouldn’t let it eat our brains.

First of all, you need to understand that while life expectancy at birth has gone up a lot, that’s not relevant to this issue; what matters is life expectancy for those at or near retirement age. When, to take one example, Alan Simpson — the co-chairman of President Obama’s deficit commission — declared that Social Security was “never intended as a retirement program” because life expectancy when it was founded was only 63, he was displaying his ignorance. Even in 1940, Americans who made it to age 65 generally had many years left.

Now, life expectancy at age 65 has risen, too. But the rise has been very uneven since the 1970s, with only the relatively affluent and well-educated seeing large gains. Bear in mind, too, that the full retirement age has already gone up to 66 and is scheduled to rise to 67 under current law.