We journalists got a gentle chiding today from Stephen Goss, chief actuary of the Social Security Administration. If you’re going to talk about a problem, it’s important to define it correctly, and we seem to spend a lot of time talking about the aging of the Baby Boomers and about Americans’ longer life expectancy. Those things do affect Social Security’s long-term solvency, but they’re dwarfed by a third factor: The sharp decrease in the birth rate since the mid-1960s. Says Goss:

The real story of the Baby Boom is not that it happened but that it stopped.

During the Baby Boom, the average American woman had 3.3 children between ages 14 and 49. Now, that’s down to 2.1 children. As the large Boomer generation moves into retirement by 2030, we’ll have about 1 retiree for every two working Americans, up from a more than 1-to-3 ratio now. After 2030, longer life expectancy will continue to change that dependency ratio, Goss says, but the big shift is all about birth rates.

Here’s the dependency ratio in graphic form, from the 2009 Social Security Trustees’ Report. The vertical axis is the number of active workers per Social Security recipient.

Goss didn’t endorse any specific policy prescriptions, but he tell a National Press Foundation audience that the policy debate might be affected by how people frame the problem:

Remember, our big change in the program is not about people living longer, which might lead you to say we should work longer. It’s about fewer kids being born.