Imagine you are sitting in first class on a plane, waiting for the plane to push off from the gate, when you see two people in uniform, the pilot and co-pilot, dash from the Jetway into the cockpit. A few seconds later, a voice comes over the intercom, saying, “This is Captain Jones, please be sure your seat belts are fastened. We’re ready for takeoff.” What crucial event could not have occurred in this scenario? The pilot and co-pilot did not go through their checklist of safety measures. Fuel tanks full? Check! Flaps up? Check!

Everybody knows obesity is a massive problem in the U.S. It rivals smoking in terms of its health hazards, according to a report in the February 2010 American Journal of Preventative Medicine. As a society, we’ve made great strides, giant leaps even, in reducing rates of smoking. Smoking bans on airplanes, in public buildings, in restaurants, have helped. So have negative ad campaigns aimed at teenagers, higher insurance premiums for smokers and higher taxes on cigarettes.

While the partisan gap in Washington is wider than it’s been at any time in living memory, the two parties do have one remarkable agenda in common. Both have proposed cuts in Medicare so drastic that they would have been politically suicidal a decade ago and may still be. Yet neither party is backing off.

Demography is not destiny, as is sometimes claimed. The human race could be wiped out by a plague or an asteroid, or transformed by some new technology. But no matter what, today's patterns of fertility, migration, and mortality fundamentally determine how much society will or can change for many generations to come.