Again, lots of good ones. I thought eigenperson offered was the best explanation, and he's the winner. But I just want to note how hard it is to pick a winner, given that all of these posts kind of riff off each other. They work as a collective and picking out the best synthesis of all the info offered is a challenge. Will do another later this week.
The US is particularly miserable at putting aside money for the future. Should we blame our paychecks or our psychology?