[chrisk]

I suppose everything's relative.  The general population has more than doubled since I was in high school, and it just feels like the number of class spots in accredited colleges has not increased at the same rate.  More people applying for not as many slots = harder to get in I think.

The general population of the USA was about half of the current population in 1920. You're a tad older than I had guessed.
USA Population History

The number of 12-17 years olds in the USA hit a peak (a local maximum) in 1974 (near the end of the Baby Boom), slightly surpassed that level for 2004-08, and isn't projected to reach that level again until 2034.
Children in USA
The overall story is that the number of 18 year olds has been surprisingly stable for a surprisingly long period of time. One might conclude that the same number of people competing for roughly the same number of slots equals not that much change in difficulty of getting admitted.

But of course it is more complicated than that.
The fraction of Americans going to college is increasing.
Enrollment Rates   (This chart is a little dated, but you can get the idea).
Some of that increase is towards the lower end of high school achievers as the idea spreads that a high school diploma isn't enough for a decent life. However, particularly among women and minority groups many more simply have college open as a path, thus putting more top end students into the mix. Of course for those groups, getting into an elite college is far easier than before (since it was close to zero probability at one point). For those who've always been in the pool, it's that much harder.

Your calculation of US population growth is off. The current population is about double the population of the mid-1950s, not 1920.

[chrisk]

I suppose everything's relative.  The general population has more than doubled since I was in high school, and it just feels like the number of class spots in accredited colleges has not increased at the same rate.  More people applying for not as many slots = harder to get in I think.

The general population of the USA was about half of the current population in 1920. You're a tad older than I had guessed.
USA Population History

The number of 12-17 years olds in the USA hit a peak (a local maximum) in 1974 (near the end of the Baby Boom), slightly surpassed that level for 2004-08, and isn't projected to reach that level again until 2034.
Children in USA
The overall story is that the number of 18 year olds has been surprisingly stable for a surprisingly long period of time. One might conclude that the same number of people competing for roughly the same number of slots equals not that much change in difficulty of getting admitted.

But of course it is more complicated than that.
The fraction of Americans going to college is increasing.
Enrollment Rates   (This chart is a little dated, but you can get the idea).
Some of that increase is towards the lower end of high school achievers as the idea spreads that a high school diploma isn't enough for a decent life. However, particularly among women and minority groups many more simply have college open as a path, thus putting more top end students into the mix. Of course for those groups, getting into an elite college is far easier than before (since it was close to zero probability at one point). For those who've always been in the pool, it's that much harder.

Your calculation of US population growth is off. The current population is about double the population of the mid-1950s, not 1920.