From what I understand, many of our kids are now doped up on either ADD/ADHD drugs or antidepressants. Offhand, that seems like it would have a negative effect. Beyond that, I haven't been in the States for 15 years so I am ignorant of any other social changes that could have been causing this.
Incidentally, we are seeing similar things in Japan. Many kids are now doped up on ADD drugs. Nowhere near as many as in the US, but it is a growing thing. I can't imagine drugging kids is a good thing. Thank God I'm not in school these days—I was a hyperactive little kid, I probably would have been doped up too.
I wonder if also the increased academic pressures are causing some issues. My mom has been a kindergarten teacher for 50 years, and she says recently every year standardized tests and academic requirements are getting more strict and more insane. She says kids today are pressured to learn much more than they used to be (and ironically they are learning much less as a result).