Why has society, (particularly americans), became "dumbed down" or lost their urge for independent thought?

It seems to me that we never question things that we are told. we tend to believe everything told to us by our government. when some studies suggest different why do we hush those who publish them and strike out in anger at those who suggest we look at them? For instance, we used to think getting under our school desks would save us from the atomic bomb. Our government tells us many things and work us into a frenzy and its all about money for them, mostly. Two examples are second hand smoke, and the ozone. Remember the K2000 panic? What ever happened to common sense and independent thinking?