I don't think the perceived "dumbing-down" of humanity is genetic or biologically-based at all. Instead, I blame the media complex and (sadly) the Internet. The media complex is entrenched in its own self-righteous moral superiority, and people tend to believe what they see on television. That means that people are less likely to do research, and less likely to become informed by their own merits. Instead, they will follow a trend or fad because they think that everyone else is doing it, whether it is true or not.
The Internet exploded into being in the mid-nineties and hasn't slowed since. Humanity went from having to research authoritative opinions in libraries to being able to Google anything and having millions of pages of information, 99% of which is pablum. Anyone can say anything about anything on the Internet, and so have an instant audience. The problem is that Humanity is starting to have trouble differentiating between Internet quackery and deliberate misinformation, and facts based on real research and experimentation. The barrier between science and pseudoscience is slowly eroding, and that leads to a misinformed public and the continuation of incorrect information and lies.
Looking at it from a broader perspective, it's almost as if there is a pendulum swinging from "enlightened" to "deliberate ignorance," as if our two-hundred years of technological evolution has pushed Humanity too far, and we are unconsciously reacting by failing to continue our intellectual progress.