Malcolm Gladwell writes about the Flynn effect, which is the tendency for IQ, whatever exactly IQ is and however exactly you measure it, to rise from generation to generation, suggesting that IQ is an environmental effect rather than a genetic one.  Flynn picture on the right from here (on the right - scroll down a bit).

Start of Gladwell’s article:

One Saturday in November of 1984, James Flynn, a social scientist at the University of Otago, in New Zealand, received a large package in the mail. It was from a colleague in Utrecht, and it contained the results of I.Q. tests given to two generations of Dutch eighteen-year-olds. When Flynn looked through the data, he found something puzzling. The Dutch eighteen-year-olds from the nineteen-eighties scored better than those who took the same tests in the nineteen-fifties - and not just slightly better, much better.

Curious, Flynn sent out some letters. He collected intelligence-test results from Europe, from North America, from Asia, and from the developing world, until he had data for almost thirty countries. In every case, the story was pretty much the same. I.Q.s around the world appeared to be rising by 0.3 points per year, or three points per decade, for as far back as the tests had been administered. For some reason, human beings seemed to be getting smarter.

Flynn has been writing about the implications of his findings - now known as the Flynn effect - for almost twenty-five years. ...

End of Gladwell’s article:

“The mind is much more like a muscle than we’ve ever realized,” Flynn said. “It needs to get cognitive exercise. It’s not some piece of clay on which you put an indelible mark.” The lesson to be drawn from black and white differences was the same as the lesson from the Netherlands years ago: I.Q. measures not just the quality of a person’s mind but the quality of the world that person lives in.

I recommend reading what’s in between as well.

Questions: Will this effect eventually stop?  Are there any cases of IQs going down over time, from one generation to the next?  Because surely an environment can get stupider, can it not?  Might that be what would be meant by the decline of a civilisation?

If so, good to able at least to believe rationally that our civilisation is still on the up and up, as I definitely believe it is.