See this article in Prospect magazine. It seems that the effect - whereby people's average intelligence, as measured by IQ tests, has been rising decade by decade - is now slowing down in many Western countries. In some cases, it's even been going into reverse in the most recent decades.
I think that explanations of the Flynn effect are rather speculative. Explanations of why it is slowing, or in some cases reversing, are equally so. We do know that the effect is mainly from those parts of IQ tests that call on formal abstract reasoning, and we know that the biggest effect is on the middle and lower ranges of IQ scores, not on people with high IQs (at least that seems to be what is being said in things I've read about it ... I don't claim to be any sort of expert in my own right).
So it looks as if modern education, media, and social forms more generally, are doing something to make ability in abstract reasoning more widely available. In The Better Angels of Our Nature, Steven Pinker has some interesting discussion of how old-timers who've been insulated from exposure to scientific thinking and logic just don't "get" what is involved in abstract categorisations, deductive arguments, and the like. But whatever it is that has produced the tendency, it's apparently happening less, or sometimes going backwards.
I have no idea what could or should be done about any of this, but it's a fascinating phenomenon.