Somewhere around the mid 90s individuals in most developed countries started to end up with lower IQs than previous generations and there are ever more results corroborating this change, whose magnitude has been rather significant. IQ results are distributed to ensure a population distribution with a median of 100 and a standard deviation of about 15 points. And of course 84% of all people fall within one standard deviation. We're seeing declines that already in the multiple points and they do not seem to be stopping.
One critical point here is that there are two 'convenient' ways to try to dismiss this. And they really amount to the same thing and suffer the same flaw. Those ways are to argue that humans have started to reach some sort of peak intelligence, or that environmental effects were driving former increases in IQ and as we reach a very healthy and wealthy society compared to past times, we're just seeing diminishing returns. The problem is both of those convenient hypothesis would lead us to expect to see an asymptotic decline in growth that would approach zero. We're not seeing that. We're seeing IQ literally decrease, not grow more slowly.
[1] - https://en.wikipedia.org/wiki/Flynn_effect#Possible_end_of_p...