If I remember correctly, researchers suspect that there was significantly decreased lead exposure around that time. I personally prefer to think of it as the Flynn Result rather than the Flynn Effect, because the second seems too much like the Flynn Cause. There are a bunch of reasons why measured IQs could increase, and some of them are there sometimes and not other times, and the measured increase fluctuates accordingly.
If I remember correctly, researchers suspect that there was significantly decreased lead exposure around that time. I personally prefer to think of it as the Flynn Result rather than the Flynn Effect, because the second seems too much like the Flynn Cause. There are a bunch of reasons why measured IQs could increase, and some of them are there sometimes and not other times, and the measured increase fluctuates accordingly.