I like finding big correlations, but not this time. The cross-national correlation between mean IQ and the total fertility rate for 178 countries is -.72. That's huge. The world gets dumber with each passing day.
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be." ~ Lord Kelvin