Only for men, though. And in the case of Sweden, specifically, IQ and fertility were positively correlated for men who were born in the postwar period.
The last time I saw an article of this nature, a commenter pointed out that conscripts may be incentived to score lower on these tests. Isn't it true that many people simply don't care to score highly on them?
This hypothesis seems reasonable to me, and yet I don't see any mention of it in the 2018 study [0] although they do address concerns about the conscript data in other respects.
In previous decades the group of people taking IQ tests might have belonged to a resource rich group.
Now that IQ tests are more commonplace, we might finally be getting more signal from the below median side of the bell curve.
The article didn't mention if these biases were accounted for, and it is a critical detail to leave out.