Recent studies has shown that exposure to flouride (which is found in toothpaste), is correlated with lower IQ scores. That might be a contributing factor.
[1] https://bmcpublichealth.biomedcentral.com/articles/10.1186/s...
[1] https://bmcpublichealth.biomedcentral.com/articles/10.1186/s...
There is, in other words, an undeniable correlation. The more interesting question is -- are there any safe levels of exposure, and what are the effects over time?
[1] https://link.springer.com/article/10.1007/s12011-008-8204-x
That rumor is false