To add onto this it seems there’s a causation of low iq with obesity but not vice versa.

I also just had this thought about declining levels of fertility in humans. If it’s being caused by external factors like plastic or any pollution we’d expect to see similar effects in all other mammals especially ones close to human populations.

I wasn’t able to find any studies backing this up with just a cursory search, but it is likely the easiest way to add or detract from the plastic hypothesis.

I do know that not all but most studies comparing fertility between decades found that when controlling for obesity the differences were not statistically significant between time periods.