An environmental cause appears likely, since this trend is being observed in many countries. Both the increase in IQ to 1975 and the drop thereafter could have environmental causes.
The increase could be due to improved nutrition following WW 2, such as better access to food overall and the iodization of salt.
For the decline, my money is on PFAS (https://www.sixclasses.org/videos/PFAS) and organohalogens more generally. Iodine is also a halogen, and all the other halogenated compounds we are pumping in the environment could interfere with iodine metabolism. These compounds are in nearly everything, and we're using ever larger quantities of them.
This seems like a pretty good hypothesis. I believe the main source of exposure to PFAS for most people is food packaging (IIRC nonstick pans don't move the needle unless you heat way above the safe range, and you'll smell the coating melting). What is the main source of exposure to organohalogens? I'm seeing information about mattresses, flame retardants, and seafood.
PFAS is in so many common consumer products you might well say "it is in everything." That outdoor jacket you're wearing? Coated in PFAS. Your stain-resistant couch? PFAS.
All textiles break and release fibres, and we inevitably end up eating them.
And if you are cooking with a non-stick pan, it is a guarantee that you are ingesting them. It doesn't have to be the PTFE itself, the emulsifiers (such as PFOA https://en.wikipedia.org/wiki/Perfluorooctanoic_acid) are more volatile and have been measured in food cooked with non-stick pans.
> There is general agreement that dietary intake is the largest source of PFAS exposure rather than inhalation or dermal contact [...] For PFOA, EFSA suggested the most important sources of chronic exposure were milk and dairy products for toddlers (up to 86% of exposure), drinking water (up to 60% for infants), and fish and other seafood (up to 56% in elderly).
> There really is a very minimal amount of residual PFOA or other perfluorinated chemicals in the nonstick pans — like, you know, thousands of folds lower than what is observed in the water or food. -- Dr. Mimi Huang, NIH
Wouldn't this be balanced out by the reduced lead in the environment? There is a fairly strong argument that removing lead has lead to the decrease in crime (due to less development damage).
I too believe it has to have environmental causes. I saw a phenomenal documentary a few years ago that went into great detail of how especially pesticides and flame retardants are the most likely cause.