Lots of possible environmental causes 1. lack of very early childhood education/stimulation / abstract thought messing with brain development 2. epigenetic

Lots of possible environmental causes 1. lack of very early childhood education/stimulation / abstract thought messing with brain development 2. epigenetic influence from parents' hard lifestyle, if you believe it is effectually hereditary in that manner (I don't think it's likely) (this is environmental in the sense it would only persist for a limited # generations) 3. parasite load 4. trauma from infectious diseases 5. severe nutritional deficiency (iodine, creatine, antioxidants, nutrients, whatever else we don't know about you can miss out on) I can't prove what I think you're implying by #1 isn't the case, but I'm quite sure that 1 and 3-5 from my list do apply.