People with O blood type have lower MAO which leads to higher criminality.
People with O blood type have slower thyroid functions which leads to slower cognitive ability.
https://pubmed.ncbi.nlm.nih.gov/6573841/
Reduced platelet MAO activity in healthy male students with blood group O
M Arató, G Bagdy, Z Rihmer, Z Kulcsár
PMID: 6573841 DOI: 10.1111/j.1600-0447.1983.tb06732.x
Full text linksCite
Abstract
The association between the two genetic markers of affective disorders, ABO blood group system and platelet MAO (monoamine oxidase) activity was studied in 70 healthy young males. The platelet MAO activity of subjects with blood type O was significantly lower than that of subjects with blood type A and with blood types A + B AB + B together. This finding could constitute a "bridge" between the two genetic approaches to affective disorders.
https://www.aboessentials.com/blood-type-o-1/
Type O's tend to have low levels of thyroid hormone and often exhibit insufficient levels of iodine, a chemical element whose sole purpose is thyroid hormone regulation. This causes many side effects such as weight gain, fluid retention and fatigue. Dr. D'Adamo does not recommend iodine supplements, rather a diet rich in saltwater fish and kelp to help regulate the thyroid gland. Bladder Wrack is also an excellent nutrient for type O's. This herb, actually a seaweed, is very effective as an aid to weight control for Type O's. 'The fucose in bladder wrack seems to help normalize the sluggish metabolic rate and produce weight loss in Type O's,' says Dr. D'Adamo.