Lead poisoning from car exhaust. Boomers and gen X inhaled A LOT of lead fumes from cars. The generation before boomers was higher intelligence because of less exposure to lead. Gen X is less intelligent than boomers because cars were even more popular and they had lead exposure from birth. Mill...

Also, 50 years ago people didn't drink battery acid on purpose to sue the manufacturer for not having a warning label. More than just intelligence at play.