We can thank all that tetraethyllead gas that was pumping lead into the air from the 20s to the 70s. Everyone got a nice healthy dose of lead while they were young. Made 'em stupid.
OP’s mom breathed nearly 20 years worth of polluted lead air straight from birth, and OP’s grandmother had been breathing it for 33 years up until OP’s mom was born. Probably not great for early development.