Not lower intelligence, just ignorance. Being deliberately kept ignorant by the ruling class. Learning anything intellectual was deliberately discouraged. The one raised by the ruling class was every bit as "intelligent"-seeming as the people who raised and educated him. It's much the same social-class point that Twain made in The Prince and the Pauper.
To get back to Pratchett, in Guards! Guards! there is some social comment about Vimes's background versus Lady Sybil's, and also Nobby Nobbs. There is more later, whenever Nobby is involved; Sir pTerry appears to have chosen to keep Nobby's family connections ambiguous.
To get back to Pratchett, in Guards! Guards! there is some social comment about Vimes's background versus Lady Sybil's, and also Nobby Nobbs. There is more later, whenever Nobby is involved; Sir pTerry appears to have chosen to keep Nobby's family connections ambiguous.
Conversely, it's possible that a plumber or electrician know very little about Latin or the Romans. That doesn't mean they're unintelligent! 
!! Of course! I was just trying to make sure you all understood the bit I was referring to.