I was a research librarian at a certain university in Cambridge, MA. It was commonly said amongst the staff that the undergraduates were very smart people -- they went to the best high schools in the country.
It was also commonly said that the grad students working on their masters degrees were pretty dense and difficult to work with. They just didn't seem to be able to think logically or do any problem solving.
And, finally, the PhD candidates -- we didn't want to talk to them, and they didn't want to talk to us. They seemed to have extraordinary trouble functioning in a world which they didn't understand and which seemed too difficult for them to understand.
But, when they had started their trek, so many years before, they had been high-functioning adults.