The pronoun they in a sentence like (1) is ambiguous:
(1) At the watchmakers shop, they buzz every morning.
They can refer to concrete people or objects, as when I am thinking for example of Peter and John or I might be having a couple of watches in mind. Another, more interesting, reading is where they picks no specific referents. On this interpretation (1) means something like (2a)
(2) a. At the watchmakers shop, some people buzz every morning
(possibly, but not necessarily by making watches buzz)
b.NOT: At the watchmakers shop, watches buzz every morning
In spite of the fact that the contextually natural interpretation would require an inanimate referent (watches), and nothing in the structure overtly signals the necessity of a human referent, as indicated by (2b) a nonhuman subject is impossible on this non-specific reading. I will show that this phenomenon is very general and stable both cross-linguistically and across various constructions. I'll provide a partial explanation and raise the possibility of a potential connection with core cognitive systems.