Does a Master or Doctoral disseration decide what my specialty is? My cousin says he has a ph.d in?

...Modern Chinese History, but I thought history ph.d and poli sci ph.d.s were just general PhD (ie ph.d. in history) and the dissertation chose) but he says he went to a college to actually get awarded a Ph>D. with the title "Modern Chinese History" on the degree? How does this work? What about specializing in economics of a particular country?