Yes

It is right to assume that one OR the other OR both wrote it is that is the credit. The following is from wikipedia: The usage of the "Lennon/McCartney" writing credits for songs written by either Lennon, McCartney, or both during the Beatles years, was an informal agreement between the two artists and was never set in contractual form. Lennon and McCartney agreed early on in their career that any song either of them wrote would bear the credits "Lennon/McCartney" with the provision that they could change it at any time to make it seem more equal (for McCartney). John Lennon even honored this agreement for his non-Beatles "Plastic Ono Band" single, "Give Peace A Chance," released in 1969. Eventually, the "Lennon/McCartney" moniker was repeated so frequently that it became the de facto standard, and recent attempts to change the arrangement of the names by McCartney for live cover recordings his of Beatles work (that were primarily or solely written by him) to "by Paul McCartney and John Lennon" met with stiff resistance from Yoko Ono, the widow of John Lennon, and from legions of Beatles fans hesitant to break from tradition. http://en.wikipedia.org/wiki/Talk:Yesterday_(song)#Lennon.2FMcCartney_credit_.22contract.22

Yeah 1 of the 2 did. They both wrote songs but in thier contract it had to say they both did.( my botfriend is a beatle freak since 1971 and knows everything)

Paul McCartney wrote it on his own, John Lennon wrote it on his own, or they both wrote it.

yes if both names are listed they both took part in writing it . If say George harrison would of helped also all three names would be listed in that manor .