If you saw the 48 Hours special on the Nativity this past week, you may recall Prof. Crossan's difficulty harmonizing the Gospel stories. For example, he says
The interesting thing is, of the four gospels, Mark and John of course have no nativity story. Only Matthew and Luke. They agree that Mary and Joseph are the parents. They agree about a virgin birth. They agree about a birth in Bethlehem. But pretty much apart from that, the stories go completely their own way.
Considering how oral traditions work, it's fascinating to me that the Gospels of Matthew and Luke agree on this much. It's no surprise at all that they cover different perspectives on these events, and, perhaps more notably, different (thought not incompatible) events. What is striking to the casual reader, however, is the near-total difference regarding the lineage of St. Joseph, going back to King David. This is the kind of difference that impressed science fiction writer Isaac Asimov, when he wrote his Guide to the Bible. Should it impress us?
When we compare their versions of the generations prior to Joseph, we find little agreement until we get to King David. From there, the lists agree with Old Testament lineages (with the exception of a possible copyists error in Luke). Both accounts name Matthan (or Matthat, in Luke's case) as Joseph's grandfather, but that could, under the circumstances, be a coincidence. The lists also agree at Zerubbabel and Shealtiel, but Matthew takes these names as immediately following the Assyrian exile, where, in 1 Chronicles 3, Shealtiel is said to be descended from Jeconiah, and we see that Matthew's list from this point is biblically based. Matthew takes Joseph through the line of Davidic kings that ruled over the kingdom of Judah. By contrast, none of Luke's names past Shealtiel seem to correspond to names in the Old Testament. Matthew finds Joseph descended from David through Solomon, while Luke finds Joseph descended through David's son Nathan, but this seems to be a mistake. It is far more likely that Solomon, who had David's other son Adonijah killed, left no rivals standing, and coincidently perhaps, Nathan was David's seer, not his son.
Luke's account gives a lineage from David to Joseph of 42 generations, while Matthew gives that lineage as only 27 generations. It is not at all clear that Luke's Zerubbabel and Shealtiel are the same as Matthew's and the Old Testament figures.
Both lineages cover the same thousand years. For Luke's lineage this gives an average inter-generational gap of about 24 years, whereas the Matthaean gap is about 38 years. Obviously, a gap of 24 years is much easier to defend than one of 38. Why the Matthaean gap? St. Matthew gives the following observation that helps us understand his account...
Thus the total number of generations
from Abraham to David
is fourteen generations;
from David to the Babylonian exile,
fourteen generations;
from the Babylonian exile to the Christ,
fourteen generations.
Matthew noticed the 14 generation gaps between Abraham and David and between David and Jeconiah. For some reason never explained, he though to cover a largely blank historical record gap with an additional 14 generations. It seems likely Matthew was trying to draw a picture of God's perfect providence, and he decided to express it using numerical relationships. According to the Jerome Biblical Commentary, the number 14 arrises in a calculation of numbers derived from David's name in Hebrew.
Though the Lukan list seems more credible arithmetically, there is evidently no independent corroboration of his genealogy. A thousand years is a long time for a family to maintain lineage records, and the devastation wrought on civil records (if there ever were any) by the invasions of Palestine in intervening centuries would seem to make such records unlikely to survive in any case. Certainly, the Babylonian captivity would have destroyed almost all family records. The biblical histories in Kings and Chronicles end just after this catastrophic event, even though these histories were probably written well after the fact. Again, according to the Jerome Commentary, it may be that Luke had a different theological purpose in his Davidic list, and the number of generations between Joseph and David was chosen to make the total number of generations from Adam to Joseph equal 77.
Thus, we must conclude that Luke's genealogy from David to Joseph is almost certainly wrong, while Matthew's is at best likely to be correct only insofar as it follows Solomon's line to Zerubbabel, following the return from the Babylonian exile.