The Birth of Messiah. In the second appendix (pg 505ff), he considers the issue of the Davidic descent of Jesus.
Brown notes that the majority of scholars think that the claim that Jesus is descended from the house of David is historically reliable. But there is also many who argue that the claim is a theologoumenon (a historicized theological assertion). The argument goes: "the Christian community believed that Jesus had fulfilled Israel's hopes; prominent among those hopes was the expectation of a Messiah, and so the traditional title 'Messiah' was given to Jesus; but in Jewish thought, the Messiah was pictured as having Davidic descent; consequently Jesus was described as 'Son of David'; and eventually a Davidic genealogy was fashioned for him" (pg 505). Those who argue this point to the example of Zadok the high priest, who rose to power but (it would seem) had to invent the pedigree in 1 Chronicles 6:1-8 that gave him the authority to exercise that priesthood.
Brown is not convinced. Like the majority of scholars, he finds the idea that Jesus' Davidic claim is historical to be more believable that the alternative. Some of the support he offers is intriguing.
1. Relatives of Jesus were known in the primitive Church. If the family was not Davidic, why would anyone have given credit to the claim? and why would they have gone along with it?
2. Why did Jesus' enemies never raise a protest to this Davidic claim if it was historically questionable? Brown explains: "One would expect to find traces of a polemic, especially on the part of the Pharisees, denying Jesus' Davidic status as falsified. But, while there are Jewish attacks on Jewish legitimacy, there is no polemic against his Davidic descent as such" (pg 507).
That brings to mind another aspect that Brown does not mention, which is that while we find frequent attack upon Jesus' origin with claims that Mary was raped or had an affair, no one seems to claim that Mary and Joseph just had premarital sex and weren't careful about pregnancy before the marriage was (at least ceremonially) consummated.
That Joseph was Jesus' natural father would seem to be the natural argument to make, for someone arguing against any supernatural element in his conception. The fact that no one was making that argument is because the whole purpose of the attack was to undermine Jesus' family claim to be the Messiah (anointed) Son of David. If Joseph was the natural father, the Davidic bloodline is sure and his Messianic claim solidified; the attack would be undermined.
3. Brown also points out that it has been claimed that St. Simeon of Jerusalem, son of Clopas and cousin of the Lord Jesus, was martyred more because he was a Davidid, not so much because he was a Christian. Brown notes that Rome was concerned local uprisings and power grabs, and thus about Davidic claims. "Hegesippus is cited to the effect that, after the capture of Jerusalem in 70, Vespasian issued an order that the descendants of David should be ferreted out, so that no member of the royal house should be left among the Jews" (pg 508).
4. Another interesting detail I had overlooked is the parallel expectation among the Jews of Qumran was of a kind of parallel priestly messiah from the tribe of Levi along with the political kingly messiah from the tribe of Judah, which is kind of what we got with John the Baptist and Jesus.
5. Lastly, Brown notes that evidence for the Davidid claim is early, first showing up in Romans 1:3 (written c. 58) and Paul here is simply quoting an older creedal formula. Would Paul have done so if there was any question about the Davidid claim? Brown explains: "To a man with Paul's training as a Pharisee, the Davidic ancestry of the Messiah would be a question of paramount importance, especially in the period before his conversion when he was seeking arguments to refute the followers of Jesus. Paul, who twice insists on his own Benjaminite descent (Rom 11:1; Philip 3:5), would scarcely have been disinterested in the Davidic descent of Jesus" (pg 508).
Brown sums up his appendix on the issue by stating, "The New Testament evidence that Jesus really was a Davidid outweighs, in my opinion, doubts to the contrary" (pg 510).