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Luke has interspersed with an account of the nativity of John the Baptist (no doubt obtained from the rival sect of John) a parallel nativity of Jesus built on John's model. Not that Luke himself was the one who composed it; it, too, was most likely pre-Lukan material. [...] Though Luke used prior sources, probably in Aramaic, for the nativities of John and Jesus, it appears he himself contributed bits of connective text to bring the two parallel stories into a particular relationship so that John should be subordinated to Jesus, whom Luke makes Jesus' elder cousin. This original, redactional material is Luke 1:36, 39-45, 56. It consists of a visit of Mary to her cousin Elizabeth, whereupon the fetus John, already in possession of clairvoyant gifts, leaps in the womb to acknowledge the greater glory of the messianic zygote. All this is blatantly legendary, or there is no such thing as a legend. Luke probably got the idea from Gen. 25:22, where according to the Greek translation of the Old Testament, the Septuagint, Rebecca is painfully pregnant with twins. [...] In this way Luke tries to harmonize the competing traditions of Jesus and John, whose cousinhood is no doubt his own invention.
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Robert M. Price (The Incredible Shrinking Son of Man: How Reliable is the Gospel Tradition?)