Keener closes the book with an appendix on memory and oral tradition. There's a lot of valuable information in it, and I recommend reading the whole thing. I'll just cite some of the highlights.
He observes that "Most memory studies so far involve contemporary western memories - which are usually considered less rigorous than ancient Mediterranean ones" (332). Here are some of his other comments:
The Gospels were written much sooner after the events they describe than were most ancient biographies, so, to be consistent, those who dismiss the Jesus tradition because they doubt the survivability of oral tradition ought to be equally agnostic or negative regarding most of what we know about ancient history....
Some scholars who appeal to modern Western psychological memory studies emphasize the deficiencies of individual memory. Certainly memory studies rightly lead us to expect eyewitnesses' recollection to be selective. But whereas based on such studies we would expect inaccuracy in as much as "20 percent of the details" of eyewitness reports behind the gospel tradition, such errors would not negate substantial memory. Moreover, even the details "are almost always consistent with the broader picture of what actually happened, even if, strictly speaking, they are errors of detail." In general, memory is more reliable than unreliable.
We typically respect modern memories, despite their tendencies and imperfections; why would we dismiss ancient ones? As N.T. Wright puts it, Jesus impacted people's memories no less than do other significant figures: "just as the friends of C.S. Lewis still bring out books of reminiscences about the great man forty or fifty years after his death, and people who worked with Winston Churchill during the war still dine out on their memories of his temper, his wit and his prodigious intake of alcohol."
Presumably most of us in our fifties or sixties can recall memorable incidents and information that we learned from four decades ago….
Although rote memorization was not the focus of higher education (which began in the mid-teens), advanced education [in the ancient world] continued to build on valued memory skills. I noted already above oratorical students' practice of memorizing model speeches….
These young adult disciples of teachers (typically in their teens) had to learn what their teacher taught and, if they became teachers themselves, were expected to pass it on. This was true whether the sages were philosophers or Jewish teachers of wisdom….
Here [in rabbinic sources] teachers expected disciples to memorize their teachings by laborious repetition. Thus a rabbi might praise a student who, instead of trying to learn on his own, merely preserved his teacher's wisdom, like a good cistern (Sipre Deut. 48.2.6)….
One might dismiss such reports [of good memory skills among ancient Jews] as pure propaganda - though many Gentiles seem to have believed it - but one should be honest about what one is doing. One is explaining away virtually all extant evidence and then complaining that no evidence supports the only position for which we have any substantial evidence - namely, that ancient disciples did normally seek to remember and pass on what they were taught….
the period of purely oral transmission that preceded the first of the Gospels cannot be more than roughly a four-decade generation, i.e., well within living memory….
If we must start with assumptions either way, should not a heavier burden of proof rest on the approach of radical skeptics, rather than with the assumption that Jesus's disciples were like other disciples in antiquity hence sought to transmit their master's teachings reliably?...
Luke attests his audience's knowledge of many written sources as well as oral sources (Luke 1:1-2), and he believed that his detailed account confirmed the stories that at least some of his audience already knew (1:3-4). Given that he wrote at a time when he had access to such information (Luke 1:2-3; Acts 21:8, 17-18), his claim should count against modern speculation, which often rests on dismissing all the evidence we have and arguing the contrary based on the silence that remains….
To assume that Jesus' disciples acted completely unlike other disciples with regard to transmitting their teachers' sayings - despite the comparatively early publication of sources about Jesus - is to value one's skepticism about Jesus more highly than the concrete comparative evidence. If some scholars exhibit a serious canonical bias [bias in favor of the canonical gospels], scholars who treat the gospel traditions as significantly less reliable than analogous traditions from antiquity appear to reflect either an anticanonical bias or a lack of direct knowledge of the analogous ancient sources. (329, 334-5, 338-9, 341-2, 346, 353-4)
(The last part in the series will be linked here when it becomes available.)