NT Studies is still working to "alter the default setting" of our views on first century literacy & orality, but Michael Bird's (truly, wonderful) post on Thursday still spent most of its energy lifting up examples of Greek and Roman literary practices. That's not bad or good, that's just what scholars at this point expect to see in an argument. Okay. But there's more to this puzzle. The larger goal here isn't just literacy at large, but the particular literacy of Jesus' earliest followers. Those male & female disciples, of course, were neither Roman nor Greek.
Jewish literacy, at large, was generally far more communal than literacy among other ancient cultures. King Philip hired Aristotle to teach his own Prince and the sons of the nobles, but any "notebooks" kept by that cohort aren't going to be read aloud to the Macedonian townspeople each Saturday. Likewise, Roman "literary elites" absolutely kept various "notebooks" for their own personal use, but that remains vastly different from the typical Hebrew experience.
It's common historical knowledge that Synagogues all over the Mediterranean were places where *literacy* was enjoyed aurally, weekly, by the entire community. In Judaism, the literate few served the literary needs of their illiterate brethren. It was not literary elites among the Jews who became known as "The People of the Book". It was everyone. This should clearly be part of scholars' new "default setting".
As NT studies continue embracing a Jesus and Paul who are more fully Jewish, I dare say it's time for "First Century Literacy" to regain more of its Jewish context as well.
Finally, in the even-more-particular case of Jesus and his disciples, I think the last key point is Moses.
Consider Jesus-as-Moses from the point of view of the twelve, or the 120:
(1) Since Exodus was a very strong part of the literary background for Jesus' earliest followers, and (2) because the twelve and the 120 all believed strongly that Moses' story had been written down for the sake of all God's people everywhere, and (3) if Jesus' disciples began to came to see him as the new Moses, leading the new Exodus, more and more as the final Passover approached... THEN all of this should have quite naturally urged several or perhaps most of the 120 to begin asking themselves, and each other, "When is one of us going to start writing some of this stuff down?"
Admittedly, that last part is my own speculation, but it's based in the broader facts, which are not.
Hebrew Literacy was an aural experience, one as ubiquitous as the Torah at the Synagogue and - more - as the Shema on their doorposts. Even to the most illiterate Jews, Literacy was a precious community institution valued as highly as any other. The Passover Night cannot even have an oral haggadah without Hebrew literacy, and no individual Jew had a single memory of Passover that failed to reflect some awareness of their religious literature's very enormous importance.
And now a word on ancient literacy statistics:
The significant stat is not that 5% or 10% of the Ancient world might have been literate. The significant statistic is that 1 out of every 1 Synagogue Jews had a special place in their heart for the value of literacy. The micro-community of Jesus' 120 disciples only required *one* notebook writer to pick up a stylus and parchment (or papyrus, or less likely a codex), but our question is not about not the six to twelve individuals who likely had the capacity to start keeping a "notebook" of Jesus' activities and engagements. The key question is about the 108 to 114 whose very presence, let alone their insistence, would have caused at least *one* of Jesus' disciples to start putting things down for posterity.
Even if the Twelve waited until Jesus' final year of Jesus' ministry to start making some notes... well, that would explain a lot, actually.
At any rate, Jewish Literacy was Communal. And to sum up the Mathematical side point, the odds that one disciple [of twelve] started writing are mathematically much, much stronger than one chance in twelve.