Take for example Professor Bartlett's experiments which were created not as laboratory experiments, but as real life tests of human memory. He asked subjects to read a story three times. Then at varying intervals he asked his subjects to write the story down. He noted specific alterations (what we call today memory distortions) to the materials. These alterations are specific and consistent. I found the same ones, and I did not have access to his experiments until after I had run my own and was writing up my analyses. This was another shock to my system.
What is fascinating is that originally I wanted to track the differences in media environments. So I had divided my subjects into five groups: oral-to-oral (OO=heard saying and orated reproduction); oral-to-written (OW=heard saying and wrote down reproduction); written-to-oral (WO=read saying and orated reproduction); written-to-written (WW=read saying and wrote down reproduction); and sources-retained (SR=read saying and wrote down reproduction but retained written "original"). In other words, I had four media environments in which composition and recall relied entirely on human memory, while one media environment allowed my subjects to consult a written source. I was confident that I would discover all sorts of differences in the first four mixed media reproductions and was ready to track them.
Much to my surprise, the differences in these four media environments were not present at all in long-term memory reproductions. In other words, in the four the media modes that relied on human memory for transmission, there was no difference in how the material looked after it was transmitted. The material underwent the same type of changes at the same rates. The differences occurred only when the written source was retained and the subjects could consult it.
In other words, from the reproductions themselves made by my subjects it was impossible to deduce whether the subject heard the saying or read the saying, wrote it down or orated it, as long as this was done from memory and not from consultation of a written source. The factor for distortion in transmission was NOT the media environment - it was dependence on human memory.
So how can we tell if the author of Thomas' parable of the Wheat and Tares was dependent on Matthew? If we are dealing with literary dependence through consultation of Matthew's text, we would expect either near verbatim reproduction or paraphrase according to the results of my experimental exercises - and we have neither of these. This is too complicated for me to go into here, so please have a read of my article and the results which I charted.
If the author of Thomas' parable was remembering Matthew's version, then the only way to know that it is Matthew's version and not some other is to detect a significant amount of Matthean development of the parable in Thomas' version. It becomes difficult at this point to detect exactly what is Matthew's development, since the concept of ONE originating parable cannot exist in an oral-rhetorical culture.
I REALLY caution all of us on this point, because I discovered in another experimental exercise that I didn't publish (yet?) that when you have twenty-five versions of a parable in front of you that all look similar, if you ask how these twenty-five versions came about, you are tempted to try to build a family-tree based on similarities in some versions. But what I'm finding is that people make the same adjustments to versions INDEPENDENTLY of each other, and these adjustments are due to the way in which our memories work, and cultural and linguistic phenomena within a given generation of people. So the twenty-five versions may be all independent versions with no relationship to each other except that all persons were present to hear a version orated (and in fact were in the case of my experimental exercise). I admit being quite shocked about this, and seeing form criticism fail miserably before my very eyes.
But if we examine the Thomasine parable carefully we see that we do not have significant commonalities with Matthean secondary developments (="rationalization" or "idiosyncrisies" as Bartlett would have phrased it). The verses that appear to me to reflect Matthew's theological interests - the dialogue of the enemy in vvs. 27-28 and the accumulated proverb in v. 30 - are not found in Thomas' version. But Thomas' version represents a condensed form of the parable, although it is impossible to conclude that this originating form is a memory of Matthew's version or some other version available to the person who composed Thomas' version (which still, based on studies of oral composition, has the characteristics of an orally-composed text).