Thank you, Donuel,

I understand that you didn't mean to point a finger at anyone, and that any field has its scoundrels.

Let me add some reasons (not all distinct from yours) why I think it's unlikely that a given dissertation is plagiarized.

First, reasons of prudence:

In my field, English, at any rate, doctoral students who teach part-time spend too much of their time detecting plagiarism and deciding how to deal with it to feel it is worth the candle: Plagiarized portions of a paper written by a novice are obvious to an expert, and this can be expected to hold at the graduate--professorial level as well as at the undergraduate--doctoral student one.

The writers of dissertations are expected to master--and cite--all the sources that provide the background for their original work. They can expect their dissertation committee to have an even broader and deeper knowledge of the relevant literature. Consequently the probability of discovering a source which is simultaneously a significant, original contribution to the field, but is unknown to one's committee is not large enough to risk.

A student writing a dissertation has already put in from three to umpteen years pursuing a narrow specialty. The benefits of successful plagiarism--if any--are slight, and the possible penalties--loss of years of work, the end of job prospects, banishment from academia--are enormous.

As you point out, the dissertation is eternally preserved at Ann Arbor, so for even the successful plagiarists, the possibility of detection and disgrace remains throughout their careers--most likely at the breakthrough moment when a hiring committee considers a candidate seriously enough to take a look at their publications.

Psychological reasons:

As I suggested above, graduate assistants and ABD teachers are occupationally conditioned to detest plagiarism and other forms of academic cheating. Plagiarism is to academia as perjury is to law: Not only a serious offense, but one that undermines the foundations of the entire institution. For a would-be career academic to base their career on a forgery would involve considerable cognitive and emotional dissonance.

Finally, dissertation authors generally choose their topics: They write about what interests and motivates them. After laboring in the vineyards for years, they have an unprecedented opportunity for them to express THEIR ideas and feelings about what is truly important in the field that they hope will be their life's work. To surrender this opportunity to a covert source--without even getting the credit for discovering it--is something I believe would stick in the craw of most academic egos.

I agree that much plagiarism--probably most--at the undergraduate level is unintentional, and it's on us teachers to do a better job of explaining the rules and conventions--which vary from community to community.

And I also realize that my arguments cannot refute credible testimony of numerous instances of plagiarism. However, a huge number of dissertations are written every year, and I feel I have provided grounds for believing the vast majority are genuine original work.