My own intuition has been that plagiarism is often due to oversight or panic. A student will fall behind and, with a deadline looming, get sloppy in the writing of a paper: a few sentences or even a paragraph makes its way into the student paper without attribution. In the rush to finish the student forgets about it, or decides it doesn't matter.
Thomas Dee and Brian Jacob had a different idea.
Some data (e.g., Power, 2009) indicate that even college students are not very knowledgeable about what constitutes plagiarism and how to avoid it, and so many instances of plagiarism may actually be accidental.
Given the stiff penalties, why don't students bone up on the rules? Dee & Jacob point out that this may be an instance of rational ignorance. That is, it's logical for students not to try to obtain better information about plagiarism; the cost of learning this information is relatively high because the rules seem complex, and the payoff seems small because the odds of punishment for plagiarism are low.
Dee and Jacob's idea: reduce plagiarism by reducing the cost of learning about what constitutes plagiarism.
Their experiment included 1,256 papers written by 573 students in a total of 28 humanities and social-science courses during a semester a selective liberal arts college. Half of the students were required to complete a "short but detailed interactive tutorial on understanding and avoiding plagiarism."
The student papers were analyzed with plagiarism detection software. In the control group, plagiarism was observed in 3.3 percent of papers. (Almost every instance was a matter of using sentences without attribution.) Students who had completed the tutorial had a plagiarism rate of about 1.3%
Thus, a relatively simple and quite inexpensive intervention may be highly effective in reducing at least one variety of plagiarism. Replicating this finding in other types of coursework--science and mathematics--would be important, as would replication at other institutions, including less selective colleges, and high schools. Even with those limitations, this is a promising start.
This paper was just published as:
Dee, T. S. & Jacob, B. A. (2012) Rational ignorance in education: A field experiment in student plagiarism. Journal of Human Resources, 47, 397-434.
(I've linked to the NBER publication above because it's freely downloadable.)
Power, L. G. (2009). University Students’ Perceptions of Plagiarism. Journal of Higher Education, 80, 643-662.