Joseph Epstein, in his recent article on plagiarism, wrote "In 30 years of teaching university students I never encountered a case of plagiarism."
That simply means that he never, ever looked. The most conservative estimate I have ever read noted that ten percent of students polled have admitted to plagiarism at some point in their lives.
I have caught seven plagiarists in the last year alone (only one last semester) and my findings led to a revised, and more strict, plagiarism policy.
In sum, if you require your students to write papers and you aren't finding plagiarism, you simply aren't looking hard enough for it.
Read the entire story here.