Call it the Big Brother of introductory computer science courses--always watching, anticipating students' every move, a little mysterious.

Every year, students in Computer Science 50: "Introduction to Computer Science" (CS50) debate whether the course's instructors really use a special program to weed out cheaters and plagiarists.

But the software is real, instructors say--and it is highly effective in tracking cheating.

"I always have students who say to me, 'Do you really have a software that checks for cheating?'" says Gordon McKay Professor of Computer Science Stuart M. Sheiber, who teaches CS50. "They think we're making this up to put the fear of God in them."

The software, which automatically scans and compares every problem set for similarities, works "astoundingly well," instructors say.

Over the last few years, CS50 has gained a reputation for sending a disproportionate number of students to the Administrative Board to answer charges of academic dishonesty.

"Of the students who have been subject to disciplinary proceedings by the Ad Board, a surprisingly high percentage have come from CS50," Sheiber says. "But it's not because CS50 students are cheating more. It's just that we're more effective in detecting cheating."

Sheiber says there is no evidence that CS50 experiences more incidents of cheating than other large courses. Students may not think twice about sharing problem set answers in large Core classes where up to a dozen different TFs separately evaluate written assignments.