B. F. Skinner has shown that people and animals can be "shaped" into just about any behaviour pattern by rewards or "reinforcements". The "law of effect" is that people and animals will unconsciously do more often whatever tends to be followed by a positive effect.

Essay and exam marking is an inexact science. The marker cannot check every passage of an essay against every source the student has read to make sure it is not a copy. And the sources usually write better than the student. So essays and exams that contain word-for-word passages from other authors invariably get higher marks than those that do not. Students are hence shaped into doing this because the marks are rewarding it.

There are warnings that verbatim use of the words of others is plagiarism, and constitutes cheating, and will be punished. But these are ineffectual against the "shaping power" of the direct rewards of doing it.

It has not been possible to give direct negative feedback to help shape students' behaviour away from plagiarism because the marker, too, is being unconsciously shaped by the integration of the better prose of other authors into student essays; apart from sloppy transitions and obvious contrasts between two different styles, there are no clues in the kind of heavily plagiarised but well-integrated essay that success in this strategy shapes students into mastering.

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The result is cheating of two kinds: one of which the student is only dimly conscious, if at all, in the form of the well-practised art of plagiarism itself; and another form of which students are even less conscious, namely, that a focus on parroting the words of others cheats them of a mastery of what those words are about.

But developments in the digitisation of texts now make it possible to remedy these problems. It is possible to give students and markers feedback about the degree of word-for-word overlap between essays/exams and source texts, using computerised P-detectors.

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Markers can at last tell the difference between a well-written essay that is the student's own and one that is a cut-and-paste job. And then they can administer the mark-rewards accordingly, "shaping" students into writing for themselves and mastering the content of the sources.

But this is all very new. The effects of this feedback must be allowed to percolate into student culture and practice. To use this detector to make a quick example would be unjust.

Not only should fair warning of the new computerised capacity to detect plagiarism be given prominently to all students, but it should first be applied to the marks "online". Marks should gradually be "weighted" by the "P" factor, which is the degree of text overlap detected by the computer, lowering them in proportion to the degree the texts are plagiarised. This will be the most effective way of "shaping" students out of plagiarism and into their own words and thoughts.

Prosecution should come later, by which time, one hopes, it will be unnecessary.

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Stevan Harnad Professor of cognitive science University of Southampton Teaching, pages -30