From Inside Higher Ed today

"Physics students who copy their classmates' work learn less than students who don't plagiarize, researchers at the Massachusetts Institute of Technology found in a study released yesterday."

Actually the study's findings are quite interesting:

Planning ahead makes a difference: "The majority of students, who copied less than 10% of their problems, worked steadily over the three days prior to the deadline, whereas repetitive copiers (those who copied >30% of their submitted problems) exerted little effort early."

Copying appears to have a greater impact on grades than learning: ..."copying homework problems that require an analytic answer correlates with a 2(σ) decline over the semester in relative score for similar problems on exams but does not significantly correlate with the amount of conceptual learning as measured by pretesting and post-testing."

Smart and dumb kids copy: Several measures of initial ability in math or physics correlated with copying weakly or not at all.

And yes you can reduce copying: Changes in course format and instructional practices that previous self-reported academic dishonesty surveys and/or the observed copying patterns suggested would reduce copying have been accompanied by more than a factor of 4 reduction of copying from ∼11% of all electronic problems to less than 3%. As expected (since repetitive copiers have approximately three times the chance of failing), this was accompanied by a reduction in the overall course failure rate.