In this month's Science:  Why Peer Discussion Improves Student Performance on In-Class Concept Questions:

When students answer an in-class conceptual question individually using clickers, discuss it with their neighbors, and then revote on the same question, the percentage of correct answers typically increases. This outcome could result from gains in understanding during discussion, or simply from peer influence of knowledgeable students on their neighbors. To distinguish between these alternatives in an undergraduate genetics course, we followed the above exercise with a second, similar (isomorphic) question on the same concept that students answered individually. Our results indicate that peer discussion enhances understanding, even when none of the students in a discussion group originally knows the correct answer.

Chronicle of Higher Education: Learning With "Clickers" Gets Better After Peer Discussions, by Ruth Hammond:

College students who use wireless handheld devices called "clickers" to register answers to instructors' questions during lectures are more likely to give correct responses after discussion with their peers, studies have found. But, researchers wondered, were students improving merely because they copied the answers of fellow students? Or had they actually gained a greater understanding of the material? The findings of a new study published in the latest issue of Science suggest that improvement after peer discussion reflects real learning.

study could be put in place without clickers, students enjoy using the device as long as they're given challenging questions, Ms. Smith says. The device is used in college classrooms across the country, especially in large lecture courses in the hard sciences and mathematics, says Jane E. Caldwell, a biology instructor at West Virginia University who has published a paper in CBE–Life Sciences Education reviewing research on clickers. She says the new paper in Science "made a great stride in pinning down the cause of improvement in performance," showing it was not just the result of "persuasion by bright students that happened to be sitting nearby."

(Hat Tip: Jim Maule, Barbara McFarland.)