When I was an undergraduate my physical chemistry professor assigned weekly problem sets to be completed outside of class. We were encouraged to work together in small groups, but each student had to submit their own work-and, of course, be responsible for knowing the material for exams. Self-selected study groups spontaneously formed within the first week of classes. Having been on study abroad the previous year, I found myself in a group with a good friend who had also been on study abroad and another friend who I knew would be a strong contributor. We met faithfully one evening a week in a study room in the chemistry department to work through problems together after we had already worked on them on our own.After several weeks a fourth person whom I did not know showed up at our study group. My first reaction was, "Hey, what is he doing here? Our group doesn't need anyone else." Against my minor (privately held) objections he started working regularly with us and I found it reassuring that he had many of the same questions that I did. But it was the differences in his approach to answering these questions that I found most useful. He ended up being a great member of our team. My first impression of this person whom I never had worked with or spoken with before, totally changed when he was given a chance to contribute. From him, and the other members of my group, I learned a tremendous amount of chemistry, and a tremendous amount about learning.But what if the fourth person had not been welcomed to the group? What if we had excluded him because he wasn't in our social circle, he wasn't of our social class, he wasn't from our country, or he wasn't of our race? Stratification of classrooms, laboratories, study groups, and so forth can occur along many lines-socioeconomic, gender, race, sexual orientation/identity, religion, cultural background, attitudes, and even by athletic team [1][2][3][4]. Social psychology research indicates that this stratification is part of the human experience and is typically implicit [1,5] but can hinder student performance and persistence [6][7][8][9][10].One challenge in any classroom is to be cognizant of the potential stratification and marginalization of students, most insidiously by other students. In lecture-based classes, it might not be apparent to the instructor that marginalization is occurring, but when students leave the classroom to work on assignments-when professors can't see-stratification and marginalization happens whether faculty acknowledge it or not [11,12]. In a lecture course, the instructor has little recourse other than to let students be responsible for taking ownership of their learning and getting involved, as appropriate, with other students. But what if they don't, or are excluded based on who they are or the color of their skin?A student-centered classroom gives instructors a unique opportunity to create learning communities in the classroom and then to observe and even influence complex social dynamics [6-8, 10, 13, 14]. Active, cooperative learning i...