Almost all of the empirical research on cooperative learning has been done with heterogeneous teams. There is strong research support that shows mixed teams outperform students working independently. Random and homogeneous do not have the same empirical base.

Same ability teams result in teams with the four lowest achieving students on same team. Random teams can create the same low ability teams completely by chance. In contrast, carefully assigned teams structure for success by teaming low students with high students, increasing the probability of successful tutoring.

A random team can result in the four biggest management/behavior problem students all on the same team. Heterogeneous teams usually avoid these pitfalls by placing a high achiever on each team. High achievers are generally (not always) less prone to discipine problems.

Heterogeneous teams help with classroom management. By having the top quarter of the class spread out, one per team, someone on each team is likely to be able to explain directions and keep the team on task.

Homogeneous teams creates winner and loser teams. I did a research study years ago on a classic cooperative learning method called TGT. TGT has a bumping system. Each week students leave their learning teams and go to tournament tables, three per table. If they win, they bring 6 points back to their team, least points gets 2 points to bring back, the other student 4. The bumping system equalizes competition because the losers go to a lower achieving table and the winners move up. Over time students all bring back the same amount of points. The surprise was that lower achieving students dropped in self-esteem. Even though everyone was bringing back the same amount of points, the low achievers knew they were going to the loser tables. It is the same with homogeneous teams. The low achieving students feel like losers. Same thing results when we have high, medium, and low reading groups. We create status and esteem problems.

Heterogeneous teams maximize the opportunity to learn different thinking skills. The more there is diversity, the more we can learn from each other. By explaining to a student who is thinking differently about a problem or issue, a student is challenged to stretch or cement his/her own learning.

Peer tutoring is often most effective when we have a more capable tutor. The more capable tutor can help the less capable partner or teammates. The great thing about peer tutoring is that the tutor often gains at least as much as the tutee. Teaching the content cements it in the minds of the tutors.

Heterogeneous teams improve race relations. When we have students of different races form strong bonds with their teammates through teambuilding and shared goals, we have gone a long way to eradicate racism. Students learn to see classmates of other races as allies, not enemies. They get to know each other for the content of their character rather than the color of their skin.

Heterogeneous teams improve everyone's social skills, especially the social skills of high achievers. By definition, high achievers have no problem with academic content. Their greatest area for growth is often in interpersonal relationships. In mixed teams, high achievers learn to coach, encourage, praise, tutor, and they learn patient waiting. Our high achievers learn to be leaders by practicing their leadership skills in their teams.

In the decades that I've been working with schools and districts, I've been advocating heterogeneous teams. We've had countless success stories of schools making dramatic gains using heterogeneous cooperative learning teams.