Ethnicity and reward structure effects on learning

The greatest benefits on learning for African American students were seen when students in small groups were asked to help each other (communal learning, no reward condition); whereas, the greatest increase in learning for white students (also working in small groups) occurred in response to offers of reward for individual performance. The study met WWC evidence standards:
http://ies.ed.gov/ncee/wwc/publications/quickreviews/QRReport.aspx?QRId=136
The original study citation is:
Hurley, E. A., Allen, B. A., & Boykin, A. W. (2009). Culture and the interaction of student ethnicity with reward structure in group learning. Cognition and Instruction, 27(2), 121–146.