3Omoto, A.M., & Snyder, M. Sustained helping without obligation, motivation, longevity of service, and perceived attitude change among AIDS vohmteers [J]. Journal of Personality and Social Psychology, 1995:68, 671-686.
4Clary, E. G., Snyder, M., Ridge, R. D., Copeland, J., Stukas, A. A., Haugen,J., et al. Understanding and assessing the motivations of volunteers:A functional approach [J]. Journal of Personality and Social Psychology, 1998:74, 1516-1530.