1. Harris, C. S., Science, 140, 812 (1963).
2. Hamilton, C. R., Amer. J. Psychol., 77, 457 (1964).
3. Pick, jun., H., Hay, J. C., and Pabst, J. (unpublished results).
4. Efstathiou, A., and Held, R. (unpublished results).
5. Craske, B. (in preparation).