1. Ainsworth, M.D.S., Blehar, M.C., Waters, E. & Wall, S. (1978). Patterns of attachment: A psychological study of the strange situation. Hillsdale, NJ: Erlbaum.
2. Berlyne, D.E. (1960). Conflict, arousal and curiosity. New York: McGraw Hill.
3. African American Fathers in Low Income, Urban Families: Development, Behavior, and Home Environment of Their Three-Year-Old Children
4. Ellis, M. (1979). Functional complexity and activity levels. In B. Sutton Smith (Ed.), Play and learning (pp. 157-173). New York: Gardner.