1. American Psychiatric Association. (2000). Diagnostic and statistical manual of mental disorders: DSM-IV-TR (text revision). Washington, DC: Author.
2. Belsky, J., & Most, R. K. (1981). From exploration to play: A cross-sectional study of infant free play behavior. Developmental Psychology, 17, 630–639.
3. Brownell, C. A., & Carriger, M. S. (1990). Changes in cooperation and self-other differentiation during the second year. Child Development, 61, 1164–1174.
4. Brownell, C. A., & Kopp, C. B. (2007). Transitions in toddler socioemotional development: Behavior, understanding, and relationships. In C. A. Brownell & C. B. Kopp (Eds.), Socioemotional development in the toddler years: Transitions and transformations (pp. 1–40). New York: Guilford Press.
5. Bryk, A., & Raudenbush, S. W. (1992). Hierarchical linear models for social and behavioral research: Applications and data analysis methods. Newbury Park: Sage.