• Conceptions of psychological development devised by Western theorists have assumed connectivity of stages, stability of acquired structures, and continuity of process, and have resisted the possibility that some emerging psychological qualities might have relatively short histories. Empirical studies of the first two years of life suggest that the enhancement of retrieval memory at 8 to 10 months of age, the emergence of a linguistic set at 15 to 18 months of age, and sensitivity to standards of competence prior to the second birthday are emergent human competences that may not necessarily have a historical link to the events of early infancy. Developmental theorists should be as receptive to sequences that are relatively discontinuous as they are to those that are gradual and cumulative.
Kagan J. The Form of Early DevelopmentContinuity and Discontinuity in Emergent Competences. Arch Gen Psychiatry. 1979;36(10):1047–1054. doi:10.1001/archpsyc.1979.01780100017001