Final answer:
Knowledge of developmental stages is crucial to identify atypical development. Developmental theories offer frameworks to understand cognitive, physical, and psychosocial milestones, and deviations can indicate discontinuous development or developmental disorders.
Step-by-step explanation:
Identifying deviations from the norm in child development requires knowledge of the developmental stages and milestones. Without this understanding, you would not have a benchmark to recognize what is typical or atypical in behavior or abilities. For example, if one is unaware that children typically start walking by the age of 12-15 months, seeing a child not walking at 16 months might not raise concern. Developmental theories provide frameworks for understanding the progression through different cognitive, physical, and psychosocial stages. Understanding these theories, such as those by Erikson or Piaget, allows for identification of atypical development such as in discontinuous development, where children don't follow the smooth progression implied in continuous development.