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The development of a child is a fascinating and complex process involving the interaction between genetic influences and vast environmental influences and experiences. Philosophers and others have spent centuries debating the influence of nature versus nurture on the development of children. Theories of child development abound, and numerous books have been written on the topic. The reader is urged to seek more in-depth information beyond what is summarized in this chapter. This chapter briefly reviews theories of child development that have had the greatest impact on pediatric physical therapy. This review is followed by an overview of the development of functional movement from birth through adolescence.

Theories of Child Development

There are numerous theories regarding all aspects of child development. Theories, by definition, are hypotheses or conjectures to explain phenomena. Therefore, they change and evolve as new information becomes available. However, some general principles of child development are now widely accepted. Development is believed to involve the interaction of three main factors. The first factor is genetic predisposition. Children inherit propensities for certain talents or abilities and genetically transmitted diseases. The second factor is the individual’s own role in development, which may include previous experiences, the willingness to persist in a difficult activity, resiliency, and the tendency to participate or not participate in activities. Finally, environmental factors, such as the family, community, and sociocultural and socioeconomic influences, as well as opportunities and experiences, play a role in development. Environmental factors are thought to play a critical role in coordinating the timing and pattern of gene expression (Fox, Levitt, & Nelson, 2010; National Academies of Sciences, Engineering, and Medicine [NASEM], 2019). Therefore, genetic predisposition, the individual’s role, and environmental factors all influence development.

There is also agreement that all of development is interdependent. Shirley (1933) first noted that children mastering motor skills such as reaching for objects, sitting alone, and walking will vocalize much less during active motor periods. Others note “the old adage that children inhibit speech development while working on new motor skills” (Tipps, Mira, & Cairns, 1981). More recently, it has been found that when an infant is learning to walk independently, there is a regression or a reorganization of reaching skills (Corbetta & Bojczyk, 2002). Others note that as infants learn a new motor skill they might temporarily ignore an already mastered skill to devote their attention to learning the new skill (Heiman, Cole, Lee, & Adolph, 2019). This has implications for physical therapists when focusing on a specific motor skill, perhaps to the disadvantage of other skills. Practicing achieved and new skills together is best for eventual integration of all skills.

For many systems and domains, development generally progresses toward greater complexity through a gradual, continuous process. For some aspects of development, children will experience stages of rapid transitions followed by periods of ...

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