Читать книгу Infants and Children in Context - Tara L. Kuther - Страница 127

Some traits illustrate a wide reaction range. Others are examples of canalization, in which heredity narrows the range of development to only one or a few outcomes. Canalized traits are biologically programmed, and only powerful environmental forces can change their developmental path (Flatt, 2005; Posadas & Carthew, 2014; Waddington, 1971). For example, infants follow an age-related sequence of motor development, from crawling, to walking, to running. Around the world, most infants walk at about 12 months of age. Generally, only extreme experiences or changes in the environment can prevent this developmental sequence from occurring. For example, children reared in impoverished international orphanages and exposed to extreme environmental deprivation demonstrated delayed motor development, with infants walking 5 months to a year later than expected (Chaibal, Bennett, Rattanathanthong, & Siritaratiwat, 2016; Miller, Tseng, Tirella, Chan, & Feig, 2008).

Motor development is not entirely canalized, however, because some minor changes in the environment can subtly alter its pace and timing. For example, practice facilitates stepping movements in young infants, prevents the disappearance of stepping movements in the early months of life, and leads to an earlier onset of walking (Adolph & Franchak, 2017; Ulrich, Lloyd, Tiernan, Looper, & Angulo-Barroso, 2008). These observations demonstrate that even highly canalized traits, such as motor development, which largely unfolds via maturation, can be subtly influenced by contextual factors.