سؤال قراءة ستيب محلول — فهم التفاصيل (تدريب 7029)

(1) As an infant’s muscles and nervous system mature, more complicated skills emerge. With minor exceptions, the sequence of physical (motor) development is universal. Babies roll over before they sit unsupported, and they creep on all fours before they walk. These behaviors reflect not imitation but a maturing nervous system; blind children too, crawl and walk. (2) But there are individual differences in the timing of this sequence. In the United States, for example, 25 percent of all babies walk by age 11 months, 50 percent within a week after their first birthday, and 90 percent by age 15 months. (3) There are also cultural differences in timing. Although such differences could be genetic, experiments reveal that experience does predict motor behavior. Consider Ugandan babies, for example, who usually walk by 10 months. Unlike babies from other cultures who spend much of the day lying in a crib, Ugandan babies experience more intimate and rhythmic physical contact while being carried on the back. Two-week-old babies wave their arms more if they can see them, suggesting that they are learning to visually control their movements. Although 3-month- old infants still cannot efficiently reach and grab something, their seemingly spontaneous kicking movements become more purposeful and coordinated when they activate a mobile. (4) But nature (genes) plays a major role, too. Identical twins typically begin sitting up and walking on nearly the same day. Biological maturation - including the rapid development of the cerebellum at the brain’s rear - creates a readiness to learn walking within 2 or 3 month of age 1. Why can Ugandan babies usually walk by 10 months?