The development of crawling between 18 months and four years.

Various quantitative and qualitative aspects of crawling were analysed in small groups of carefully selected normal toddlers of 1 1/2, two, 2 1/2, three, 3 1/2 and four years of age in a cross-sectional design. Between two and three years of age, major changes occurred in all observed aspects, resulting in differentiated, adaptive and efficient crawling. It is proposed that these changes are based on so-called adaptive variability, an age-related developmental process which seems to be a prerequisite for the development of cognition.