Three Decades of Infant Motor Development: Cohort Effects in Motor Skill Onsets

Infant motor development is affected by the sociocultural context in which it takes place. Because societal and cultural practices are dynamic, this exploratory study examined whether the ages at which infants typically learned to crawl, cruise, and walk changed over the past 3 decades. We compiled archival data from 1,306 infants born between January 31, 1992, and December 10, 2021. Parents originally reported milestone onsets in interviews and by using diaries. For each motor milestone, a linear regression model predicted the onset age using birth date. Segmented regression analyses inspected changes in slopes over time. Covariates included rural/urban housing, gestation age, season of birth, and birth weight. Infants' average crawling, cruising, and walking onset ages changed over time. After controlling for the covariates, infants' crawling onset age steadily increased until 2012, after which crawling onset age decreased. Infants' cruising onset age increased from 1991 to 2001, after which cruising onset age remained stable. After controlling for the covariates, infants' walking onset increased until 2015, after which walking onset age decreased. Thus, when infants were born explained a small but significant amount of variability in infant motor skill onset. While the current study showed that motor development changed over the years, motor development is just a model system for development more generally: Cohort effects may be pervasive across developmental domains. Using motor development as a model system for studying change suggests that generational effects due to a changing society may be pervasive across developmental domains. Public Significance StatementFrom 1992 to 2021, on average infants learned to crawl, cruise, and walk at later ages. Trajectories of infant and child development are susceptible to broad social and cultural changes. Updating developmental norms to account for within-culture and cohort-level changes may be necessary.