The field of motor development has a long tradition of documentin

The field of motor development has a long tradition of documenting individual differences. Studies have documented between-subjects variability in supine kicking, manual and pedal lateralization, fluctuations between unimanual and bimanual reaching preferences, crawling strategies, strategies for the acquisition of pulling-to-stand,

and many others (Adolph, Vereijken, & Denny, 1998; Atun-Einy et al., 2011; Berger et al., 2011; Corbetta & Bojczyk, 2002; Corbetta & Thelen, 1996; Gesell & Ames, 1947; Jacobsohn et al., 2012 Thelen, Ridley-Johnson, & Fisher, 1983). We continue in that tradition by describing three trajectory profiles of infants’ reaching preferences: Strong unimanual, Fluctuations in preference, and No preference. Most infants fit the overall and expected group pattern of fluctuations between unimanual and bimanual reaching preferences over the course of the study. However, as in previous studies of the developmental trajectory of

reaching preference learn more (Corbetta & Bojczyk, 2002), we also identified a subset of infants who did not fit the group average. Historically, variability in a data set was seen as a nuisance that was deemed best to ignore. More recently, variability has been frequently conceptualized as a behavioral pattern that facilitates finding the most efficient and successful solution to the problem of acquiring new motor skills (Adolph et al., 1998; Oakes & Plumert, 2002; Piek, 2002; Snapp-Childs & Corbetta, 2009). However, because infants had previously solved the problem of manual differentiation, but then adopted

a less adaptive solution, this study, along with others describing the individual variation in the expression see more of bi- and unimanual Ribonucleotide reductase reaching (e.g., Thelen & Corbetta, 2002), seems to be describing a different phenomenon in the case of variability in the trajectory of infants’ return to bimanual reaching. Rather than reflecting individual problem-solving strategies, in this case, the examination of the individual developmental trajectories may serve as a direct and effective way to understand the processes that lead to overall population trends (Jacobsohn et al., 2012). For example, previous work has shown that when infants switch from a quadrupedal to a bipedal stance, they need to restrict their motor patterns until they have more fully mastered the new locomotor skill (Babik, 2010; Berger et al., 2011; Corbetta et al., 2006). Returning to a well-practiced bimanual reaching pattern in the context of the transition from manual to pedal balance control may serve a similar stabilizing function. This new finding illustrates a more general developmental trend where novices, such as infants during the transition to a new locomotor skill, limit joint movements or the repertoire of executed behavior when they first acquire new skills that require coordination (e.g., Atun-Einy et al., 2011; Berger et al., 2011; Harbourne & Stergiou, 2003; Vereijken & Waardenburg, 1996).

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