Developing language in a developing body: genetic associations of infant gross motor behaviour and self-care/symbolic...

Mastering gross motor abilities in early infancy and culturally defined actions (e.g. self-care routines) in late infancy can initiate cascading developmental changes that affect language learning. Here, we adopt a genetic perspective to investigate underlying processes, implicating either shared or "gateway" mechanisms, where the latter enable children to interact with their environment.

Selecting heritable traits (h², heritability), we studied infant gross motor (6 months) and self-care/symbolic (15 months) skills as predictors of 10 language outcomes (15-38 months) in genotyped children from the Avon Longitudinal Study of Parents and Children (N ≤ 7,017). Language measures were combined into three interrelated language factors (LF) using structural equation modeling (SEM), corresponding to largely different age windows (LF_15M, LF_24M, LF_38M, 51.3% total explained variance). Developmental genomic and non-genomic relationships across measures were dissected with Cholesky decompositions using genetic-relationship-matrix structural equation modeling (GRM-SEM) as part of a multivariate approach.

Gross motor abilities at 6 months (h² = 0.18 (SE = .06)) and self-care/symbolic actions at 15 months (h² = 0.18 (SE = .06)) were modestly heritable, as well as the three derived language factor scores (LFS_15M-h² = 0.12 (SE = .05), LFS_24M-h² = 0.21 (SE = .06), LFS_38M-h² = 0.17 (SE = .05)), enabling genetic analyses. Developmental genetic models (GRM-SEM) showed that gross motor abilities (6 months) share genetic influences with self-care/symbolic actions (15 months, factor loading λ; λ = 0.22 (SE = .09)), but not with language performance (p ≥ .05). In contrast, genetic influences underlying self-care/symbolic actions, independent of early gross motor skills, were related to all three language factors (LFS_15M-λ = 0.26 (SE = .09), LFS_24M-λ = 0.28 (SE = .10), LFS_38M-λ = 0.30 (SE = .10)). Multivariate models studying individual language outcomes provided consistent results, both for genomic and non-genomic influences.

Genetically encoded processes linking gross motor behaviour in young infants to self-care/symbolic actions in older infants are different from those linking self-care/symbolic actions to emerging language abilities. These findings are consistent with a developmental cascade where motor control enables children to engage in novel social interactions, but children's social learning abilities foster language development.