Adaptive behaviour refers to the everyday skills that individuals are expected to have to function independently, based on their age and societal norms. Currently, we know little about the role of genetic and environmental factors in parent-rated adaptive behaviours in early infancy. The aim of this study was to investigate the aetiological factors that influence individual variability in different adaptive behaviour domains at 5 months, and the degree of genetic and environmental influences that are unique and shared across these domains. We analysed data from the Vineland Adaptive Behaviour Scale (VABS-II) motor domain and combined domain of socialization and communication (social-communication) using a multivariate twin modelling approach. Participants were a community sample of monozygotic and dizygotic twins assessed at 5 months of age (n = 594). The results show high shared environmental influence on both motor (0.67) and social-communication (0.78) domains with 45% shared variance. Both had low, but significant heritability estimates (0.21 and 0.12, respectively) but did not share genetic variance. No statistically significant associations were found between polygenic scores for autism, ADHD, schizophrenia, depression, and bipolar disorder, and either of the adaptive behaviours measured here. Our results highlight the importance of shared environmental factors in the development of social-communication and motor skills in infancy, whether it is through social interaction with caregivers, or the stimuli and opportunities presented at home. Summary During development structural arm length representation is underestimated, while the functional arm length representation is overestimated. Underestimation of structural arm length is driven by an underestimation of hand length, as forearm length is accurate. Structural hand length is underestimated, supporting that underestimation of hand length is a characteristic of human body representation. The opposite pattern of results between structural and functional arm representation suggests the existence of multiple independent representations of the body.