In the first eighteen months of life, children's accomplishments in language and motor development are striking and synchronous. Language and motor abilities emerge through biological and environmental prompts and facilitators, without explicit instruction. Early language (Fenson, Dake, Reznick, Bates, Thal & Pethick, Reference Fenson, Dale, Reznick, Bates, Thal and Pethick1994) and motor milestones (Darrah, Senthilselvan & Magill-Evans, Reference Darrah, Senthilselvan and Magill-Evans2009) are broadly universal, although growth trajectories vary widely in both domains (Reilly et al., Reference Reilly, Bavin, Bretherton, Conway, Eadie and Cini2009). Asynchronies in motor and language abilities are observed in developmental disorders such as cerebral palsy and specific language impairment (SLI).

In the Descartian and Piagetian traditions, the relationship between mind and body has long been the subject of intense philosophical debate and empirical scrutiny. Iverson (this issue) encourages us to consider emergent motor abilities and early sensorimotor experience as part of the resource mix for language acquisition and cognitive development. Her view is that early motor experiences and accomplishments transform the infant's world and are a developmental means for language and cognitive development. She expresses the view that ‘the development of language should be viewed in the context of the body in which the developing language system is embedded’ (p. 000).

Recently, Zubrick, Taylor, Lawrence, Mitrou, Christensen & Dalby (Reference Zubrick, Taylor, Lawrence, Mitrou, Christensen and Dalbyin press) proposed a conceptual framework for understanding the acquisition, accumulation, transformation and loss of human capability from early childhood onwards. Within this framework, developmental mechanisms are described as prompts, facilitators and constraints. Developmental prompts are necessary for the acquisition of a particular ability and include biology, expectations and opportunities. Facilitators are helpful for the development of ability but are not essential – they provide leverage for developmental prompts. Constraints impede or diminish the influence of developmental prompts and facilitators. Constraints vary in magnitude and effect, resulting in low ability levels within the normal range through to ability levels in the clinical range.

Zubrick and colleagues applied this model to the development of social skills. However, it has application in and across all domains of development. The application of this model to the study of child development allows for developmental ends such as walking to be viewed as developmental means for other abilities such as talking. According to this model, motor development would be viewed as a facilitator of language development. Iverson expresses a similar view.

In this model, prompts for language development include biology, expectations and opportunities. Normal brain maturation, normal neurodevelopment, social interaction, shared activity and adult input are all necessary for normal language development. Children depend on social interaction and shared activity with adults to learn how to use language and to map words and sentences to agents, actions, objects, attributes, locations and intentions (Carpenter, Nagell & Tomasello, Reference Carpenter, Nagell and Tomasello1998).

The study designs required to study children's early development directly and in great detail necessarily limit observation, measurement and interpretation to the most proximal influences on children's development, that is children themselves and their caregivers. These informative studies have yielded valuable information about the onset, timing and convergence of emergent language and motor abilities. These studies cannot answer questions about the complex interplay between proximal (i.e. child, maternal, family) and distal (i.e. school, community, society) influences on children's development, as in the Bronfenbrenner tradition (Bronfenbrenner, Reference Bronfenbrenner1979).

To understand the interactions between proximal and distal influences on child development, multiple study designs and sources of data are needed. Prospective population-based longitudinal studies with large numbers of children play an important role in helping disentangle multiple factors that influence children's development (Taylor & Zubrick, Reference Taylor and Zubrick2009). This is accomplished using multivariate methods, such as multivariate logistic regression that is adjusted for the effects of other predictors in the models.

Recently, Bavin and colleagues (Reference Bavin, Prior, Reilly, Bretherton, Williams and Eadie2008) examined the prediction of vocabulary development at ages 1 ; 0 and 2 ; 0 from performance on the CDI gesture subscales (Fenson et al., Reference Fenson, Dale, Reznick, Thal, Bates and Hartung1993) at ages 0 ; 8 and 1 ; 0. The close relationship between gesture and expressive vocabulary development in the first two years of life has been well documented in small studies. Bavin and colleagues confirmed this relationship in their large epidemiological sample of Australian children. Their results provided general support for the relationship between gesture and expressive vocabulary development. Their data showed that the CDI gesture subscales administered at age 0 ; 8 contributed 14·3% of the variance in vocabulary production at age 1 ; 0, with low prediction of individual components of the gesture subscales. The same pattern held at age 2 ; 0, with 14·5% of the variance in vocabulary production explained by performance on the CDI gesture subscales at age 1 ; 0.

Iverson points us to the small literature that reports subclinical levels of motor impairment in children with SLI (Bishop, Reference Bishop2002). Late language emergence at age 2 ; 0 is one of the hallmarks of SLI. In studies of small numbers of otherwise healthy children with and without late onset of language, lower levels of motor development have been reported for late talkers relative to controls (Zubrick, Taylor, Rice & Slegers, Reference Zubrick, Taylor, Rice and Slegers2007), extended this finding. Recently, Zubrick et al. used multivariate methods to investigate a comprehensive range of candidate child, maternal and family influences on late language emergence in an epidemiological sample of 1766 children at age 2 ; 0. From a vast set of candidate child, maternal and family predictors of late language emergence, the significant predictors in order of magnitude were: developmental lag in personal–social skills, developmental lag in gross motor skills, male gender, developmental lag in adaptive motor skills, developmental lag in fine motor skills, positive family history of late language emergence, the presence of one or more siblings, suboptimal foetal growth and lower child age. The results of this study suggested a strong role for neurobiological mechanisms in the late onset of language. It is not known whether this multidimensional developmental lag is part of the phenotype or the aetiology of late language emergence.

Recent gene discoveries in language, speech and reading impairment (Rice, Smith & Gayan, Reference Rice, Smith and Gayan2009; SLI Consortium, 2004; Smith, Pennington, Boada & Shriberg, Reference Smith, Pennington, Boada and Shriberg2005; SLI Consortium, 2002; Vernes, Newbury & Abrahmas, Reference Vernes, Newbury and Abrahmas2008) have inexorably led the field to re-engage with maturational theories of language development (cf. Muller, Reference Muller2005) and reconsider the role of genetically regulated biological timing mechanisms in language acquisition and language impairment.

Finally, an understanding of human development, beyond the epoch of childhood, can inform our thinking about early development. The kind of physical exploration that Iverson describes is prompted by children's cognitive capability and facilitated by adult expectations, opportunities and input. The means–end relationships between cognition, language and motor development change across childhood and indeed over the life-course. In the first two years of life, cognitive and motor abilities can be viewed as a developmental means for language acquisition. Once children have a language system, language is a means for cognitive development. Late in the life-course, normal aging and disease processes (e.g. dementia, stroke) can constrain cognition, language and motor function to different degrees. In a sense, these abilities that were so well synchronized early in life become dissociated.

There are formidable scientific and clinical boundaries around different domains of development in childhood. Iverson has successfully penetrated the boundaries around the study of motor and language development in infancy. In doing so, she has provided us with an enriched account of developing language in a developing body.