Colwyn Trevarthen claimed that babies have an intrinsic motivation to communicate and engage with others that he called intersubjectivity.Around the ninth month of life this motivation changes and passes from person-to-person dyadic (Primary Intersubjectivity) to triangulate a person-person-object relationship (Secondary Intersubjectivity). Despite the scientific consensus on this developmental leap, few empirical studies explore the trajectory of this more complex form of intersubjectivity between the second and third year of life. 115 free play sessions from 27 mother-child dyads (13 girls and 14 boys) between 9 and 37 months were filmed and were categorized based on the Level of Intersubjective Attunement Scale.The data was analysed using a Linear Mixed Effects Model. Resultsshow both a population trajectory of the levels of intersubjective attunement and random individual differences. We discuss these results in relation to the binomial typical developmental route and interindividual variability.

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Babies are born with an innate drive or intrinsic motive formation with which to communicate and share meanings with others and that some authors have called intersubjectivity (S. Bråten & C. Trevarthen, 2007; C. Trevarthen, 1974, 2001). Around the ninth month of life, this motivation changes and passes from a person-to-person dyadic (primary intersubjectivity) to a person-person-object relationship (secondary intersubjectivity). S. Bråten and C. Trevarthen (2007) also proposed a third form or layer of intersubjectivity known as tertiary intersubjectivity. One hundred fifteen free-play sessions of 27 mother-child dyads (13 girls and 14 boys ages 9-37 months) were filmed and categorized using the Level of Intersubjective Attunement Scale (LISA-T; M. Pérez Burriel & M. Sadurní Brugué, 2014; M. Sadurní Brugué & M. Pérez Burriel, 2012). Results from these nine hierarchical levels are presented, following a developmental sequence or population trajectory around an interindividual variability. In this article, we propose viewing these age-related levels as windows of achievement of intersubjective milestones. The statistical analysis suggested a redesign of the LISA-T levels of intersubjectivity; thus, results from this redesign and the debate on the implications of these transitions in infant mental health development are presented.