Researchers have traditionally relied on institutionally-defined, native speaker conceptions of linguistic competence to define whether or not individual learners are multilingual. However, to better understand the relationship between language and identity in the context of additional language learning, there is a need to widen the definition of multilingualism by including a larger repertoire of semiotic competences and a wider range of levels of linguistic competence. To date, little systematic research has investigated the relationship between different key variables influencing language learners' construction of their multilingual identity (MI). Based on surveys completed by 1338 learners in seven secondary schools in England, this paper reports on a structural equation modelling of key latent variables influencing MI. The analysis suggests that MI is more strongly correlated with direct experiential contact with languages and with social interaction than with explicit beliefs about language learning. Similarly, the influence of parental beliefs is greater than those of teachers or friends. The paper also considers the significance of potential differences between EAL and non-EAL respondents and urban and rural school contexts. Having conducted SEM we offer support for our theorisation of MI as an outcome latent variable composed of three main components, namely, experience, evaluation and emotion.