In many so-called canonical null subject languages, null and overt subject pronouns have contrasting referential preferences: null subjects tend to maintain reference to the preceding subject while overt pronominal subjects do not. We propose that children acquire this contrast by initially restricting their attention to 1st and 2nd person pronouns, whose reference is simpler to infer compared to 3rd person pronouns. We provide supporting evidence from spontaneous production and comprehension in Mexico City Spanish, showing that (i) the null/overt contrast is in principle acquirable from exclusively observing the referential preferences of 1st and 2nd person subject pronouns in caretaker speech; (ii) children themselves condition subject pronoun expression on pronoun reference in the 1st and 2nd persons before doing so in the 3rd person; and (iii) children use the null/overt contrast in comprehension at a similar age when they begin making this distinction in production.