What do subject pronouns do in discourse? Cognitive, mechanical and constructional factors in variation

In languages with variable subject expression, or "pro-drop" languages, when do speakers use subject pronouns? We address this question by investigating the linguistic conditioning of Spanish first-person singular pronoun yo in conversational data, testing hypotheses about speakers' choice of an expressed subject as factors in multivariate analysis. Our results indicate that, despite a widely held understanding of a contrastive role for subject pronouns, yo expression is primarily driven by cognitive, mechanical and constructional factors. In cognitive terms, we find that yo is favored in the presence of human subjects intervening between coreferential 1sg subjects (a refined measure of the well-described phenomenon of "switch-reference"). A mechanical effect is observed in two distinct manifestations of priming: the increased rate of yo when the previous coreferential first singular subject was realized as yo and when the subject of the immediately preceding clause was realized pronominally. And evidence for a particular yo + cognitive verb construction is provided by a speaker-turn effect, the favoring of yo in a turn-initial Intonation Unit, that is observed with cognitive (but not other) verbs, which form a category centered around high frequency yo creo 'I think'