This paper is the first part of Alonso Pascua (in press). Both address the question as to why Jespersen’s Cycle (JC) evolved differently in Romance, focusing on the analysis of negation patterns in two West Iberian languages: Spanish and Asturleonese. The hypothesis that JC gets blocked when speakers are provided with evidence that negation is complex, but activated when this evidence is unavailable, will be tested. It will be contended that the alternation of two forms for negation in Old Spanish, triggered by adjacency with object clitics (OCs), provided speakers with evidence of the complex nature of negation and contributed to the blocking of JC. Conversely, it will be argued that the lack of such an alternation in Asturleonese hampered the speaker’s awareness of this complexity and favoured the activation of JC. This evidence will be used to support the hypothesis tested and thus contribute to a better understanding of the long-standing question of JC triggers.