Erlewine, Michael Yoshitaka and Theodore Levin, 2021. “Philippine clitic pronouns and the lower phase edge.”
Linguistic Inquiry 52:2, pages 408–425.

Pronominal paradigms in Philippine-type languages show a robust and curious gap: in transitive clauses pivot arguments and non-pivot agents may have bound pronominal forms, appearing as second position clitics, but pronominal non-pivot themes must be full, free pronouns. This gap teaches us about the organization of the lower phase edge. As cliticization involves a syntactic dependency between the host and argument position and all syntactic dependencies are constrained by phases, the gap is explained if pivots and non-pivot agents are specifiers of the phase head, making them the only DPs accessible for operations from outside of the phase.