Why are bound morphological forms useful for comparing Papuan languages?

Bound morphological forms are considered the most reliable guide for determining genetic relationships among Papuan languages because they are highly resistant to borrowing.1

In the linguistically complex and fragmented environment of New Guinea, languages frequently exchange basic vocabulary, grammatical patterns, and discourse styles, which makes it difficult to distinguish between inherited traits and borrowed ones. Because bound forms—such as the verbal prefixes used to mark the object of a transitive verb—are rarely borrowed between languages, their presence in multiple languages serves as strong evidence of a shared genetic origin. For example, the use of specific prefixes for the first person singular and second person singular across languages from the Huon Peninsula to the highlands of Papua indicates a genetic relationship that is unlikely to have resulted from borrowing.1