dispositive

dispositive \dih-SPOZ-i-tiv\, adjective:

involving or affecting disposition or settlement: a dispositive clue in a case of embezzlement.

Perhaps it had been a hallucination, or it was a false memory. Boggs had even predicted that later Ellis would doubt the incident had even happened—which seemed dispositive toward the dead man's reality…
-- Nick Arvin, The Reconstructionist, 2012

Looks and charm were often dispositive, the more attractive partner sailing on to other waters.
-- Louis Begley, About Schmidt, 1996

Dispositive comes from the word dispose meaning "to put in a particular place."