The aftermath of an individual in a group of friends dating a nut-case girlfriend or boyfriend. The significant other will intentionally or unintentionally control the group member's entire life and eventually stomp out anyone he or she sees as 'unfit' based on arbitrary criteria. Comes from Yoko Ono's effect on the Beatles, where her attitude towards the rest of the band and influence over John Lennon resulted in animosity and the eventual estrangement of John and Yoko from the group.
friend1: Why don't we see Mark anymore?
friend2: The Yoko Effect, Mark is our John and his Yoko hates us