When all or most of the males in a group or population fall for the same girl. This will generally cause the other girls in the group to irrationally dislike the girl of interest, as all their male friends will talk about her constantly.
Annette: So Jess just won't stop talking about this girl.
Andrew: Anyone I know?
Annette: Her name is Lauren, dark hair, freshman...
Andrew: Oh yeah, Hot Lauren. She's amazing. She smiled at me the other day, so I think I might have a chance. I wrote her this poem...
Annette: Oh god, it's the Lauren Effect.