Drinking the Kool-Aid" is an expression used to refer to a person who believes in a possibly doomed or dangerous idea because of perceived potential high rewards. The phrase typically carries a negative connotation. It can also be used ironically or humorously to refer to accepting an idea or changing a preference due to
popularity,
peer pressure, or
persuasion. In recent years, it has evolved further to mean extreme dedication to a cause or purpose, so extreme that one would "drink the
Kool-Aid" and die for the cause. That's what happens when you don't have an open mind, or have a bias.