The Law of Unintended Consequences was introduced by option pricing expert Robert Merton's father, sociologist Robert K Merton, and states that almost all actions have material unintended consequences. These are almost always bad, such as when Prohibition in the US (outlawing alcohol) increased organized crime, how rent controls created slums, how regulators tend to promote the interests of those they are regulating rather than consumers, or offering peace and abetting violence and coercion (eg, Munich 1938, Jimmy Carter).

But there are good some unexpected consequences, such as the medieval policy of setting up large hunting reserves for the nobility preserved green space, often as parks, throughout England. Wartime sinking of ships in shallow waters created artificial coral reefs. And Viagra, which was introduced to grow hair, until users kept noting it made them horny.


I would like to add this good unintended consequence: women tend to more sexual on and around Halloween parties. Halloween became popular as a way for children to have fun pretending they were comic book characters and get candy. Of course, eventually adults have taken it over. Halloween parties when I was young and unmarried were very good, mainly because it made women frisky. As this classic Onion list shows, young women predictably go slutty on Halloween, and the costume can't help but spill into attitude. My armchair psychological analysis is that women figure since it's a night of acting out of character, they can act like a whore without the usual reputational consequences because "they were just kidding".

Everyone wins.