People hold mental models of the way the world works.  Approximations that are “good enough” to get through day to day life.  We follow rules of thumb as shortcuts, freeing our brains from figuring out anew how to do everything.  But when those models are based on obvious logical fallacies, the best you can hope for is that they do no harm.

A classic example of this is the thermostat.  Depending on how a heating system works, the controller for it might determine the rate at which heat is output, or it might only set the point at which heat output stops because the room is warm enough.  The vast majority of residential systems work on the latter principle, and stop when told to, after pumping out heat at a fixed rate to get to that point.

And yet, people persist in turning the thermostat up to 90 although they will be back at it, trying to make it stop, well before the room is that uncomfortable.  Because the bad mental model of the thermostat that they hold tells them, More is better, more is faster.

Here’s another, more complex example.  An elevator bank has four elevators.  The two elevators on the right serve floors 1 to 13.  The two on the left serve floors Basement to 13.  (The basement is a good destination for many people, especially at lunchtime, because there’s a restaurant down there).  Call buttons are installed on the right with Up and Down, on the left with Up, Down and Basement.  The intent of the Basement button is, when you want to go to the basement, the only elevator you are interested in taking is one of the left-hand pair.  So pressing it calls only one of those.  Pressing Down calls any of the four.

Now watch: people will walk up to the elevator and press both Down and Basement when they want to go down.  The mental model at work here is, if I press more buttons, an elevator will arrive more quickly.  More is better, more is faster.  However, since the Down button alone will summon whichever of the four elevators can get there soonest, this benefit is imaginary.

Not harmless, though.  When the rightmost elevator arrives, the person who has pressed both buttons gets on.  But because that elevator doesn’t satisfy the request for the Basement, a minute later, the leftmost elevator stops at the same floor and waits in vain for its new passenger.  Possibly a group of hungry people inside look around awkwardly for him, and then are mildly annoyed when nobody is there.

OK, so the harm here is tiny, probably not even worthy of the word “harm”.  But it’s exemplary of two things I have noticed about bad mental models.  One is, that they can cause harm.  If someone turned the thermostat up to 90 and then left, waste of energy and damage to the room’s contents are possible results.

The other thing I have noticed is, that the harm caused might not be to the person with the bad model.  After all, the only person guaranteed not to be inconvenienced by pressing both buttons is… the person who pressed both buttons!  This is something economists call an “externality.”  More about externalities – and how they apply to matters of security – in future posts.