See <a href="http://en.wikipedia.org/wiki/Parable_of_the_broken_window" rel="nofollow">http://en.wikipedia.org/wiki/Parable_of_the_broken_window</a><p>In a nutshell, it's the story of kids playing baseball and accidentally hitting a ball through someone's window. The fallacy is that by breaking the window, they've helped out the economy. The homeowner will now pay the glazier to fix it, whose wife will now have money to buy a new dress, etc.<p>It's a fallacy because it ignores the productivity that the money would have resulted in had it been used for its original intent. Perhaps his car needs new tires, or whatever -- but there was something that the homeowner would have preferred but for the necessity created by the ball.<p>By analogy, perhaps the hypothetical second parent, who is now staying home, might instead be a doctor helping people, or a teacher, or even a garbage collector. Whatever, it's something that we would have gotten had it not been for this disruption.