Here’s the normal story. Picture you are in a room with 10 people. Each of them has a slice of cake. How much you are willing to pay for a slice of the cake is the ‘marginal utility’ of having it, and the more cake you have the less any more cake is worth to you. You’d be willing to pay a $1 for the first slice of cake, but you’d only be will to pay 90 cents for the second slice. You’d only be willing to pay 10 cents for the 9th slice, and a penny for the 10th slice. Eating the 10th slice of cake in that room would probably make you sick, hence you want it a lot less than the first slice, which is delicious. That’s declining marginal utility.
Now picture you are in a room with 10 people screaming. You hate it when people scream, and you can pay a person to get them to stop screaming. Would you pay in a similar way to the cake example? Would you pay a $1 to get the first person to stop screaming, and a penny for the 10th person to stop screaming?
No. Getting one person to stop screaming would make very little difference in how much you dislike being in the room. Modern psychology tells us you might not even notice it. You’d probably only pay a penny to get that first guy to stop screaming. However getting the second guy to stop screaming might be worth 10 cents. And the last guy, the difference between some screaming and no screaming, might be worth the full dollar to you. The more quiet it got, the more a marginal difference in how quiet it is would be worth to you. There’s increasing returns to this good; the 10th guy not screaming is worth more than the first guy not screaming, which is the exact opposite dynamic of the 10th cake being less delicious than the first.
-- http://rortybomb.wordpress.com/2009/12/03/persistence-of-pov...