Law of Diminishing Marginal Utility

This law states that marginal utility declines as the consumer acquires additional units of a given product.

For example, suppose you absolutely love hamburgers. So you go to a burger joint and order one burger. That burger gives you 30 utils of satisfaction. You finish off that burger while still standing at the counter. The amazed cashier asks if you want another. "Sure!" you say and buy another - but this gives you only 23 utils. You eat this burger in about five bites, when the cashier asks if you want yet another. What the hell, you think, I'm getting a little full but I just love those burgers! This one however only gives you 4 utils.

This particular burger takes awhile to finish with several moments where you stop to let things settle. Now you're full. When you finish this third burger, the cashier yet again asks you if you'd like another. Well, considering you've spent almost all your money, are really full, and have to use the bathroom, the utils you'd receive from that next burger would be a whopping negative 10. So you decline.

A principle by which an equal amount of something will have less effect depending on how much has already been sampled. This works on all sorts of levels. Imagine chocolate. At first one small square of Cadbury's Dairy Milk is sufficient to get a kick. But after that, eating one at a time is not enough to get a similar kick. It has to be two the next time, and maybe even three the next. Until you feel sick.

More seriously it is also the principle which underpins the idea of increasing sex and violence in films. We like being titillated - but in order to be titillated we must see more sex or more violence than we did last time. The same amount will not have the same effect twice. Hence films get more sexually explicit and more violent. I'm not sure I agree with the conclusions which are usually drawn from this, but it's the way the reasoning goes.

This law explains why the halves of scone in a Devon cream tea have to have more and more jam and cream on them.