Nudging / manipulation

Nudging is about presenting choices in a way that makes people do what you want them to – while still leaving them with the freedom to choose for themselves.
One way of nudging is setting the default, you present people with options, but one of them has been pre-chosen. Most of us will simply accept the default.

This has been used to make school kids make more healthy choices in canteens – but the same mechanism can obviously also be used to make consumers spend more than they would otherwise. 

Here’s an example, from Disney in Orlando. It’s always a bit of a hassle to calculate tips in the US, but this restaurant helps you by calculating the amount for you. They have two suggestions: 18% and 20%. All you need to do is fill in the amount.
…But what happened to 15%? As long as I can remember, 15% was the norm. Now, if you want to pay 15% you are not normal, in fact it really sticks out that you are trying to go against a norm that you have been presented with.
Personally, I get really stubborn when I sense that I’m being manipulated. It’s no longer a friendly transaction, when you sense that the other party is trying to trick you.
They got 15%.