Consistency
Robert Cialdini, in his book “Influence: The Psychology of Persuasion”, talks about an experiment he did. They asked a group of people to put a huge sign in their yards that said “Drive slow; kids playing”. The overwhelming majority of the people said no. A second group of people were asked to put a tiny sticker in their windows with the same text. Almost everyone agreed. Then they were asked to put the giant sign in their yards. A large percentage agreed.
 

I think it's more about trust than anything else.  Folks like to test small, no matter even if it's free.  

 
Contrast
The principle of contrast states that “People see things as good or bad depending on what’s around them”. For example, a regular house looks beautiful next to an ugly one but the same house looks ugly next to a gorgeous mansion. 

This one is funny.  He goes on to say that you should have a bad offer on your website.   Not being a seasoned marketer myself, I gotta try this Laughing.

The rest of them 9 are here:

http://www.theoutsourcingcompany.com/blog/internet-marketing/9-weird-psychological-tricks/