The most effective call to action for ads

I have been working in Google for a while and we studied different call to action words/sentence in order to make ads more effective.

One of the best call to action is : "buy now". You should use it when planning an advertising campaign to generate sales. But why does it work ?

The reason lies in a study conducted by four Princeton psychologysts aimed at scanning subjects' brain as they were presented with a choice: short term gratification versus delayed rewards. The "Buy Now" option caused an unusual flurry stimulation in the limbic areas of students - a whole grouping of brain structures primary responsible for our emotions, as well as for the formation of memory. The more the students were emotionally excited about something, the greater the chances of their opting for the immediate.
Most of our brain is dominated by automatic processes, rather than deliberate thinking. A lot of our decisions (to click on the ad in this example) are driven by emotions, not by rational thoughts.