Some evolutionary psychologists argue that in our distant past, those who selfishly pursued their own individual interests would be less likely to survive to pass on their genes. this argument lies at the heart of as a source of altruistic behavior.

The reciprocity norm

Explanation:

The reciprocity norm is also called the rule of reciprocity. It is the norm in the society where if you do something for someone then it has been expected to get return the same favor from another person. You feel obligated to do the same in return to the person. This principle is applicable in marketing because marketers use several strategies to convince the customer so the customer can purchase their products. There are some direct strategies such as sales, coupons, discounts, etc. Some have been using psychological tricks that normal person not aware of about that.  

b) the bill goes to the president.

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