In a group setting, one member could take advantage of the help that he/she receives from the other group members, thereby exploiting the reciprocity norm to his/her advantage.
Some researchers believe that instead ________ evolved.

a. gratitude
b. diffusion of responsibility
c. bystander effect
d. volunteering

Reciprocity is defined as the exchange of goods or services out of one's kindness to another. This mutual and friendship exchange of goods and services has improved the species survival according to researchers. Robert Trivers, who is the originator of the theory of reciprocal altruism states that gratitude is born through these act of mutual kindness.

Other researchers suggest that gratitude is what drives us to act back and return the act of kindness where we have received it,it drives us to show our appreciation.

This act of gratitude is what encourages the other person to actual do us favors again.

So according to researchers reciprocity is what drove the evolution of gratitude.