Often times while passing through a street you will come across people, who will do you a favor like giving you an unexpected gift, and no matter how much you resist it, they will not take it back; on contract they will provide you an option for another favor in return like a small donation so that you can repay them. Or there will be another form of it, you will be able to easily recall instances when your class teacher throws very hard homework for your summer holidays , and then there is a lot of crying in the class, “No, Mam, this is too much”, the teacher then instantaneously throws another option which you all agree, as it is less hectic. In reality this second option was the real homework that the teacher wanted you all to do, but to break your resistance for not doing the homework she used a reciprocation-concession strategy. Robert Cialdini in his book Influence, gives many more similar examples of this which we encounter in our daily lives.
In games, like in many MMORPGs, you will see a pro player often gifting you some special tricks or powers to help you move forward in the game, so rather than leaving the game no matter how boring you are you stick around the game under the pro for some time; as you will be thinking that the person has given his/her personal time helping me level-up, it will be rude if I just stop playing, hence I should continue for a while. This is a clear case of the trap of reciprocity.
A good gamification designer who wants to increase engagement, should harness on this basic human nature to reciprocate, while designing for users who are less likely to adopt easily. If they get a meaningful gift within a system, which helps the newbie in an easy on-boarding, they will be compelled to give back internally for the effort the other player has put in helping the new player to survive the initial few days and hence increasing the adoption of your gamification design.
The reciprocity is not only limited to a new and pro relation, but can be a powerful mechanic at any level for increasing engagement. Even gifting between a pro -pro game increase the time they spend on the system, alone, when compared when they interact under the reciprocity condition.
The best part of reciprocity, is you are not enforcing as a sticky PBL, but harnessing on the innate human desire to give back to those who have helped you, thereby reducing your chances of developing a poor gamification design.