Receiving something unprompted predisposes us to return the favor.
Have you ever received unexpected help and felt the need to return the favor? People feel compelled to treat others the way others treat them.
A study from the University of Bristol tested whether reciprocity could boost attendance at a job fair. The invitation was sent by text message, and some people received a message that also said "I've booked you a seat, good luck." Mentioning that gesture, and wishing the recipient luck, significantly increased attendance at the event, compared to invitations that only mentioned the day and time.
In the market, Netflix, Spotify, UberEats, and Rappi offer free trial months for the same reason. Receiving a service at no cost increases the likelihood that we'll pay for it later, compared to equally good alternatives that didn't offer anything first.
Reciprocity is a social norm. We feel obligated to respond to another person's action with an equivalent action, whether it's a kind gesture or a hostile one.
Source: Sanders, M. & Kirkman, E. (2014). I've Booked You a Place. Good Luck: A Field Experiment Applying Behavioural Science to Improve Attendance at High-Impact Recruitment Events. Centre for Market and Public Organisation, Working Paper 14/334.
A small, genuine gesture before asking for something (a free sample, a useful piece of information at no cost, a personalized message) tends to generate more willingness to return the favor than asking directly without giving anything first.
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