Reciprocity
We’re hardwired to return kindness received. Considered the most powerful global social rule, our initial actions can be highly persuasive in affecting others’ judgements and decisions thereafter.
Related studies:
Jacob, C., Guéguen, N., & Boulbry, G. (2015). Effect of an unexpected small favor on compliance with a survey request. Journal of Business Research, 68, 56–59.