We are more likely to comply with requests from strangers if we believe we share seemingly uncommon, incidental characteristics (e.g. first name, birthday, etc.), according to a 2004 research study published in the Personality and Social Psychology Bulletin (pdf):
Four studies examined the effect of an incidental similarity on compliance to a request. Undergraduates who believed they shared a birthday (Study 1), a first name (Study 2), or fingerprint similarities (Study 3) with a requester were more likely to comply with a request than participants who did not perceive an incidental similarity with the requester. The findings are consistent with past research demonstrating that people often rely on heuristic processing when responding to requests and with Heider’s description of unit relationships in which perceived similarities lead to positive affect. Consistent with the unit relation interpretation, participants did not increase compliance when hearing about an incidental similarity with someone other than the requester or when they believed the feature they shared with the requester was common.
Wondering why there were two birthday-related posts today? Today I am 25!