“To the best of our knowledge, this everyday behavior has not been systematically studied before…” 
The behavior in question is ‘door-holding’, and Joseph Santamaria at Pennsylvania State University and David Rosenbaum, Distinguished Professor of Psychology (also at Penn State) have been investigating this under-studied phenomenon.
The team used a digital video camera to record the door-holding behaviors of 148 individuals who passed through the door which leads to the patio of the Hetzel Union Building of Penn State University. Analysis of the footage – taking note of the number of times that the door was held open for another, and for how long – broadly confirmed the investigators’ hypothesis that :
“…holding a door open for another person, reflects the door holder’s expectation that the person for whom he or she holds the door shares the belief that the total effort expended by the two of them will be less than the summed efforts of the two individuals acting on their own.”
But noting too that some other etiquette-related behaviors might be considered effort-independent :
“Some forms of etiquette do not concern physical effort (e.g., napkin folding), and some forms of social effort reduction are not about etiquette (e.g., team sports).”
Their paper Etiquette and Effort : Holding Doors for Others is published in Psychological Science May 2011 vol. 22 no. 5 584-588 and can also be read in full here
Also see: Interpretations of automatic door behavior. (Really Magazine, Dec. 09)