From Landry, Lange, List, Price, and Rupp, "Toward an Understanding of the Economics of Charity: Evidence from a Field Experiment," National Bureau of Economic Research Working Paper 11611 (September 2005):
In measuring factors that affect charitable donation rates, "we find that a one standard deviation increase in female solicitor physical attractiveness ... is roughly equivalent" in its positive effect on participation to moving from the least favorable to the most favorable incentive approach that they tried. "This result is largely driven by increased participation rates among households where a male answered the door."
Still, my favorite NBER empirical paper of all time remains Joel Slemrod's justly infamous paper in which he found that death is tax-responsive (people die more when it is more tax-favorable to do so). Joel carefully noted that his data did not permit him to determine whether the tax-responsiveness pertained to actual times of death or merely to reported times of death.