Economists at the University of Southampton found that participants in a study on earning and charity were less likely to donate after receiving a bigger bonus. "Our findings suggest that receiving higher pay due to good luck is not generating a stronger need to 'give back to society,'" explained Dr. Mirco Tonin, co-author of the study. "This is probably because people instinctively attribute their high pay or bonuses to being a reward purely for their own skills and effort, even if there is actually an element of luck involved. As such, they feel entitled to the money." Tonin and study co-author Dr. Michael Vlassopoulos conducted their psychological experiment by recruiting a 104 participants to work for a low, fixed hourly wage. After performing a period of data entry functions, half the workers were awarded a low bonus, while the others were given a bonus more than double in size. The participants were not aware that their bonuses were being randomly assigned -- the amount based on chance, not performance. At the end of the experiment, participants were asked to give to charity. While 37 percent of low-bonus earners donated, only 21 percent of those more-handsomely rewarded parted with their earnings. The results build on previous studies that suggest humans attribute good fortune to ability rather than luck. Other studies have shown that wealthy Americans give away only 1.3 percent of their income, while the poorest donate 3.2 percent. Dacher Keltner, a psychology professor at the University of California, Berkeley, has studied giving habits in depth. "In just about every way you can study it, our lower-class individuals volunteer more, they give more of their resources -- they're more generous," he told NPR last year. Tonin and Vlassopoulos's paper was recently featured by Germany's Institute for the Study of Labor.