Ed O’Brien from the University of Chicago and Samantha Kassirer from Northwestern University gave research participants five dollars to spend on themselves or on others, but there was a catch: the money had to be spent on the exact same thing. Unsurprisingly, over time, buying the same thing over and over again quickly got old, but interestingly, buying that same thing repeatedly for others didn’t.
Here’s how Professor O’Brien described the findings:
If you want to sustain happiness over time, past research tells us that we need to take a break from what we’re currently consuming and experience something new. Our research reveals that the kind of thing may matter more than assumed: Repeated giving, even in identical ways to identical others, may continue to feel relatively fresh and relatively pleasurable the more that we do it.
The Association for Psychological Science has a good summary in The Joy of Giving Lasts Longer Than the Joy of Getting.