but since the subjects are "colleagues", we can assume they have similar income/wealth.

However, other factors such as relationships with their family/friends should also be controlled because maybe those who are happy are simply those who have a lot of friends and family. They are the type of people who treat each other well and they probably receive more money or "free lunch" in return as well. Most likely it’s not the part of giving, but the part of mutual support, that makes these people happy.

In short, the research falls short and the conclusions are flawed.