The study had 470 people in it and they had four different groups. The first group did random acts of kindness to other people. The second group did random acts of kindness to the world (i.e participate in a recycling drive). The third group did random acts of kindness to themselves. The fourth group did neutral acts. Each of the groups measured their happiness after a four week period.
What they were able to find was those that did random acts of kindness to other people, over the course of four weeks, had the biggest boost in mental health and had the biggest improvement in their own happiness.
This goes to show that doing kindness acts to other people can in fact be beneficial for your own mental health.