Yes, generosity causes happiness.

VEDANTAM: If getting treats was what made the children happy you would expect they would be happiest when they received treats, not when they shared their treats. And remember these were toddlers. They hadn't yet learned rules about generosity. You might expect them to be naturally selfish. Aknin says she was surprised by what she saw.

AKNIN: Children smiled significantly more when they were giving treats away than when they received the treats themselves. But what we thought was particularly exciting was that children actually smiled significantly more when they gave away their own treat than an identical treat provided by the experimenter.

VEDANTAM: Giving up something that belonged to them, their own treasure, this is what seemed to children happiest. Other experiments have found the same thing with adults.