"Marcus's life changed dramatically when Aro brought home his newly turned younger sister, Didyme, along with the first members of the guard - vampires who were drawn to Didyme's aura of happiness. None of these early subordinates lasted more than a few centuries; they were still experimental at that point. Didyme's gift had always been with her, though subtler during her human years, and so she was used to many suitors."
According to Stephenie Meyer, this power was only possessed by Didyme, Aro's younger sister, which made the Volturi members around her happy during her time of existence. But the most benefactor of this talent of hers was Marcus, one of the Volturi leaders, who became her mate. This power seemingly released an aura of happiness that naturally affected everyone around her wherever she went.
Happiness induction also can bring happiness to Didyme, as well as those around her. However, Jasper's emotional manipulation, can only control the emotions of others.
After she became a vampire, this power manifested much more clearly, allowing those around her to be easily affected by her aura.
Similar abilities
Corin's ability to make others content and happy with their situations, though it has a side effect. Anyone constantly exposed to her power will feel physically uneasy if they are not exposed to it for an extended length of time.
Jasper Hale's ability to manipulate others' emotions is in certain ways more powerful than Didyme's, because he can influence the emotions in any way he pleases. Like Didyme, his power only works while he is present.
