- "Jasper has the ability to both feel and manipulate the emotions of those around him."
- ―Entry on Jasper's gift[src]
A vampire with this ability can sense the feelings of others and alter them according to his will. With it, Jasper can resolve a tense situation peacefully or, (like Chelsea with her gift) make people coexist more easily, as Jasper did with his newborn army. It can also be used to enlighten someone's mood; Edward described it as a subtle ability, and is very useful in resolving conflict. It is said that Jasper feels horrible because he could feel Edward's negative emotions when he tried to attack Bella in New Moon.
He can also manipulate the level of intensity people feel emotions, as shown when Jasper calmed Bella so much that she fell asleep in Twilight.
Given the way he was able to influence the mood of the Quilette shape-shifters in Eclipse, it is obvious that his power is not limited to humans and vampires alone.
This power can work potently through a mental shield because it physically changes the emotions i.e. manipulating endorphins, adrenaline and pheromones instead of providing a false alarm to the mind.
With practice, there is possibility that he can overload his targets with emotions as a way of attack.
This ability is limited, as shown in Breaking Dawn, when Jasper and Emmett come to Bella's house to take Edward for a "bachelor party". Jasper reassures Bella with his power (or rather takes away her insecurity at the idea), but as they leave, she states she felt her calm slipping away. This shows that the emotion he creates can only last for as long as he is around; when he is no longer present, the influence wears off almost immediately. This limitation was also explained in Eclipse, when Jacob told Bella that Jasper got rid of everyone's anger, which did not resurface until after he was gone.
Jasper was quite charismatic as a human - he could make others view things his way. It was via this charisma that he persuaded the Confederate Army's superiors to let him join the cause. Throughout the next three years, he unintentionally used this skill to persuade his co-soldiers and officers to see things his way, and was eventually appointed to major at an early age.
Upon transformation, this charisma changed into a power to physically sense and control other people's emotions.
While serving in Maria's army, Jasper's power was useful in controlling the newborn soldiers and keeping them calm. However, he sensed the pain of every person he killed (human or vampire). Overtime, that pain built until it became unbearable. When he came across Alice in 1948, he felt an unfamiliar emotion for the first time: hope. Her happy feelings impacted him so strongly that he easily reciprocated her love for him. And then, by practicing to live on animal blood, he was freed from the emotions that depressed him. After joining the Cullen family, the happy atmosphere his new family created provided him a peaceful existence.
- "Jasper is very interesting. He was quite charismatic in his first life, able to influence those around him to see things his way. Now he is able to manipulate the emotions of those around him―calm down a room of angry people, for example, or excite a lethargic crowd, conversely. It's a very subtle gift."
- ―Edward's description on Jasper's gift.[src]
In Twilight, Edward describes Jasper's ability to Bella when they first started dating. While hiding from James, Jasper feels Bella's distress about James and the Cullens' involvement to protect her and tries to console her about it. Later in Phoenix, he uses his power to calm her to the point that she falls asleep. However, when he tries to use it again, she refuses and brushes him off.
- "Jasper, no fair with the mood-control thing."
- ―Bella's complain at Jasper's influence.[src]
In the New Moon film, he changes Bella's mood to make her agree to attend her own 18th birthday party that Alice had planned for her. Bella moans at Jasper for the mood change, to which Jasper, Edward and Alice laugh.
In Eclipse, Bella mentions not being able to tell if Jasper was making the atmosphere tense or whether she was just feeling it. He helps resolve a tight situation caused by Paul, Emmett and Rosalie during their pursuit Victoria, though the pack members feel their anger rising again after he left.
When Bella asks Alice why she can see her future and Jasper can affect her emotions when other vampires' powers have no effect on her, Alice answers that her power is to see the future affected by people's decisions, but not the thoughts and reasons behind them; and Jasper can actually manipulate people's emotions.
At the beginning of Breaking Dawn, Jasper senses Bella's wedding jitters and temporarily calms her down.
After Bella became a newborn, Jasper is constantly at her side, initially to change her mood in case something upset her; seeing how a newborn's temper and thirst are highly uncontrollable. Later, he feels her endlessly happy mood, which makes him gravitate toward her without thinking.
- Chelsea's ability to manipulate relationships bears a resemblance to this power, though she can bind the emotional bonds between people permanently while Jasper can only control the emotions of those around him for as long as he is present.
- Corin's talent is similar to Jasper's, though she can only create a feeling of contentment in others. Her power is seemingly long lasting and addictive.
- Didyme's happiness induction is very similar to this power, although it was limited to spreading joy while Jasper can inflict any emotion he wishes.
- Fred's gift of physical repulsion is similar to Jasper's in the sense that he can change the way someone feels, physically. Though, like Didyme, it is limited to only one emotion: repulsion.