Final answer:
Jerome's improved performance when running with his teammates is an example of social facilitation, where the presence of others boosts his competitive drive and effort.
Step-by-step explanation:
The phenomenon that Jerome experiences, wherein he performs better when running with his teammates than training alone, despite being a loner by nature, is a classic example of social facilitation. Social facilitation is the tendency for individuals to perform differently, typically better, when in the presence of others than when alone. It operates under the principle that the presence of others can increase physiological arousal, which, depending on the task, can improve performance, especially if the tasks are simple or well-rehearsed. In Jerome's case, running is a familiar task where the presence of his teammates likely provides motivation and competitive arousal, leading to improved performance.