In 1965, Robert Zajonc developed the stern activation theory, by proposing his generalized drive hypothesis for social facilitation. Zajonc's generalized drive hypothesis was the first theory that addressed why the presence of others increased performance sometimes yet decreased it at other times. He argued that the presence of others serves as a source of arousal, and heightened arousal increases the likelihood of an organism to do better on well-learned or habitual responses. For this reason, arousal improves performance on simple, or well-learned tasks, but impairs performance on complex, or not well-learned tasks. Zajonc's reasoning was based on the Yerkes-Dodson law, which holds that performance works like an inverse "U" function. This means that an individual's optimal drive is higher for simpler, or well-practiced tasks, and that the same individual's optimal drive is lower for more complex, or less-practiced tasks. The presence of other people further arouses us and increases our drive level, and so an individual's performance will be enhanced if a task is simple (because of the high levels of energy) but diminished if the task is complex. He tested his theories by having people complete word association tasks alone and again in the presence of others, and found that the tasks were done much faster while in the presence of others.