The Yerkes – Dodson Law suggests that performance and arousal are directly related. In simpler terms, increase in arousal to a certain level can help to boost performance. Once the arousal crosses the optimal level, performance of the individual starts to diminish.
Motivation can be defined as a reason (or reasons) which leads an individual to act in a certain way. Motivation is the force that causes an individual to take action and initiate, guide and maintain behaviors. There can be variety of different forces at play behind the cause of motivation. Over the years researchers and psychologists have come up with variety of different theories to explain the idea of motivation.
Expectancy theory(orExpectancy theory of motivation) proposes an individual will behave or act in a certain way because they are motivated to select a specific behavior over other behaviors due to what they expect the result of that selectedbehavior will be. (Wikipedia)
There have been various different approaches to explaining motivation. Based on these, there are different theories of motivation. One of them is Instinct Theory of Motivation which completely focuses on the biological approach to motivation.