Motivation in the workplace has been a topic of debate since the inception of the employer-employee relationship, the discussion frequently centers on the financial aspects. Although it is a logical assumption that higher wages should infer higher output it is not always the case. I think that there are other factors at play, for example, psychological motivators such as part ownership and company leadership.
A good reason to hold this view is that part ownership is based on the concept that if you have a personal investment in the future of the company you are likely to want to ensure the success of the entity. Facebook, for instance, is thirty percent owned by its employees through stock options. Owning a portion of the company where you work instills a sense of ownership which motivates one to think of the company as their own. If you have control over the destiny of the company it will undoubtedly motivate you to think twice when performing your daily tasks.
Another point to consider is that leadership is often cited as a critical factor for motivating employees and can have a profound effect on the workplace. Consider Steve Jobs, he cultivated a legion of followers and employees by demanding that his company made products the employees would be proud of. Here the staff is motivated by non-monetary values so increasing their salaries would have a negligible effect.
To conclude, Leadership and part ownership are clear examples of increasing productivity via other means than simply increasing wages. I strongly disagree that higher wages would result in staff working harder.