Final answer:
Worker productivity in the Hawthorne studies increased with both greater and less illumination, a phenomenon known as the Hawthorne effect, which represents the influence of being observed on participant performance.
Step-by-step explanation:
In the Hawthorne studies, it was found that worker productivity increased with both greater and less illumination. This counterintuitive finding is best explained by the Hawthorne effect, which suggests that the increase in performance was due to the workers being aware that they were being observed by researchers or supervisors, rather than the physical changes in their environment. Thus, the correct answer to the question regarding the Hawthorne studies is that worker productivity (C) increased with both greater and less illumination.
The Hawthorne effect illustrates how psychological and social factors can have a more significant impact on worker productivity than the physical work environment. As such, it's a crucial concept in organizational psychology and study design, indicating that the mere presence of researchers or changes being observed can affect the behavior and performance of participants.