The statement that 'Value stocks generate higher returns during bad economic times' is not a valid explanation for the value premium. While it is true that value stocks may perform better during economic downturns due to their inherent stability and lower valuation, this is not the primary reason behind the value premium. The value premium is primarily attributed to the higher risk associated with value stocks, which demands a higher return as compensation. This risk arises from factors such as the inflexibility of value firms and their possession of more unproductive capital during bad times, as well as the underestimation of their growth prospects by investors. Furthermore, investor psychology, including biases such as mental accounting and loss aversion, also plays a significant role in the existence of the value premium. Therefore, while value stocks may indeed generate higher returns during bad economic times, this is not the primary reason behind the value premium, making choice D the correct answer.

Choice A is incorrect. This statement accurately represents a reason behind the existence of the value premium. Value firms are indeed less flexible and have more unproductive capital during bad times, which can lead to higher risk and therefore higher expected returns as compensation for this risk.

Choice B is incorrect. This statement also accurately represents a reason behind the existence of the value premium. Behavioral finance theories suggest that investors' psychological biases — such as mental accounting and loss aversion — contribute to the value premium.