有效需求不足是由消费需求和投资需求不足造成的,而消费需求和投资需求不足又是由边际消费倾向、资本边际效率和流动性偏好三个基本的心理规律决定的。在经济正常运行的情况下,由于存在储蓄行为,所以消费的增加一般小于收入的增加,且随着收入的增加,消费与收入之间的缺口会日益扩大,因此仅仅依靠消费需求无法实现社会的扩大再生产,这客观需要投资来弥补这一缺口,投资不仅本身是有效需求的一部 Insufficient effective demand is caused by the lack of consumer demand and investment demand. However, consumer demand and investment demand are determined by three basic psychological rules: marginal propensity to consume, marginal efficiency of capital and liquidity preference. In the case of normal economic operation, the increase in consumption is generally smaller than the increase in income due to the existence of savings behavior. As the income increases, the gap between consumption and income will increase. Therefore, it is impossible to realize the social Expanding reproduction, which objectively requires investment to fill this gap, invests not only in itself an effective demand