Keynes believed that how much money you have determines how much you spend, while Duesenberry added that how much others have also influences your spending. However, newer theories say people plan for the future and spend based on that, not just on current income. This means government spending doesn't have a big impact on the economy. Keynes and Duesenberry's ideas were pushed aside because they focused on psychology and society, not on future planning like the newer theories do.