John's income is the only influence that is being allowed to change

John's income is the only influence that is being allowed to change because it directly determines how much money he has available to spend on goods and services. It is important to keep in mind that there are other factors that can influence an individual's spending habits, such as inflation, interest rates, government regulations, and taxation policies, but these factors do not directly influence the amount of money an individual has available for spending. Thus, John's income is the only influence that is being taken into consideration when looking at how his spending habits may change over time.