Economists can predict what consumers in general will do because all humans seek D. BENEFIT MAXIMIZATION.
Consumers want to get the value for their money. They buy goods that are worth the amount they are willing to spend.
For example, in comparing two products, product A is cheaper but has a shorter life-span. Product B is pricier but lasts longer than Product A.
Given the choices, I'd prefer to buy Product B because I will benefit form it for a longer time than Product A.