A scientist who wants to predict the way in which consumers(消费者) will spend their money must study consumer behavior. He must obtain data both on the resources of consumers and on the motives that tend to encourage or discourage money spending. If an economist were asked which of three groups borrow most—people with rising incomes, stable incomes, or 'decreasing incomes—he would probably answer, those with decreasing incomes. Actually, in the years 1947~1950, the answer was: people with rising incomes. People with decreasing incomes were next and people with stable incomes borrowed the least. This shows us that traditional assumptions(假设) about earning and spending are not always reliable. Another traditional assumption is that if people who have money expect prices to go up they will hasten to buy. If they expect prices to go down, they will postpone buying. But research surveys have shown that this is not always true. The expectations of price increases may not stimulate buying. One typical attitude was expressed by the wife of a mechanic in an interview at a time of rising prices. 'In a few months', she said, 'we'll have to pay more for meat and milk we'll have less to spend on other things'. Her family had been planning to buy a new car but they postponed this purchase. Furthermore, the rise in prices that has already taken place may be disliked and buyers' resistance may be produced. This is shown by the following typical comment 'I just don't pay these prices they are too high'. The investigations mentioned above were carried out in America condition most helpful to spending appears to be price stability. If prices have been stable and people consider that they are reasonable, they are likely to buy. Thus, it appears that the common business policy of maintaining stable prices is based on a correct understanding of consumer psychology(心理学). According to the passage, if one wants to predict the way people spend their money, he should______.