原文 Bank Credit Standards By Mitchell BerlinBankers and the business press often speak of cycles in bank credit standards, periods in which banks lending standards are too lax, followed by periods in which standards are too stringent. In this view, bank lending policies tend to amplify fluctuations in GDP；easy money during the upturn sows the seeds of tight money episodes in the downturn. But this pattern is also consistent with variations in bank lending driven by changes in borrowers default risk over the business cycle or changes in the demand for loans, which rises and falls with GDP. To make sense of the idea of a lending cycle, we must uncover a systematic reason for banks to make unprofitable loans in an upturn and to forgot profitable loans in a downturn. I emphasize that the tendency must be systematic to distinguish the idea of a credit cycle from the truism that loans made near the peak of an expansion are more likely to go bad simply because bankers (just like economists and other businessmen) have difficulties predicting downturns. What is the evidence for an independent effect for changing bank lending standards that is, a systematic reason why banks might be too lax or too stringent? And what factors might explain this type of behavior? Economists have proposed a number of plausible models of a bank lending cycle, emphasizing changes in bank capital, competition, or herding behavior. To date, only the channel relating changes in bank capital to lending standards has firm empirical support.The available evidence is too weak to give us much confidence in assigning an important role for other theories of bank lending standards. WHAT ARE CREDIT STANDARDS? It is helpful to be a little clearer about what we mean by a change in bank credit standards. Lets begin with a straightforward prescription from investment theory: A profitmaximizing bank should make any loan with a positive net present value (NPV). The NPV of a loan is just the sum of discounted future repayments (principal plus interest) on the loan minus the loan amount. Future repayments must be discounted for two different reasons: First, $10 in the bank now is worth more than $10 paid a year from now. After all, the bank could receive a years interest by purchasing Treasury bills on the $10 paid back tomorrow. Second, the bank recognizes that the borrower may default in the future, so the bank may never receive some future payments. The firm may have a healthy balance sheet at the time the loan is made; a year from now, the borrowing firm may suffer financial setbacks and may be unable to pay back its loan. Using this framework, we can define a change in bank credit standards as a change in a banks loangranting decisions for some reason other than a change in the NPV of the loan. We can define a credit cycle as a systematic tendency to fund negative NPV loans during an expansion and a systematic tendency to reject positive NPV loans during a contraction. Since bankslending decisions also involve the pricing and design of loan contracts, a credit cycle might also take the form of a systematic tendency to relax or tighten loan terms by more than would be justified by changes in borrower risk. Conceptually, it is not too difficult to define a credit cycle. Empirically, it may be much harder to tell whether one has occurred. For example, think about some of the things that happen in an economic downturn. As economic conditions become more difficult, more firms experience economic difficulties and the probability that a firm will default increases. This reduces the NPV of a given stream of repayments and would probably induce the bank to raise the loan rate, impose new contractual restrictions, or refuse to make the loan at all. While these actions might be interpreted as a tightening of standards by an outside observer or by an aggrieved borrower, credit standards havent changed according to our definition. Figures 1a and 1b illustrate the distinction between the effects of a tightening of credit standards and the effects of an increase in credit risk. Figure 1a shows a probability distribution of loan applicants NPVs. The profit-maximizing rule for a bank is to make a loan as long as its NPV is positive (the sum of the shaded regions). If the bank tightens its credit standards, for example, making only loans with an NPV greater than $A, the bank will make a smaller number of loans (just the darker region). Figure 1b illustrates the effects of a downturn: Loans become riskier and the distribution of NPVs shifts to the left. But this figure shows a bank that retains the profit-maximizing rule. Note that the number of loans made falls in this case also (from the sum of the shaded regions to just the darker region). Slightly more subtly, in a downturn many loans often go bad at once. Typically, a bank will charge a borrower a higher loan rate if the borrower is likely t