Rational people think at the margin

Economists use the term marginal changes to describe small incremental adjustments to an existing plan of action.In many situations,people make the best decisions by thinking at the margin．

Suppose,for instance,that you asked a friend for advice about how many years to stay in school.If he were to compare for you the lifestyle of a person with a Ph.D.to that of a grade school dropout,you might complain that this comparison is not helpful for your decision.You have some education already and most likely are deciding whether to spend an extra year or two in school.To make this decision,you need to know the additional benefits that an extra year in school would offer (higher wages throughout life and the sheer joy of learning) and the additional costs that you would incur (tuition and the forgone wages while you're in school) .By comparing these marginal benefits and marginal costs,you can evaluate whether the extra year is worthwhile．

As another example,consider an airline deciding how much to charge passengers who fly standby.Suppose that flying a 200-seat plane across the country costs the airline $100,000.In this case,the average cost of each seat is $100,000/200,which is $500.One might be tempted to conclude that the airline should never sell a ticket for less than $500.In fact,however,the airline can raise its profits by thinking at the margin.Imagine that a plane is about to take off with ten empty seats,and a standby passenger is waiting at the gate willing to pay $ 300 for a seat.Should the airline sell it to him? Of course it should.If the plane has empty seats,the cost of adding one more passenger is minuscule.Although the average cost of flying a passenger is $500,the marginal cost is merely the cost of the bag of peanuts and a can of soda that the extra passenger will consume.As long as the standby passenger pays more than the marginal cost,selling him a ticket is profitable．

As these examples show,individuals and firms can make better decisions by thinking at the margin.A rational decision maker takes an action if and only if the marginal benefit of the action exceeds the marginal cost．

理性人考慮邊際量

生活中的許多決策涉及對(duì)現(xiàn)有行動(dòng)計(jì)劃進(jìn)行微小的增量調(diào)整,經(jīng)濟(jì)學(xué)家把這些調(diào)整稱為邊際變動(dòng)。在許多情況下,人們可以通過(guò)考慮邊際量來(lái)作出最優(yōu)決策。

例如,假設(shè)你請(qǐng)教一位朋友,你應(yīng)該在學(xué)校上多少年學(xué)。如果他給你用一個(gè)擁有博士學(xué)位的人的生活方式與一個(gè)完全沒(méi)有上完小學(xué)的人進(jìn)行比較,你會(huì)抱怨這種比較無(wú)助于你的決策。你已經(jīng)受過(guò)某種程度的教育,并要決定是否再多上一兩年學(xué)。為了作出這種決策,你需要知道,多上一年學(xué)所帶來(lái)的額外收益和所花費(fèi)的額外成本。通過(guò)比較這種邊際收益與邊際成本,你就可以計(jì)算出多上一年學(xué)是否值得。

再舉一個(gè)考慮邊際量如何有助于作出決策的例子。考慮一下一個(gè)航空公司決定對(duì)等退余票的乘客收取多高的價(jià)格的例子,假設(shè)一架200個(gè)座位的飛機(jī)橫越國(guó)內(nèi)飛行一次,航空公司的成本是10萬(wàn)美元。在這種情況下,每個(gè)座位的平均成本是10萬(wàn)美元除以200,即500美元。有人會(huì)得出結(jié)論: 航空公司的票價(jià)絕不應(yīng)該低于500美元。但航空公司可以通過(guò)考慮邊際量而增加利潤(rùn)。假設(shè)一架飛機(jī)即將起飛時(shí)仍有10個(gè)空位。而在登機(jī)口等退余票的乘客愿意支付300美元買一張票。那么航空公司應(yīng)該賣給他票嗎? 當(dāng)然應(yīng)該。如果飛機(jī)有空位,多增加一位乘客的成本是微乎其微的。雖然一位乘客飛行的平均成本是500美元,但邊際成本僅僅是這位額外的乘客將消費(fèi)的一包花生和一罐汽水的成本而已。只要等退余票的乘客所支付的錢大于邊際成本,賣給他機(jī)票就是有利可圖的。

正如這些例子說(shuō)明的,個(gè)人和企業(yè)通過(guò)考慮邊際量將會(huì)作出更好的決策。只有一種行動(dòng)的邊際收益大于邊際成本時(shí),一個(gè)理性決策者才會(huì)采取這項(xiàng)行動(dòng)。