Economists love to tell each other stories about perverse incentives. The “cobra effect” is a favourite. It describes an attempt by the British Raj to rid Delhi of its cobras by paying a bounty for each cobra skin, thus encouraging a thriving cobra-farming industry. The cobra story is probably an urban myth — or a policy wonk’s version of one — but there is more evidence of a very similar scheme for Hanoi’s rats in the early 1900s. Rat tails brought a bounty from the colonial government, and soon enough the city was crawling with tailless rats who had had their valuable tails clipped before being released to breed.
經濟學家們喜歡互相講述關於反向激勵的故事，其中「眼鏡蛇效應」是他們的最愛。這個故事講述了英屬印度(British Raj)試圖通過爲每一張眼鏡蛇皮支付賞金來清除德里的眼鏡蛇，結果卻催生了一個蓬勃發展的養蛇產業。眼鏡蛇的故事很可能只是一個都市傳說——或者說是政策專家版本的都市傳說——但有更多證據表明，20世紀初在河內曾有過非常類似的老鼠計劃。那時殖民政府懸賞收購老鼠尾巴，結果很快整座城市都爬滿了斷尾的老鼠——它們的寶貴尾巴被剪下後又被放回去繁殖。