經濟學 1968

公地悲劇

加勒特·哈丁

人人可用、無人擁有之物，會被每個理性的人一同用到枯竭。

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In depth · the introduction

為什麼一片公共漁場會被打撈一空，私家魚塘裡的魚卻不會？

核心想法

有些東西屬於所有人、又不屬於任何一個人——全村共用的一塊草場、一處漁場、那片開闊的空氣。哈丁的論斷令人不安：這些共享之物往往會被毀掉，毀掉它們的不是惡人，而是做著合情合理之事的普通人。我若往公共牧場多放一頭牛，這頭牛的好處全歸我；而草被多啃的損耗，卻由大家分攤。於是「再多放一頭」永遠是聰明之舉——既然人人都一樣聰明，草便被啃成了光禿禿的泥地。

他把這稱作「公地悲劇」，並主張它「沒有技術解」：再巧的圍欄或化肥，也修不好一個根植於激勵本身的問題。真正的解藥，都在於改變「誰、以何種方式」取用資源的規則。

它是如何誕生的

哈丁是加州大學聖巴巴拉分校的生態學家。1968 年，他把一場學會主席致辭改寫成一篇短文，發表在《科學》雜誌上。他寫作的背景，是戰後人口的激增與方興未艾的環保運動——瑞秋·卡森的《寂靜的春天》六年前剛剛問世——而他真正憂心的，是人口的增長，他視之為隱在污染與匱乏背後的那個公地問題。牧場，是他讓這套邏輯令人難忘的方式；它奏效了：這一語脫口而出，進入了日常用語，成為現代科學中被引用最多的觀念之一。

它為何重要

它給一個無處不在的問題起了名字、配了圖畫——凡是人們共享某種難以圍起來的東西，這問題就會冒頭：被過度捕撈的海洋、被抽乾的含水層、被污染的河流，以及如今這片人人都可往裡排碳、卻正在變暖的大氣。一旦你看清了這結構，便可去尋藥方——而藥方幾乎從來不是「再努力些」或「別那麼貪」，而是改變規則：所有權、配額、許可，或帶有牙齒的共同約定。

一個可以想像的畫面

想像一家自助餐廳，帳單由在場所有人平攤。你多取的每一盤都美味、且對你幾乎免費——成本被抹勻到整桌人身上。於是人人都點過了頭，食物告罄，共擔的帳單卻越滾越大。要是這頓飯只由你自己買單，你點起來便會有分寸。差別不在你的品性，而在「下一盤由誰付錢」。

它的位置

哈丁的悲觀，是一場漫長爭論的一極。托馬斯·馬爾薩斯（1798，見本館）曾警告：人口的增長會超過資源；哈丁則為污染的時代更新了這份憂慮。爭論的另一極，站著埃莉諾·歐斯壯（1990，也在本館）——她實地走訪，找到了把公地可持續治理了數百年的社群，表明毀滅是一種風險，而非宿命。在亞當·斯密對市場的信念，與「需要國家」的主張之間，公地標出了二者都可能照拂不到的那片地帶。

The original document

Original source text

Garrett Hardin · Science, New Series, Vol. 162, No. 3859 (13 December 1968), pp. 1243–1248 · based on a presidential address to the Pacific Division of the AAAS, Utah State University, Logan, 25 June 1968.

The summary

Hardin opens by arguing that some problems admit no technical fix — no improvement in technique or yield will solve them, because the difficulty is one of values, not of means.

The population problem has no technical solution; it requires a fundamental extension in morality.

The pasture

To show why, he draws a now-famous picture of a grazing common shared by many herdsmen.

Picture a pasture open to all.

Each herdsman, he argues, rationally asks one question — and the arithmetic always points the same way:

What is the utility to me of adding one more animal to my herd?

The gain from the extra animal is his alone (a benefit of nearly +1); the cost — overgrazing — is shared among all the herdsmen (a fraction of −1). So the rational herdsman adds another animal, and another. Since each reasons alike, the conclusion is collective.

Therein is the tragedy. Each man is locked into a system that compels him to increase his herd without limit—in a world that is limited.

Ruin is the destination toward which all men rush, each pursuing his own best interest in a society that believes in the freedom of the commons. Freedom in a commons brings ruin to all.

From pasture to pollution and population

Hardin extends the same logic to pollution (the commons as a sink into which each discharges waste) and, centrally, to human population growth, which he regarded as the commons problem behind all the others. His remedy is not appeals to conscience — which he holds to be self-eliminating — but agreed-upon constraint, “mutual coercion, mutually agreed upon by the majority of the people affected.”

The remedy — and its shadow

Applied to population, Hardin reaches a conclusion many readers have since rejected:

Freedom to breed is intolerable.

(On honesty: Hardin’s later writing on population, immigration and aid carried coercive and, at times, explicitly racist views; this line is quoted not to endorse it but because the essay’s reception cannot be understood without it — see the Expert “limits” section.)

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Garrett Hardin · University of California, Santa Barbara · Science · 13 December 1968