鋒利的利他主義的另一端

 原著 Holly Arrow, 韓順生 譯
 (原文: Holly Arrow, The Sharp End of Altruism. Science 26 October 2007:Vol. 318. no. 5850, pp. 581 – 582)
這兩種社會你更喜歡哪一個: 一個社會由自私而寬容的自由交換者組成, 另一個由相互幫助的鬥士組成. 如果你認為利他主義和寬容更有價值, 那麽它們都不完美. 在Choi 和 Bowles用計算機模擬的世界裏, 寬容且利他的社會是少見的 (1). 相反, 利他主義總是和對外敵對相互促進, 而戰爭是這個過程的動力也是結果. 對於 同胞, 利他主義者不怕犧牲, 是 俠客; 而外族人所遇到的是利他主義行為鋒利的另一端.
從進化的角度看, 利他主義是難以理解的. 有人用血緣的利他主義 (2) 和互利的利他主義 (3) 來解釋這個迷. 就是說去幫助有血緣關係的人或者會給回報的人. 兩者都不能解釋在不能得到回報時, 人們還會以很大的代價幫助素不相識的人. 戰爭英雄是一個例子, 用他或他的親屬可以得到間接利益來解釋這種極度的利他主義是很難的.越來越多的研究工作試圖用對個體和族群共同的生存選擇來解釋利他主義.
在這樣多層次的模擬過程中 (4), 進化的結果決定於個體競爭與族群競爭的相對影響. 對個體的選擇促使利他主義的行為從基因庫中消失. 對族群的選擇產生相反的作用, 即相對由更多自私人組成的族群, 由更多利他主義者組成的族群更利於生存. 對於大多數物種, 對個體的選擇更重要. 然而強大的族群選擇力對人類更適用. 有證據表明族群間的暴力殺死了我們人類不少的祖先 (5), 這使人懷疑戰爭是強而有力的族群選擇動力. 由於人們根據是否屬於同一族群在戰爭中相互殘殺, 所以戰爭是很強的候選因素.
在 Choi 和 Bowles 的模擬中, 二十個人群一起生存數千代. 每一個成員有二個基因, 每一個基因有二個類型. 一個基因要麽是寬容 (T), 要麽是 地域性 (P). 另一個基因要麽是利他的 (A) 要麽不是 (N). 除了少數突變, 後代都繼承父母的特征. 利他主義者以自己的代價幫助自己族群的成員. 非利他主義者不會這樣作. 寬容的成員與外族人交換而獲利. 地域性的成員卻不會. 在一個族群中, 高比例的地域性成員會限製所有成員的交換機會.
在所有可能的四種特征組合中, TN是最有利的, 這些自我本位的商人從外族人漁利 並接受利他主義者的捐獻. 犧牲最大的組合是PA. 這些慈善的鬥士捐獻給同袍, 並冒險保護平民, 去給同族的後代征服新的領地. 個體選擇有利於TN組合. 而戰爭的順利則有利於由更多PA成員組成的族群.其他兩個特征的組合是PN 惡霸, 他們即自私又仇視, TA慈善家, 他們與其他人交換, 也捐獻給別人.
在每一代, 這些族群隨即配對, 下麵的結果決定於寬容型成員與鬥士型成員的比例.如果兩個族群都主要由寬容型成員組成, 寬容型成員從交換中獲益.  如果在任何一個族群中寬容型成員不占大多數, 那麽和平交換的可能就急劇下降. 代而替之的是無效的對峙或是戰爭. 如果兩個族群含有相當數目的鬥士, 結果就是對峙. 力量越不平衡, 戰爭的可能就越大. 戰爭可能一方得勝也可能 平分秋色. 一部分鬥士會因戰爭而死亡. 一方勝利後, 另一方的平民也會被殺. 戰後, 勝方大量生育, 後代遷徙到征服的土地上.
這些進化的族群在兩種情況下達到穩定: 或者當自私的商人 (TN) 占絕大多數, 或者當慈善的鬥士 (PA) 占絕大多數. 少數的惡霸 (PN) 和更少的慈善家 (TA) 可以共存於中. 交換的社會是和平的, 而對峙與戰爭在鬥士的社會更常見, 即使不太經常的戰爭, (10% – 20% 的遭遇) 也可以維持相當高度的地方性利他主義. 間斷性戰爭對英雄主義的影響 (6) 同樣表明戰爭作為一個強大的選擇力不需要持續進行.
Choi 和 Bowles 的模擬結果--利他主義與地方主義的結合—是與行為研究的結果相一致的. 比如, 在兩難選擇的實驗中, 如果遊戲中存在多個小組, 自私的選擇就會大大減少 (7). 對外族的存在的意識可以促進利他主義的行為, 這種現象更多見於女性 (8). 這也與戰爭對作為鬥士的男人有更大的選擇壓力相一致. 有意思的是, 女人的利他主義盡管相對不受族群間敵對的影響, 也仍然是很高的. 利他主義的進化途徑在兩性中是不同的.
這個進化途徑的所有因素中應該包括文化的進化. 在另一個研究中, Bowles 等顯示社會規範能通過促進同一而支持利他主義 (9). 在目前的模擬中, 鬥士占多數的族群實施貿易壁壘. 但是這個是預先設置的. 自由貿易能不能勝過獨立主義的地區保護? 懲罰懦夫的規範是否和戰爭與利他主義共同進化?
這個模擬的發現啟示戰爭的遺產是寬容與利他主義間的不相容. 然而, 跨文化的研究使人樂觀. 在一個研究中, 來自十五個社會的人玩捐獻的遊戲 (10). 平均慷慨的程度與一個社會的市場交換總量, 經濟合作相關. 通過在模擬中加入可變的規範, 可以進一步探索寬容的利他主義的社會的潛在活力.
對相互交織的人類的要給與, 要交換, 要攻擊外族人的衝動的理解. 能幫助我們提倡有利於社會的行為, 同時把利他主義的鋒利的另一端放入鞘內.
參考文獻:
1.    J.-K. Choi, S. Bowles, Science 318, 636 (2007).
2.    W. D. Hamilton, J. Theor. Biol. 7, 1 (1964).
3.    R. L. Trivers, Q. Rev. Biol. 46, 35 (1971).
4.    E. Sober, D. S. Wilson, Unto Others: The Evolution and Psychology of Unselfish Behavior (Harvard Univ. Press, Cambridge, MA, 1998).
5.    L. H. Keeley, War Before Civilization; The Myth of the Peaceful Savage (Oxford Univ. Press, New York, 1997).
6.    H. Arrow, O. Smirnov, J. Orbell, D. Kennett, in Conflict in Organizational Teams: New Directions in Theory and Practice, L. Thompson, K. Behfar, Eds. (Northwestern Univ. Press, Evanston, IL, 2007), pp. 113-142.
7.    G. Bornstein, I. Erev, Int. J. Conflict Manag. 5, 271 (1994).
8.    M. Van Vugt, D. De Cremer, D. P. Janssen, Psychol. Sci. 18, 19 (2007).
9.    S. Bowles, J.-K. Choi, A. Hopfensitz, J. Theor. Biol. 223, 135 (2003).
10.    J. Henrich et al., Am. Econ. Rev. 91, 73 (2001).
11.    I thank J. Orbell, K. Henry, and B. H. Rogers for helpful comments, and O. Smirnov and D. Kennett for earlier discussions on altruism and war.



原文: The Sharp End of Altruism
                 Holly Arrow



Which would you prefer: a society of selfish but tolerant freetraders, or a warrior society in which people help one another but are hostile to outsiders? If you value both altruism and tolerance, neither seems ideal. Societies of tolerant altruists, however, are exceedingly rare in the simulation presented by Choi and Bowles on page 636 of this issue (1). Instead, altruism flourishes only in the company of outgroup hostility (parochialism), with war as both the engine of this coevolutionary process and its legacy. For a compatriot, the  altruist who risks his life is a shining knight, whereas the outsider encounters the sharp end of this altruism.

From an evolutionary perspective, altruism--acts that benefit others at a personal cost--is puzzling. Some influential theories that address this puzzle are kin altruism (2), the tendency to help blood relations; and reciprocal altruism (3), the tendency to help people who are likely to return the favor. Neither explains generosity to non-kin when costs are high and reciprocation unlikely. Heroism in warfare is an example. Explaining such extravagant altruism via indirect benefits to altruists and their kin has proved difficult. A growing body of work seeks instead to explain altruism with models that include selection on both individuals and groups.

In such "multilevel" models (4), the evolutionary outcome depends on the relative impact of competing pushes and pulls at individual and group levels. Individual selection pushes counterproductive behaviors like altruism out of the gene pool. Group selection exerts a contrary pull, favoring groups with many altruists over groups of more selfish folk. In most species, individual selection wins out. For humans living in small groups, however, a strong group selection pull is plausible. Evidence that intergroup violence killed a nontrivial proportion of our ancestors (5) has fueled interest in war as a force for robust group selection. War is a strong candidate because people kill each other based on group membership.

In Choi and Bowles' simulation, 20 small groups of agents interact over thousands of generations. Agents have two genes, each with two alleles. They are either tolerant (T) or (P) and either altruistic (A) or not (N). Offspring inherit their parents'traits, with occasional random mutations. Altruists help fellow group members at a personal cost; non-altruists do not. Tolerant agents have lucrative exchanges with outsiders; parochial agents do not. A high proportion of parochials in groups restricts trading opportunities for all.

Among the four possible combinations of traits, TN is the most profitable. These self-interested traders profit both from contact with outsiders and from the donations made by altruists. The most costly combination is PA. These generous warriors make donations and also risk their lives to protect noncombatants and conquer new territory for the group's offspring. Individual selection favors the T and N alleles over the P and A alleles. Victory in war favors groups with more PA types over those with fewer. The other two trait combinations are PN bullies, who are both hostile and selfish, and TA philanthropists, who both trade and donate to others.

In each generation, groups are randomly paired. What happens next depends on the proportions of tolerant types and warriors in the paired groups. If two highly tolerant groups are paired, tolerant members reap the benefits of trade. If the proportion of tolerant types drops below a strong majority in either group, however, the likelihood of peaceful trade plummets. Instead, the groups have either an unproductive standoff or a war. If both groups have the same numbers of warriors, a standoff results. War becomes increasingly likely the greater the imbalance of power, and wars end in a victory or a draw. Some proportion of warriors are killed regardless of outcome. In a victory, however, many civilians on the losing side are also killed, and offspring from a postwar baby boom among the victors migrate into the conquered territory.

The societies that evolve are stable in two conditions: when either selfish traders (TN) or generous warriors (PA) are the dominant type. A few PN bullies and even fewer TA philanthropists can coexist within trader or warrior regimes. The trading regime is peaceful. Standoffs and wars are more common in the warrior regime, but even infrequent war--10 to 20% of encounters--can maintain high levels of parochial altruism. Similar findings for the impact of intermittent war on the evolution of heroism (6) suggest that war need not be "constant" to act as a powerful selective force.

The convergence of altruism and parochialism in Choi and Bowles' simulation is consistent with links between the two found in behavioral studies. Selfish choices in social dilemma experiments, for example, diminish markedly when the game is embedded in an intergroup context (7). The boost in altruism caused by awareness of an outgroup is also more marked among women than men (8), consistent with war exerting stronger selective pressure on males as warriors. Interestingly, altruism levels for women, although relatively unaffected by intergroup hostility, were still high. It appears that the relative importance of alternative evolutionary pathways to altruism may differ for men and women.

A full accounting of such pathways must include cultural evolution. In other work, Bowles and colleagues show how norms can support altruism by promoting conformity (9). In the current simulation, warrior-rich groups enforce a trading ban. However, this norm is predetermined. An obvious extension would be to allow norms to evolve. Can pro-trade norms outcompete more isolationist parochial norms? Do norms that punish cowards naturally coevolve with war and altruism?

The simulation findings suggest that one legacy of war is an inherent tension between tolerance and altruism. Cross-cultural studies, however, provide grounds for optimism. In one study, people from 15 small-scale societies played a donation game (10). Average generosity correlated with the amount of market exchange and economic cooperation typical in the society. By adding mutable norms to the simulation, the potential viability of societies of tolerant altruists could be further explored.

A better understanding of how our impulses to give, to trade, and to attack outsiders are intertwined should help in the quest to promote pro-social behavior while keeping the sharp end of altruism sheathed.