对于我所提出的“乳房的等级多偶制起源假说”（为引用方便起见，我刚给它取了个名字），不少朋友提出的质疑是：“按你的说法，乳房的起源晚于人类社会的等级分化，这实在难以置信。”

这一质疑抓住了问题的要害，确实，我当初提出这一想法时，也感觉到这是该假说最大风险所在，但也正因为如此，这很可能也是该假说的真正价值所在，如果它能引导我们向这个方向探索，并发现许多先前未被察觉的事实，或把许多此前被忽视的凌乱事实组织到一个简单的框架中，那它就有望成为拉卡托斯所说的“进步的纲领”，相反，如果推演开去发现处处碰壁，就该被抛弃。

那么，从我的假说，能推演出些什么呢？下面是我已经想到的几条：
1）等级多偶制的出现远远早于人类走出非洲的时间，我认为它的出现时间不会比智人的成种时间晚很多（如果不是更早的话），我猜想，它大致出现于人类获得成熟的语言能力之后几万年内。对此猜测，许多人感到惊讶，这是因为它与历史学家和社会学家们早先留给我们的印象迥然不同，但是，这些印象所依据的，并未得到实际数据的支持，毋宁说是这些学者基于其理论所做的猜测（和我一样）。
2）影响乳房和其他女性第二性征（比如腰臀比和容貌）的基因，不仅可沿母系遗传，也可沿父系遗传。
3）当我们沿着人口的家谱树向上回溯时，沿父系家谱树回溯的收敛速度，比沿母系家谱树回溯的收敛速度，要快得多，即，分子人类学的Y染色体亚当的年代比线粒体夏娃的年代，要晚得多。这一点已被基因测序的结果所证实（亚当比夏娃晚几万年），稍后我会介绍有关研究。
4）重男轻女的偏好，起源于贵族阶层。

下面着重说明等级多偶制。我说的等级制度，并不需要像日耳曼封建系统那般严格精确，其作为逆向性选择之动力来源，只须满足下列条件：
1）它是等级多偶的，而不是独占多偶的（详见下）；
2）等级地位（无论是财富还是权力）是可继承的，至少多数情况下可望继承一代以上，否则正反馈机制所依赖的隔代优势就不能成立。
3）配偶数量与社会等级地位和财富数量强相关。

我对等级多偶的定义是：一个群体（部落）内，具有稳定的配偶关系，每个男（雄）性按其等级拥有显著不同的配偶数量，这一等级与其在群体内的社会地位或财富数量强相关。与等级多偶相对的另一种多偶制是独占多偶（和等级多偶一样，是我取的名字），即，每个群体内只有一个成年雄性个体的配偶模式，此模式见于海豹、非洲狮、大猩猩和家鸡，等。

人类的多偶制都是且只能是等级化的，因为人类男性需要合作共存于一个群体内，所以不可能是独占多偶，因此对于人类，凡存在多妻制的地方，必有等级多偶，这样，对等级多偶起源时间的考察，就转变成对继承制和多妻制起源时间的考察。

稍后继续……

对于我在关于乳房的帖子里提出的“富贵男性对女性美貌构成选择压力”这一观点，MiniTrue质疑道：

皇帝，拥有享之不尽的雌性资源，预计的后代数量也远高于人均水平。但对于皇帝的女人们来说，即使是宠妃，恐怕这点也没优势。雌性的性选择压力小，五万年的时间绝对不够造成在人类这里看到的重大差别。

宠妃相比于农妇，的确未必会有更多子女（准确的说，第一代优势并不明显），但一旦她有了个儿子，却能为她带来比农妇多得多孙子女（理由你已经知道了），即，第二代优势十分巨大。这就是为什么女性如此渴望（统计学意义上）嫁入富贵之家。这一隔代起效的遗传优势，正是性选择的妙处所在，它构成了“性状-偏爱”的正反馈。

逆向性选择的隔代优势，同时也解释了重男轻女偏好的起源，如果我的解释成立，重男轻女的价值观起源于贵族阶层，理由是：男孩优势，只有对有能力获得大量交配机会并能把足够多孩子养大的贵族，才能发挥其作用，对于穷人，生女孩更保险，女孩好歹能嫁出去，即使嫁个穷光蛋，而穷人家的男孩则很可能沦为光棍。

注：我用了“伯爵”、“贵族”等词汇来表达等级结构中的上层男性，不必拘泥于这些词的狭窄含义。

我原本以为这（非洲单一起源说）已经成为常识，所以未加说明，看来我还是疏忽了。

首先，我说的走出非洲的人类特指现代智人（Homo sapiens，有时候简称现代人）这一物种，智人的祖先直立人（Homo erectus）的某些分支曾与一百多万年前（很可能不止一次）走出非洲，但它们都没在非洲以外留下后代。的确还有一些学者（中国学者居多）仍在坚持现代人多起源说，但他们貌似年纪都不小了（还记得库恩吗？）。

据分子人类学研究，智人在十几万或二十万年前成种之后，大概分两次走出非洲，第一次是约8万年前从埃塞俄比亚跨越红海入阿拉伯半岛东南沿海，沿印度洋北岸——也门-阿曼-伊朗-印度-马来半岛-印尼，最终到达澳洲，这些就是通常所称“棕色人种”，大部分已经灭绝，其较纯粹的后代是澳洲土著和安达曼人；第二次是约5万年前，从东非出发，经苏丹-埃及进入黎凡特地区（即以今以色列为中心的地中海东岸新月形地区），然后在那里出发扩散到整个欧亚大陆，其中东方的一支在一万多年前穿过白令海峡（冰川期中是地峡）进入美洲。

感谢xing_zh提供的资料，如果想完整了解这一题目，可阅读斯宾塞·韦尔斯的著作《出非洲记：人类祖先的迁徙史诗》，网上有中文电子版。另外，李辉的《走向远东的两个现代人种》一文也值得一读，其中一段概述摘录于下：（他估算的第一次出非洲是十万年，我更倾向8万年这个数字）

现在我们可以勾画棕色人种和黄色人种在远东地区的迁徙历史了。
A. 棕色人种大约十万年前走出非洲，至少在五万年之前已经来到东南亚；黄色人种则是大约五万年前的时候开始向东迁徙；
B. 大约四万年前，棕色人种已散布于东北亚到澳洲的广大地区，而黄色人种则进入了东南亚；
C. 大约三万年前，黄色人种在东南亚、印度尼西亚次大陆和中国南方进一步扩张，而棕色人种可能有一部分已经来到美洲；
D. 大约两万年前，黄色人种几乎征服了东亚和东南亚的大部分地区，使该地区的棕色人种锐减；
E. 大约一万年前，黄种人进入美洲，并成为美洲的主宰；
F. 在数千年内，黄种人向中亚、北欧、印度洋和太平洋诸岛扩张，东亚最偏远地区的棕色人种也渐渐与黄种人融合。

我关于活塞运动的表述，说的并非我自己的观点，而只是对罗宾·贝克在《精子战争》中所表达观点的理解。我认为贝克的思想值得推荐，但《精子战争》一书的重点在用理论描绘一个设想世界，而不是对如何得到这个理论的介绍，因而也（至少我看到的中译本）没有列出所依据的参考文献，考虑到这一点，我在去年将其列入我的《动物行为学推荐书目》时，特别强调了“姑且读之，不要太信”。

虽然《精子战争》是通俗作品，但作为曼彻斯特大学的行为学家，罗宾·贝克却是该领域货真价实的专家，我找到了一份他的著作清单：

Sperm Wars: The Science of Sex
Sex in the Future : The Reproductive Revolution and How it Will Change Us
Baby Wars: The Dynamics of Family Conflict
Fragile Science: The Reality Behind the Headlines
Designing the future : the computer transformation of reality
Human navigation and the sixth sense
The Mystery of migration
Fantastic Journeys: The Marvels of Animal Migration
The evolutionary ecology of animal migration
Bird Navigation:The Solution of a Mystery?
Mystery of Migration (A Studio book)

Amazon上有几篇关于书评：Editorial Reviews on Sperm Wars: The Science of Sex

马来西亚行为学家Otto Steinmayer的较长篇书评：The best book on the subject

关于精子竞争的wiki条目：Sperm competition

我在上一篇里介绍了我对人类女性丰满乳房起源的看法，wuxianghong大师不以为然：“社会等级制度至多有几万年历史，其进化学的选择意义有多大呢？” 五万年大约相当于2000代，这在物种分化树上的确很短，但对于种内进化，并不算太短，特别是对于由性选择驱动的单一体表特征，可以说很长了。 性选择是一种基于正反馈机制进化加速器，道金斯在《盲钟表匠》（The Blind Watchmaker）一书第8章（EXPLOSIONS AND SPIRALS）中详细阐述了性选择的正反馈（[[positive feedback]]）机制：当一个性状成为性选择的对象时，不仅这一性状成为雄性的优势，对这一性状的偏爱也成为雌性的优势，并且，这一对“性状-偏爱”相互加速，即，拥有这一性状的雄性获得更多交配机会，而拥有对这一性状之偏爱的雌性更可能得到拥有这一性状的儿子，因而她的儿子们将有更多机会给她带来孙子女，这样，一旦某一性状在择偶中获得微弱的偏爱，便会迅速得到传播。 道金斯指出，性选择常常可以在数十个世代中显著改变一个性状。另外，他还指出，“军备竞赛”（[[arms races]]）——即捕食者与被捕食者之间的进化竞赛——是另一个具有类似效果的正反馈加速器：更好的进攻武器选择出更好的防御装置，反之亦然。 回到乳房问题。富贵男性对美貌女性的性选择，其进化效果类似于人类对家养动物的选育，家养动物因人工饲养条件而部分摆脱了自然选择的压力，因而相比自然条件下能容纳高得多的变异率，从而加速选育过程。一般认为，家犬与狼的分化也只有几万年，看看京八和德国牧羊犬，你就知道选育可以有多快（好玩的是，基因测序显示，京八与狼的亲缘关系，比德牧更近）；同样，贵族为妻子（们）提供的优渥条件也可部分放松自然选择的压力，从而进一步加速性选择的速度。 人类走出非洲不超过8万年，而北欧人与东亚人的分化，也只有大约3万年，但现在欧亚澳美人与非洲人、北欧人和东亚人，其体征差异，已经十分可观，其中，女性乳房的形态，也大异其趣。

abada在最近的帖子里还讨论了“女性丰满的乳房的遗传价值何在”的问题，并提到了Nikolas Lloyd在<进化心理学：为什么女性有乳房>中的一段话。实际上，据我所知，那段话并非Lloyd给出的解释，而是他的一个设问。

乳房问题的确非常困难，我为此困惑了很久，2005年9月，我认为我找到了答案，见<权力与乳房：论男性的第三性征与女性的第二性征>，去年见到Lloyd的文章后我便迅速转载到我文章后面，并评论道：

我感觉Lloyd的贡献在于：全面回顾了乳房问题，澄清了许多错误解释，然后提出了自己的几点很有价值的看法，我认为主要有三点：1）把乳房永久化和排卵期隐藏联系起来是很好的主意，而后者已经有共识。2）把乳房和女性最佳策略“富贵老公+风流情人”联系起来，也很有启发。3）他的“一步翻转”理论挺有意思。弱点是，对乳房最初开始永久化的连续进化优势论证不足，达尔文主义的核心原则之一是连续性，即仅仅证明最终产品有优势是不够的，必须证明进化的连续过程中每一步都有优势。

但遗憾的是，Lloyd仍然把重点放在解释“为什么乳房会吸引男性？”，但在我看来，真正的困难是：“为什么女性需要吸引男性？”——

很多人试图回答“为什么乳房会吸引男性”，却很少追问“为什么女性需要吸引男性”？
这才是关键问题，因为这是件非常例外的事情，动物界很少有雌性吸引雄性的情况，一般都是反过来。目前为止还没看到有人尝试回答这个问题，而在我看来，只有社会等级结构能作出解释。

对这个“真正的困难”，我的回答是：社会等级化和固定配偶家庭，让上层男性有了选择女性的机会，简言之：性选择创造了权力，权力创造了等级，等级创造了逆向性选择，最终创造了丰乳肥臀和美貌。今年7月，我得到了一个更好的表述：

男性控制资源的能力和交配机会的不对称，诱使那些控制大量资源的男性用交配对象的质量换取交配机会的数量，从而引发了逆向性选择。

在动物界，这种不对称很少见，雄性对领地、食物源等的控制和其交配机会是相称的，一只公鸡控制一片领地后，便拥有了该领地内的全部母鸡，但人类不同，因为有了等级结构，一个伯爵可以控制方圆数百里的领地，但也只有几个妻妾，即便最好色的贵族，其情人的数量也只是领地内女性数量的极小部分，所以，伯爵们就会用质量换取数量，选择最佳交配对象，这就导致了逆向性选择。

关于乳房问题的讨论细节，见这个帖子：http://www.vankeweekly.com/Forum/AnswerTopic.aspx?ForumID=37&TopicID=150737

 

abada在最近的帖子里讨论了“男性在性交时的活塞运动是否有遗传价值”的问题，并认为这是精子战争的组成部分，作用在于将其他男性先前留在女性生殖道里的精液泵出去。罗宾·贝克在《精子战争》一书里的确给出了这一解释，但abada的引述不太完整，按贝克的说法，把前男的精液泵出去，不仅降低了前男精子进入卵子的机会（因而增加自己精子进入卵子的机会），同时也因为清理了生殖道粘膜上的精子通道，从而为自己的精子游向卵子增加了便利。

精子战争的一个重要战法是：将精子大军分工为前锋与后卫，前者拼命往前冲，后者慢悠悠走并择机横尸于生殖道粘膜的各处细沟道中，造成堵塞，让后男的精子不易通过。所以我认为，上面第二点更重要。

对“泵出前男精子”的解释，bitstream质疑道：“我怀疑它会把别人的精液送得更深。。。而且，一个女人两次交配间隔的时间有这么短，以至于在第二次时体内还有那么多上一个男人的精液吗？”

对第一点质疑，你仔细看一下女性内生殖器的构造就会释然，子宫在阴道上面，形成一个角度，而且子宫颈口是收敛的，活塞运动不大可能把精液“溅”上去。精子从阴道进入子宫颈只有两个途径：要么自己历经千难万险向上爬过宫颈口，要么当女性高潮时，子宫颈做有节律的升降和张缩运动（贝克叫它“撑伞”运动（tenting）），主动把它蘸吸上去。

对第二点质疑，精子在女性体内的保质期很长，最长可达5天，精子射入后，其最活跃因而最易受孕的状态出现在大约两天之后。女性甚至可以利用这一点，在事后自己制造一次高潮，来提高某男精子进入卵子的机会。

（按：新年伊始用这么一篇来吓唬报业同仁，似乎不太吉利，也不太厚道，呵呵）

报业失去了什么？
辉格
2008年12月29日

上周五，纽约时报集团宣布挂牌出售波士顿红袜棒球队17.5%股份，就在之前两天，它刚刚宣布了近年来最糟糕的业绩，不仅传统的印刷广告收入继续加速下滑，连此前一直在增长的网络广告也开始下降。为了应付收入下滑和现金流枯竭的局面，纽约时报安排了一系列资产出售；早在去年初，它已出售了包括九家电视台在内的广电业务；上月中旬，它抵押了去年刚刚建成启用的总部大楼来换取贷款；在出售红袜队股份之后，它很可能继续出售《波士顿环球报》和About.com。很明显，如果在未来几个季度中无法遏制收入下滑趋势，纽约时报集团将面临与芝加哥论坛报集团同样的破产命运。

近年来，在互联网和新媒体冲击下，美国传统报业早已陷入困境，金融危机或将成为打垮它的最后一击。那么，究竟什么因素使得报业在网络冲击下如此不堪？要知道，并非所有传统媒体都是如此，通讯社和电视网在网络时代就活得很好，甚至还在增长。从这一对比中，我得出的判断是：在印刷媒介到电子媒介的变迁中，必定有一种报业原本拥有的核心资源丧失了。它是什么？

我们每天从外部世界获得的信息，可分为两类，一种是出于我们即时的需要而主动求索的，一种是别人认为我们可能会需要而主动向我们推送的，传统媒体做的便是后一种。一个人每天愿意接受他人推送来的信息的量，十分有限，少于这个量的信息，他会视为享受，而多于这个量，就是负担了。一份报纸要做的，就是在你能接受的那个容量限制下，装进那些最能引起你兴趣的信息；这些信息，它可以从通讯社采购，也可以自己去采编。

如果这份报纸只给你一个人看，那报社就是你的情报顾问；但是多数人雇不起情报顾问，所以报纸必须达到最小发行量来支撑它的采编团队；它能否做到这一点，取决于它能否从成本可行的发行半径内的识字人口中，找出足够大的共同兴趣集合。在报业发展早期，每个城市和地区都会有一批办报人来尝试，在经历过一番淘汰之后，当竞争格局稳定下来，那些成功找到足够大的共同兴趣集，从而突破了最小发行量的报纸，便生存了下来。

一份报纸一旦有了稳定发行量，便获得了这样一种地位：它成了读者了解外部世界的窗口。人们在茶余饭后或坐车等电梯时，翻阅一下报纸，便通过这个窗口获知这世界又发生了些什么；此外，有些人还会订阅一两份杂志，来满足自己的独特信息需求，由于杂志的成本结构对最小发行量的要求更低，因而可以针对更狭窄的共同兴趣。由于受到最小发行量门槛的保护，窗口地位一旦获得便很难撼动，于是窗口成为一种独特的资源，报社可以将其中一部分租给广告商，这构成了其运营收入的大部分。

但是，窗口资源并非恒定不变，每当新技术改变采编、印刷、投送和信息展示等任何一个环节的成本结构时，报业总是会受到冲击。实际上，上世纪互联网发展之前，已经有了两次大冲击。第一次是电视，它开辟了全新的窗口，但报纸凭借信息密度高、可选择性随机阅读和可利用零星时间阅读这三大优势，虽然被电视夺走大块份额，但仍争得了生存空间；第二次是数字化出版和图文传输技术，扩大了报纸的有效投送半径，使得大批地区性报纸难以独立生存，要么萎缩为地方小报，要么依附于全国性报业集团。

这次互联网和新媒体的冲击无疑更加猛烈，新闻门户、内容网站、订阅邮件和RSS阅读器，都在努力成为新的窗口，实时网络新闻在时效性上压过了日报，报纸赖以抵御电视的三大优势中，前两个已经丧失：多媒体和互动技术在表达能力和信息密度上都远胜报纸，选择性和随机阅读也更加灵活，而第三个优势，随着高分辨率智能手机的普及，和手持阅读器的发展，以及未来电子纸的出现，也正在或即将丧失。

旧窗口已经瓦解，新窗口有待建立，报业的未来（如果还有未来的话），必须在新的技术基础、业务模式和成本结构上彻底重建，立志于未来者，应将新媒体视为机会，而非洪水猛兽。

我最初对自由意志这个问题发生兴趣是在看约翰·埃克尔斯的《脑的进化：自我意识的创生》时，去年，Matt Ridley的Genome: the autobiography of a species in 23 chapters（《基因组：一个物种的23章自传》）一书再次激起我的兴趣，该书第22章谈论了这个问题，新浪读书上有该书的节译本，其中包括了第22章的译文。下面是该章原文：（注：关于“休谟之叉”一词究竟指什么，好像有不同说法，所以我暂时放弃这一术语）

CHROMOSOME 2 2  Free Will

Hume's fork: Either our actions are determined, in which case we are not responsible for them, or they are the result of random events, in which case we are not responsible for them.
                     Oxford Dictionary of Philosophy

As this book is being completed, a few months before the end of a millennium, there comes news of a momentous announcement. At the Sanger Centre, near Cambridge - the laboratory which leads the world in reading the human genome - the complete sequence of chromosome 22 is finished. All 15.5 million 'words' (or so - the exact length depends on the repeat sequences, which vary greatly) in the twenty-second chapter of the human autobiography have been read and written down in English letters: 47 million As, Cs, Gs and Ts.

Near the tip of the long arm of chromosome 22 there lies a massive and complicated gene, pregnant with significance, known as HFW. It has fourteen exons, which together spell out a text more than 6,000 letters long. That text is severely edited after tran­scription by the strange process of RNA splicing to produce a highly complicated protein that is expressed only in a small part of the prefrontal cortex of the brain. The function of the protein is, generalising horribly, to endow human beings with free will. Without HFW, we would have no free will.

The preceding paragraph is fictional. There is no HFW gene on chromosome 22 nor on any other. After twenty-two chapters of relentless truth, I just felt like deceiving you. I cracked under the strain of being a non-fiction writer and could no longer resist the temptation to make something up.

But who am 'I'? The I who, overcome by a silly impulse, decided to write a fictional paragraph? I am a biological creature put together by my genes. They prescribed my shape, gave me five fingers on each hand and thirty-two teeth in my mouth, laid down my capacity for language, and defined about half of my intellectual capacity. When I remember something, it is they that do it for me, switching on the CREB system to store the memory. They built me a brain and delegated responsibility for day-to-day duties to it. They also gave me the distinct impression that I am free to make up my own mind about how to behave. Simple introspection tells me there is nothing that I cannot help myself doing. There is equally nothing that says that I must do one thing and not something else. I am quite capable of jumping in my car and driving to Edinburgh right now and for no other reason than that I want to, or of making up a whole paragraph of fiction. I am a free agent, equipped with free will.

Where did this free will come from? It plainly could not have come from my genes, or else it would not be free will. The answer, according to many, is that it came from society, culture and nurture. According to this reasoning, freedom equals the parts of our natures not determined by our genes, a sort of flower that blooms after our genes have done their tyrannical worst. We can rise above our genetic determinism and grasp that mystic flower, freedom.

There has been a long tradition among a certain kind of science writer to say that the world of biology is divided into people who believe in genetic determinism and people who believe in freedom. Yet these same writers have rejected genetic determinism only by establishing other forms of biological determinism in its place - the determinism of parental influence or social conditioning. It is odd that so many writers who defend human dignity against the tyranny of our genes seem happy to accept the tyranny of our surroundings. I was once criticised in print for allegedly saying (which I had not) that all behaviour is genetically determined. The writer went on to give an example of how behaviour was not genetic: it was well known that child abusers were generally abused themselves as children and this was the cause of their later behaviour. It did not seem to occur to him that this was just as deterministic and a far more heartless and prejudicial condemnation of people who had suffered enough than anything I had said. He was arguing that the children of child abusers were likely to become child abusers and there was little they could do about it. It did not occur to him that he was applying a double standard: demanding rigorous proof for genetic explanations of behaviour while easily accepting social ones.

The crude distinction between genes as implacable programmers of a Calvinist predestination and the environment as the home of liberal free will is a fallacy. One of the most powerful environmental sculptors of character and ability is the sum of conditions in the womb, about which you can do nothing. As I argued in the chapter on chromosome 6, some of the genes for intellectual ability are probably genes for appetite rather than aptitude: they set their pos­sessor on a course of willing learning. The same result can be achieved by an inspiring teacher. Nature, in other words, can be much more malleable than nurture.

Aldous Huxley's Brave new world, written at the height of eugenic enthusiasm in the 1920s, presents a terrifying world of uniform, coerced control in which there is no individuality. Each person meekly and willingly accepts his or her place in a caste system - alphas to epsilons - and obediently does the tasks and enjoys the recreations that society expects of him or her. The very phrase 'brave new world' has come to mean such a dystopia brought into being by central control and advanced science working hand­in-hand.

It therefore comes as something of a surprise to read the book and discover that there is virtually nothing about eugenics in it. Alphas and epsilons are not bred, but are produced by chemical adjustment in artificial wombs followed by Pavlovian conditioning and brainwashing, then sustained in adulthood by opiate-like drugs. In other words, this dystopia owes nothing to nature and everything to nurture. It is an environmental, not a genetic, hell. Everybody's fate is determined, but by their controlled environment, not their genes. It is indeed biological determinism, but not genetic determin­ism. Aldous Huxley's genius was to recognise how hellish a world in which nurture prevailed would actually be. Indeed, it is hard to tell whether the extreme genetic determinists who ruled Germany in the 1930s caused more suffering than the extreme environmental determinists who ruled Russia at the same time. All we can be sure of is that both extremes were horrible.

Fortunately we are spectacularly resistant to brainwashing. No matter how hard their parents or their politicians tell them that smoking is bad for them, young people still take it up. Indeed, it is precisely because grown-ups lecture them about it that it seems so appealing. We are genetically endowed with a tendency to be bloody-minded towards authority, especially in our teens, to guard our own innate character against dictators, teachers, abusing step­parents or government advertising campaigns.

Besides, we now know that virtually all the evidence purporting to show how parental influences shape our character is deeply flawed. There is indeed a correlation between abusing children and having been abused as a child, but it can be entirely accounted for by inherited personality traits. The children of abusers inherit their persecutor's characteristics. Properly controlled for this effect, studies leave no room for nurture determinism at all. The step­children of abusers, for instance, do not become abusers.1
The same, remarkably, is true of virtually every standard social nostrum you have ever heard. Criminals rear criminals. Divorcees rear divorcers. Problem parents rear problem children. Obese parents rear obese children. Having subscribed to all of these assertions during a long career of writing psychology textbooks, Judith Rich Harris suddenly began questioning them a few years ago. What she discovered appalled her. Because virtually no studies had controlled for heritability, there was no proof of causation at all in any study. Not even lip service was being paid to this omission: correlation was being routinely presented as causation. Yet in each case, from behaviour genetics studies, there was new, strong evi­dence against what Rich Harris calls 'the nurture assumption'. Studies of the divorce rate of twins, for example, reveal that genetics accounts for about half of the variation in divorce rate, non-shared environmental factors for another half and shared home environ­ment for nothing at all.1 In other words, you are no more likely to divorce if reared in a broken home than the average - unless your biological parents divorced. Studies of criminal records of adoptees in Denmark revealed a strong correlation with the criminal record of the biological parent and a very small correlation with the criminal record of the adopting parent — and even that vanished when con­trolled for peer-group effects, whereby the adopting parents were found to live in more, or less, criminal neighbourhoods according to whether they themselves were criminals.

Indeed, it is now clear that children probably have more non­genetic effect on parents than vice versa. As I argued in the chapter on chromosomes X and Y, it used to be conventional wisdom that distant fathers and over-protective mothers turn sons gay. It is now considered much more likely to be the reverse: perceiving that a son is not fully interested in masculine concerns, the father retreats; the mother compensates by being overprotective. Likewise, it is true that autistic children often have cold mothers; but this is an effect, not a cause: the mother, exhausted and dispirited by years of unre­warding attempts to break through to an autistic child, eventually gives up trying.

Rich Harris has systematically demolished the dogma that has lain, unchallenged, beneath twentieth-century social science: the assump­tion that parents shape the personality and culture of their children. In Sigmund Freud's psychology, John Watson's behaviourism and Margaret Mead's anthropology, nurture-determinism by parents was never tested, only assumed. Yet the evidence, from twin studies, from the children of immigrants and from adoption studies, is now staring us in the face: people get their personalities from their genes and from their peers, not from their parents.1

In the 1970s, after the publication of E .O. Wilson's book Sociobiol­ogy, there was a vigorous counter-attack against the idea of genetic influences on behaviour led by Wilson's Harvard colleagues, Richard Lewontin and Stephen Jay Gould. Their favourite slogan, used as a tide for one of Lewontin's books, was uncompromisingly dogmatic: 'Not in our genes!' It was at the time still just a plausible hypothesis to assert that genetic influences on behaviour were slight or non­existent. After twenty-five years of studies in behavioural genetics, that view is no longer tenable. Genes do influence behaviour.
Yet even after these discoveries, environment is still massively important - probably in total more important than genes in nearly all behaviours. But a remarkably small part in environmental influ­ence is played by parental influence. This is not to deny that parents matter, or that children could do without them. Indeed, as Rich Harris observes, it is absurd to argue otherwise. Parents shape the home environment and a happy home environment is a good thing in its own right. You do not have to believe that happiness determines personality to agree that it is a good thing to have. But children do not seem to let the home environment influence their personality outside the home, nor to let it influence their personality in later life as an adult. Rich Harris makes the vital observation that we all keep the public and private zones of our lives separate and we do not necessarily take the lessons or the personality from one to the other. We easily 'code-switch' between them. Thus we acquire the language (in the case of immigrants) or accent of our peers, not our parents, for use in the rest of our lives. Culture is transmitted autonomously from each children's peer group to the next and not from parent to child - which is why, for example, the move towards greater adult sexual equality has had zero effect on willing sexual segregation in the playground. As every parent knows, children prefer to imitate peers than parents. Psychology, like sociology and anthropology, has been dominated by those with a strong antipathy to genetic explanations; it can no longer sustain such ignorance.2
My point is not to rehearse the nature-nurture debate, which I explored in the chapter on chromosome 6, but to draw attention to the fact that even if the nurture assumption had proved true, it would not have reduced determinism one iota. As it is, by stressing the powerful influence that conformity to a peer group can have on personality, Rich Harris lays bare just how much more alarming social determinism is than genetic. It is brainwashing. Far from leaving room for free will, it rather diminishes it. A child who expresses her own (partly genetic) personality in defiance of her parents' or her siblings' pressures is at least obeying endogenous causality, not somebody else's.

So there is no escape from determinism by appealing to socialisa­tion. Either effects have causes or they do not. If I am timid because of something that happened to me when I was young, that event is no less deterministic than a gene for timidity. The greater mistake is not to equate determinism with genes, but to mistake determinism for inevitability. Said the three authors of Not in our genes, Steven Rose, Leon Kamin and Richard Lewontin, 'To the biological determinists the old credo "You can't change human nature" is the alpha and omega of the human condition.' But this equation - determinism equals fatalism — is so well understood to be a fallacy that it is hard to find the straw men that the three critics indict.3
The reason the equation of determinism with fatalism is a fallacy is as follows. Suppose you are ill, but you reason that there is no point in calling the doctor because either you will recover, or you won't: in either case, a doctor is superfluous. But this overlooks the possibility that your recovery or lack thereof could be caused by your calling the doctor, or failure to do so. It follows that determin­ism implies nothing about what you can or cannot do. Determinism looks backwards to the causes of the present state, not forward to the consequences.

Yet the myth persists that genetic determinism is a more implac­able kind of fate than social determinism. As James Watson has put it, 'We talk about gene therapy as if it can change someone's fate, but you can also change someone's fate if you pay off their credit card.' The whole point of genetic knowledge is to remedy genetic defects with (mostly non-genetic) interventions. Far from the dis­coveries of genetic mutations leading to fatalism, I have already cited many examples where they have led to redoubled efforts to ameliorate their effects. As I pointed out in the chapter on chromo­some 6, when dyslexia was belatedly recognised as a real, and possibly genetic, condition, the response of parents, teachers and govern­ments was not fatalistic. Nobody said that because it was a genetic condition dyslexia was therefore incurable and from now on children diagnosed with dyslexia would be allowed to remain illiterate. Quite the reverse happened: remedial education for dyslexics was developed, with impressive results. Likewise, as I argued in the chapter on chromosome 11, even psychotherapists have found gen­etic explanations of shyness helpful in curing it. By reassuring shy people that their shyness is innate and 'real', it somehow helps them overcome it.
Nor does it make sense to argue that biological determinism threatens the case for political freedom. As Sam Brittan has argued, 'the opposite of freedom is coercion, not determinism.'4 We cherish political freedom because it allows us freedom of personal self­determination, not the other way around. Though we pay lip service to our love of free will, when the chips are down we cling to determinism to save us. In February 1994 an American named Stephen Mobley was convicted of the murder of a pizza-shop man­ager, John Collins, and sentenced to death. Appealing to have the sentence reduced to life imprisonment, his lawyers offered a genetic defence. Mobley came, they said, from a long pedigree of crooks and criminals. He probably killed Collins because his genes made him do it. 'He' was not responsible; he was a genetically determined automaton.

It to be thought that he had none. So does every criminal who uses the defence of insanity or diminished responsibility. So does every jealous spouse who uses the defence of temporary insanity or justifiable rage after murdering an unfaithful partner. So does the unfaithful partner when justifying the infidelity. So does every tycoon who uses the excuse of Alzheimer's disease when accused of fraud against his shareholders. So indeed does a child in the playground who says that his friend made him do it. So does each one of us when we willingly go along with a subtle suggestion from the therapist that we should blame our parents for our present unhappiness. So does a politician who blames social conditions for the crime rate in an area. So does an economist when he asserts that consumers are utility maximisers. So does a biographer when he tries to explain how his subject's character was forged by formative experiences. So does everybody who consults a horoscope. In every case there is a willing, happy and grateful embracing of determinism. Far from loving free will, we seem to be a species that positively leaps to surrender it whenever we can.5

Full responsibility for one's actions is a necessary fiction without which the law would flounder, but it is a fiction all the same. To the extent that you act in character you are responsible for your actions; yet acting in character is merely expressing the many deter­minisms that caused your character. David Hume found himself impaled on this dilemma, subsequently named Hume's fork. Either our actions are determined, in which case we are not responsible for them, or they are random, in which case we are not responsible for them. In either case, common sense is outraged and society impossible to organise.

Christianity has wrestled with these issues for two millennia and theologians of other stripes for much longer. God, almost by defin­ition, seems to deny free will or He would not be omnipotent. Yet Christianity in particular has striven to preserve a concept of free will because, without it, human beings cannot be held accountable for their actions. Without accountability, sin is a mockery and Hell a damnable injustice from a just God. The modern Christian consensus is that God has implanted free will in us, so that we have a choice of living virtuously or in sin.

Several prominent evolutionary biologists have recently argued that religious belief is an expression of a universal human instinct — that there is in some sense a group of genes for believing in God or gods. (One neuroscientist even claims to have found a dedicated neural module in the temporal lobes of the brain that is bigger or more active in religious believers; hyper-religiosity is a feature of some types of temporal-lobe epilepsy.) A religious instinct may be no more than a by-product of an instinctive superstition to assume that all events, even thunderstorms, have wilful causes. Such a super­stition could have been useful in the Stone Age. When a boulder rolls down the hill and nearly crushes you, it is less dangerous to subscribe to the conspiracy theory that it was pushed by somebody than to assume it was an accident. Our very language is larded with intentionality. I wrote earlier that my genes built me and delegated responsibility to my brain. My genes did nothing of the sort. It all just happened.

E. O. Wilson even argues, in his book Consilience,6 that morality is the codified expression of our instincts, and that what is right is indeed - despite the naturalistic fallacy — derived from what comes naturally. This leads to the paradoxical conclusion that belief in a god, being natural, is therefore correct. Yet Wilson himself was reared a devout Baptist and is now an agnostic, so he has rebelled against a deterministic instinct. Likewise, Steven Pinker, by remaining childless while subscribing to the theory of the selfish gene, has told his selfish genes to 'go jump in a lake'.

So even determinists can escape determinism. We have a paradox. Unless our behaviour is random, then it is determined. If it is determined, then it is not free. And yet we feel, and demonstrably are, free. Charles Darwin described free will as a delusion caused by our inability to analyse our own motives. Modern Darwinists such as Robert Trivers have even argued that deceiving ourselves about such matters is itself an evolved adaptation. Pinker has called free will 'an idealisation of human beings that makes the ethics game playable'. The writer Rita Carter calls it an illusion hard-wired into the mind. The philosopher Tony Ingram calls free will something that we assume other people have — we seem to have an inbuilt bias to ascribe free will to everybody and everything about us, from recalcitrant outboard motors to recalcitrant children equipped with our genes.

I would like to think that we can get a little closer to resolving the paradox than that. Recall that, when discussing chromosome 10, I described how the stress response consists of genes at the whim of the social environment, not vice versa. If genes can affect behaviour and behaviour can affect genes, then the causality is circu­lar. And in a system of circular feedbacks, hugely unpredictable results can follow from simple deterministic processes.

This kind of notion goes under the name of chaos theory. Much as I hate to admit it, the physicists have got there first. Pierre-Simon de LaPlace, the great French mathematician of the eighteenth cen­tury, once mused that if, as a good Newtonian, he could know the positions and the motions of every atom in the universe, he could predict the future. Or rather, he suspected that he could not know the future, but he wondered why not. It is fashionable to say that the answer lies at the subatomic level, where we now know that there are quantum-mechanical events that are only statistically pre­dictable and the world is not made of Newtonian billiard balls. But that is not much help because Newtonian physics is actually a pretty good description of events at the scale at which we live and nobody seriously believes that we rely, for our free will, on the probabilistic scaffolding of Heisenberg's uncertainty principle. To put the reason bluntly: in deciding to write this chapter this afternoon, my brain did not play dice. To act randomly is not the same thing as to act freely — in fact, quite the reverse.8

Chaos theory provides a better answer to LaPlace. Unlike quantum physics, it does not rest on chance. Chaotic systems, as defined by mathematicians, are determined, not random. But the theory holds that even if you know all the determining factors in a system, you may not be able to predict the course it will take, because of the way different causes can interact with each other. Even simply deter­mined systems can behave chaotically. They do so partly because of reflexivity, whereby one action affects the starting conditions of the next action, so small effects become larger causes. The trajectory of the stock market index, the future of the weather and the 'fractal geometry' of a coastline are all chaotic systems: in each case, the broad outline or course of events is predictable, but the precise details are not. We know it will be colder in winter than summer, but we cannot tell whether it will snow next Christmas Day.
Human behaviour shares these characteristics. Stress can alter the expression of genes, which can affect the response to stress and so on. Human behaviour is therefore unpredictable in the short term, but broadly predictable in the long term. Thus at any instant in the day, I can choose not to consume a meal. I am free not to eat. But over the course of the day it is almost a certainty that I will eat. The timing of my meal may depend on many things — my hunger (partly dictated by my genes), the weather (chaotically determined by myriad external factors), or somebody else's decision to ask me out to lunch (he being a deterministic being over whom I have no control). This interaction of genetic and external influences makes my behaviour unpredictable, but not undetermined. In the gap between those words lies freedom.

We can never escape from determinism, but we can make a distinction between good determinisms and bad ones - free ones and unfree ones. Suppose that I am sitting in the laboratory of Shin Shimojo at the California Institute of Technology and he is at this very moment prodding with an electrode a part of my brain some­where close to the anterior cingulate sulcus. Since the control of 'voluntary' movement is in this general area, he might be responsible for me making a movement that would, to me, have all the appear­ance of volition. Asked why I had moved my arm, I would almost certainly reply with conviction that it was a voluntary decision. Professor Shimojo would know better (I hasten to add that this is still a thought experiment suggested to me by Shimojo, not a real one). It was not the fact that my movement was determined that contradicted my illusion of freedom; it was the fact that it was determined from outside by somebody else.
The philosopher A. J. Ayer put it this way:9

If I suffered from a compulsive neurosis, so that I got up and walked across the room, whether I wanted to or not, or if I did so because somebody else compelled me, then I should not be acting freely. But if I do it now, I shall be acting freely, just because these conditions do not obtain; and the fact that my action may nevertheless have a cause is, from this point of view, irrelevant.
A psychologist of twins, Lyndon Eaves, has made a similar point:10
Freedom is the ability to stand up and transcend the limitations of the environment. That capacity is something that natural selection has placed in us, because it's adaptive ... If you're going to be pushed around, would you rather be pushed around by your environment, which is not you, or by your genes, which in some sense is who you are.

Freedom lies in expressing your own determinism, not somebody else's. It is not the determinism that makes a difference, but the ownership. If freedom is what we prefer, then it is preferable to be determined by forces that originate in ourselves and not in others. Part of our revulsion at cloning originates in the fear that what is uniquely ours could be shared by another. The single-minded obses­sion of the genes to do the determining in their own body is our strongest bulwark against loss of freedom to external causes. Do you begin to see why I facetiously flirted with the idea of a gene for free will? A gene for free will would not be such a paradox because it would locate the source of our behaviour inside us, where others cannot get at it. Of course, there is no single gene, but instead there is something infinitely more uplifting and magnificent: a whole human nature, flexibly preordained in our chromosomes and idio­syncratic to each of us. Everybody has a unique and different, endogenous nature. A self.

我在<关于自由意志和奴役>一文中，提到了休谟对自由意志问题的诘问，刚刚发现它的原始出处，出自其《人性论》（A Treatise of Human Nature, 1739）第二卷，第三章（论意志与直接情感），第一节（论自由与必然）和第二节（论自由与必然（续）），下面是部分段落（摘自商务印书馆1982年版）：

休谟认为，如果我们能对无生命世界进行因果推断，那就没有理由阻止我们对人的行为也这么做：

……根据经验证明，我们的行为与我们的动机、性情、环境，都有一种恒常的结合……

在人类的行动中，正像在太阳和气候的运行中一样，有一个一般的自然规程。有些性格是不同的民族和特殊的个人所特有的，正如有些性格是人类所共有的一样。我们关于这些性格的知识是建立在我们对于由这些性格发出的各种行为的一致性所作的观察上面的；这种一致性就构成了必然性的本质。

然后休谟回答了一种反对意见：人的行为虽然有原因可循，但其捉摸不定和变化无常的特征无可否认：

我所能想到躲避这个论证的惟一方法，就是否认这个论证所依据的人类行为的一致性。只要各种行为和行为者的境况和性情有一种恒常的结合和联系，那么我们不论如何在口头上不承认必然性，而在事实上就承认这回事了。有人或许会找到一个借口，来否认这个有规则的结合和联系。因为，人类的行为不是最为捉摸不定的么？还有什么比人类的欲望更为变化无常的呢？还有什么动物比人类不但更为违背正常理性，而且更为违背自己的性格和性情的呢？一个小时，一个刹那，就足以使他从一个极端变到另一个极端，就足以推翻他费了极大的辛苦和劳动才确定下来的事情。必然性是有规则的、确定的。人类的行为是不规则的、不确定的。因此，人类行为并不是由必然发生的。

休谟回答是：自然同样捉摸不定、反复无常，但我们却并不因此摒弃因果律：

对于这个说法，我答复说，在判断人类的行为时，我们必须依照我们对外界对象进行推理时所凭借的那些原理。当任何一些现象恒常而不变地结合在一起时，它们就在想像中获得了那样一种联系，以至使想像毫不犹疑地由一个现象转移到另一个现象。不过在此以下还有许多较低极的证据和概然性，而且单独一个相反的实验也不足以完全破坏我们的全部推理。心灵把各种相反的实验互相对消，从多数中减去少数，根据剩下的那种程度的信据或证据进行推理。即使当这些相反的实验的数目完全相等时，我们也不消除原因和必然的概念：我们仍然假设，这种通常的反对是由相反的、秘密的原因的作用而发生的，并且断言，所谓机会或中立性只是由于我们知识的缺陷而存在于判断中间，并不存在于事物自身，事物自身在任何情形下都是一律地必然的，虽然在现象上并不是一律地恒常的或确定的。没有任何一种结合比某些行为与某些动机和性格的结合更为恒常而确定的了；如果在其他情形下那种结合是不确定的，那也不超过于物体的活动方面所发生的情况，而且我们根据心灵活动的不规则性所推出的任何结论，没有一条不可以同样地根据物体活动的不规则性推出来的。

接着，最精彩的一段出现了，这是休谟对自由意志论者的致命诘问，大意是：如果自由意志是因果决定的对立面的话，那么疯子就是最自由的了，因为对他们的行为最难作出因果推断：

疯人们一般被认为是没有自由的。但是如果我们根据他们的行为加以判断，这些行为比理智清楚的人的行为有较小的规则性和恒常性，因而是较为远离于必然性的。因此，我们在这一点上的思想方式是绝对矛盾的，但它只是我们在自己的推理中（尤其是在现在这个问题上）通常所运用的这些胡涂的观念和含混的名词的自然结果。

现在我们必须表明，动机和行为之间的结合既然像任何一些自然活动的结合一样、具有同样的恒常性，所以它在决定我们由一项的存在推断另一项的存在方面对于知性的影响也是一样的。如果这一点显得是对的，那么凡是加入各种物质活动的联系和产生中的任何已知的条件，没有不可以在心灵的一切活动中发现出来的；因此，我们如果认为物质方面有必然性，而认为心灵方面没有必然性，那就不能不陷于明显的矛盾。

最后，（在第二节中）休谟指出，自由意志论的糊涂，源自于对“免于暴力强制的自由”和“中立的自由”（即自由意志）的混淆，我才发现，原来休谟不仅提出了问题，也给出了答案！

我相信，我们可以给自由学说的流行提出下面三个理由，虽然这个学说不论在任何一个意义下都是荒谬和不可理解的。第一，当我们已经完成任何一种行动以后，虽然我们承认自己是被某些特殊观点和动机所影响，可是我们难以说服自己是被必然所支配的，是完全不可能作出另外一种行为的；必然观念似乎涵摄我们所知觉不到的某种力量、暴力和强制。很少有人能够区别自发的自由（如经院中所称）和中立的自由。很少有人能够区别与暴力对立的自由和意味着必然与原因的否定的那种自由。第一种意义甚至是这个名词的最常见的含义；我们所注意保存的既然只有那种自由，所以我们的思想主要就转向了它，而几乎普遍地把它与另一种自由混淆了。

第二，甚至关于中立的自由，人们也有一种虚妄的感觉或经验，并把它作为自由真正存在的论证。不论是物质的或心灵的任何活动的必然性，严格地说，并不是主动因的一种性质，而是可以思考那种活动的任何有思想的或有理智的存在者的性质，并且就是人的思想由先前对象来推断那种活动的存在的一种确定的倾向：正像在另一方面，自由或机会只是那种确定倾向的不存在，只是我们感觉到的一种漠然，可以随意由一个对象的观念转到或不转到另一个对象的观念。现在我们可以说，在反省人类行为时，我们虽然很少感到那样一种漠然或中立，可是通常有这种事情发生就是在完成那些行为本身时，我们感觉到与此类似的某种状况：一切相关的或类似的对象既然都容易被互相混同，所以人们就以这一点作为关于人类自由的一种理证的证明，甚至作为一种直观的证明。我们感觉到，在多数场合下，我们的行动受我们意志的支配，并想像自己感觉到意志自身不受任何事物支配；因为当人们否认这点、因而我们被挑激起来亲自试验时，我们就感觉到意志容易地在每一方面活动，甚至在它原来不曾定下来的那一面产生了自己的意象。我们自己相信，这个意象或微弱的运动，原来可以成为事实自身；因为如果否认这一点，则我们在第二次试验时会发现它能够如此。但是所有这些努力都是无效的；不论我们所能完成的行为是怎样任意和不规则，由于证明我们自由的欲望是我们行动的惟一动机，所以我们就永远不能摆脱必然的束缚。我们可以想像自己感觉到自己心内有一种自由；但是一个旁观者通常能够从我们的动机和性格推断我们的行动；即使在他推断不出来的时候，他也一般地断言说，假如他完全熟悉了我们的处境和性情的每个情节，以及我们的天性和心情的最秘密的动力，他就可以作出这样的推断。而依照前面的学说来说，这正是必然的本质。