好英语网生不生孩子，也许你会受周围的人影响
There is a town in western Japan named Nagi that’s famous for making babies. Its fertility rate in 2021 was 2.68 lifetime births per woman, compared with 1.3 for Japan as a whole, according to an article in The Wall Street Journal that my Opinion colleague Jessica Grose recently cited. Delegations from elsewhere in Japan and abroad have come to Nagi to learn its secret formula. Is it the free medical care for all children? The affordable child care? The cash gifts to new mothers?
在日本西部有一个叫奈义的镇子，以生娃著称。根据我的观点栏目同事杰西卡·格罗斯近日引用过的一篇《华尔街日报》文章，2021年这里的生育率是每名女性生育2.68个孩子，相比之下日本总体的数字是1.3。日本各地乃至海外纷纷派人到奈义取经。是因为所有孩子都有免费医疗吗？成本低廉的保育？刚生育的母亲可以得到现金馈赠？
I’ve been considering another theory. Maybe people in Nagi are having babies because other people in Nagi are having babies. That would be what economists call a “peer effect.” We are social animals and we take our cues from family, friends and sometimes even passers-by. Peer effects could help explain the decline in fertility as well: It could be that in most of the world, people are having fewer babies partly because other people are having fewer babies. That would explain why so many towns and countries are putting in Nagi-like pronatalist measures but not getting Nagi-like results.
我在考虑另一种解释。也许奈义人要孩子是因为其他奈义人要孩子。也就是经济学家所说的“同群效应”。我们是社会性动物，会受家人、朋友、有时甚至包括路人的影响。同群效应也可以解释生育率的下降：可能在世界上大多数地方，生育的减少部分是因为其他人生的孩子数量减少了。这就解释了为什么很多城镇和国家也有跟奈义一样的生育激励措施，但没能产生奈义那样的效果。
Researchers have claimed to find peer effects on obesity, smoking and drinking, so it’s plausible that they influence fertility. We all know of siblings, in-laws and friends who have their first children around the same time. George Akerlof, a Nobel laureate economist from the University of California, Berkeley, wrote in a 1997 paper in the journal Econometrica that “social decisions — such as the demand for education, the practice of discrimination, the decision to marry, divorce and bear children, and the decision whether or not to commit crimes — are not simple choices based primarily on individual considerations.”
研究人员称在肥胖、吸烟和饮酒方面都能找到同群效应，因此影响生育也说得通。我们都认识某些同胞、姻亲和朋友是差不多同时生下第一个孩子的。加州大学伯克利分校经济学家、诺贝尔奖得主乔治·阿克洛夫在1997年发表于《计量经济学》的一篇论文中说，“社会性决策”——比如教育需求、歧视举动、是否结婚、离婚和要孩子、是否犯罪——并非主要基于个人考量的简单抉择。
I came across two problems as I looked into peer effects, though. One is that it’s not clear what to do about them. Let’s say there are two equilibria, one low-fertility and one high-fertility, and both are the result of peer effects. How would a government planner or a church or a nonprofit or anyone else flip a society from the low-fertility equilibrium to the high-fertility equilibrium?
不过在对同群效应的研究中，我遇到了两个问题。一是不太确定该如何对待这种效应。假设有两种平衡——低生育率和高生育率，都是同群效应的结果。那么政府规划者或教会或非营利组织或任何别的什么人该如何把社会从低生育率平衡切换到高生育率平衡呢？
Another problem is whether peer effects are real. Other explanations for high or low fertility are also plausible, after all. Maybe friends behave the same way because they’re alike, not because one influences the other. Or maybe they’re all influenced by some outside factor rather than each other. Dozens of such factors have been suggested: Working parents don’t get the support they need. Young people can’t afford a dwelling that’s big enough for children, or they’re saddled with student debt. The opportunity cost of having children is higher for people with higher education and better earnings prospects. Or maybe women simply can’t find suitable mates.
还有一个问题是同群效应是否真的存在。毕竟还有一些理论也能对生育率的高或低给出合理的解释。也许朋友之间有类似的行为是因为他们本来就是相似的人，并非对彼此有什么影响。还有许多类似的因素被提出来过：在工作的父母没有得到充分的支持。年轻人无力负担足够容纳孩子的居住空间，或被学生债所累。受教育程度越高，收入前景越好，要孩子的机会成本就越高。还有也许根本只是女性找不到合适的伴侣。
In studying peer effects, “The scope for spurious correlation in peer analysis is wide,” Joshua Angrist, an economist at the Massachusetts Institute of Technology who is also a Nobel laureate, warned in 2014 in an article in the journal Labour Economics titled “The Perils of Peer Effects.”
麻省理工学院经济学家、诺贝尔奖得主约书亚·安格里斯特在2014年发表在《劳动经济学》(Labour Economics)杂志上的论文《同群效应的危害》中警告，在研究该效应时，“同群分析中很容易出现假性相关。”
Scholars who aren’t careful can get caught going around in circles. If there is a peer effect, the same women causing it are also being affected by it, making causality almost impossible to isolate. As Angrist put it, there needs to be “a clear separation between the subjects of a peer effects investigation and the peers who provide the mechanism for causal effects on these subjects.” (Here’s a link to an early working paper version.)
学者们一不小心就有出现原地转圈的情况。如果存在同群效应，造成这种效应的女性同样也会受到这种效应的影响，这使得因与果几乎不可能区分开来。正如安格里斯特所说，需要“明确区分同群效应调查的对象，和为这些对象提供因果效应机制的同伴”。（这里是一个早期工作底稿版本的链接。）
Even scholars who avoid that circularity can run into trouble determining the direction of causality. One solution is the instrumental variables approach, of which Angrist is a master, as I wrote in 2021 after he shared the Nobel. To study the relationship between military service during the Vietnam War and subsequent earnings, he looked at draftees’ lottery numbers. Men with low numbers were more likely to serve, and there was no risk that people who drew low numbers were systematically different from those who drew high ones.
即使是避免了转圈的学者，在确定因果关系的方向时也会遇到麻烦。一个解决方案是使用工具变量，安格里斯特是这方面的大师，正如我在2021年的一篇文章中写的那样，当时他刚获得了诺贝尔奖。为了研究越南战争期间的兵役与随后收入之间的关系，他研究了应征入伍者的抽签号码。号码越小的人越有可能入伍，而且抽到小号码者与抽到大号码者之间不存在系统性差异的风险。
Angrist wrote me an email on Thursday to explain why he’s cautious about asserting peer effects: “People who live, work, and study near one another — peers — tend to be similar. But there are all sorts of reasons for that, few, if any, causal. For instance, my neighbors on both sides and I all drive late-model German luxury cars. But I don’t think my (or their) peer pressure is to blame for this. In fact, they got theirs before I even moved in.”
安格里斯特周四给我写了一封电子邮件，解释了他为何对同群效应的主张持谨慎态度：“在一起生活、工作和学习的人——同群者——往往是相似的。但这背后有各种各样的原因，就算其中存在因果关系，也是很少的。例如，我和我的左邻右舍都开最新款的德国豪车。但我不认为我或他们的同群压力是造成这种情况的原因。事实上，他们在我搬进来之前就有了这样的车。”
I read several papers on peer effects on fertility with Angrist’s caveats in mind. One, by Jason Fletcher and Olga Yakusheva, looked at American teenagers and found that a 10 percentage point increase in pregnancies of classmates is associated with a 2 to 5 percentage point greater likelihood of a teenager herself becoming pregnant.
带着安格里斯特的警告，我读了几篇关于同群效应对生育影响的论文。其中一项研究是由杰森·弗莱彻和奥尔加·雅库舍娃进行的，他们对美国青少年进行了研究，发现同学怀孕率每增加10个百分点，青少年自己怀孕的可能性就会增加2到5个百分点。
Disentangling causality is “a really hard problem,” Fletcher, an economist at the University of Wisconsin’s La Follette School of Public Affairs, told me. He and Yakusheva, who is at the University of Michigan School of Nursing, tried to identify causality by using a trick similar to the one Angrist used to study the impact of military service on veterans’ earnings. They needed a factor that was correlated with a girl’s likelihood of becoming pregnant, but with no risk that the factor could be caused by the girl (reverse causality). They found two such factors, called instrumental variables: whether her classmates began to menstruate early and whether the classmates were themselves children of teenage mothers. Those factors made the classmates more likely to become pregnant, which in turn influenced the fertility outcomes of the girls being studied.
弗莱彻是威斯康星大学拉福莱特公共事务学院的经济学家，他告诉我，理清因果关系是“一个非常困难的问题”。他和在密歇根大学护理学院工作的雅库舍娃试图确定因果关系，使用的方法与安格里斯特研究兵役对退伍军人收入影响时差不多。他们需要一个与女孩怀孕可能性相关的因素，但该因素不可能是由女孩造成的（反向因果关系）。他们发现了两个这样的因素，被称为工具变量：她的同学是否很早就开始来月经，以及这些同学自己是不是母亲在十几岁时生下的。这些因素使这些同学们更有可能怀孕，这反过来又影响了被研究女孩的生育结果。
Amalia Miller at the University of Virginia is one of four scholars who studied fertility in workplaces in Denmark. If two women in an office got pregnant around the same time, it was hard to tell if A influenced B or B influenced A. They came up with the idea of looking at the fertility of A’s sister, realizing that if A’s sister got pregnant, A herself was more likely to get pregnant. That was their instrumental variable. Miller and her research partners found that for more-educated women, a colleague of the same education level having a baby increased their own chance of having a baby. Less-educated women were less likely to have a baby when a colleague of the same education level had one. The negative peer effect for less-educated women “could come from a desire to distinguish oneself from one’s peers,” among other factors, they speculated.
弗吉尼亚大学的阿玛莉亚·米勒是研究丹麦职场生育率的四位学者之一。如果一个办公室里的两个女人同时怀孕了，很难说是A影响了B还是B影响了A。她们想到要看A的姐妹的生育情况，意识到如果A的姐妹怀孕了，A自己更有可能怀孕。这是她们的工具变量。米勒和她的研究伙伴发现，对于受教育程度更高的女性来说，如果一位具有相同教育水平的同事生了孩子，她们自己生孩子的几率就会增加。对于受教育程度较低的女性来说，当相同教育程度的同事生了孩子，她生孩子的可能性较小。他们推测，受教育程度较低的女性的负面同群效应有多个因素，其中包括“可能是出于渴望让自己在同群者中显得不同”。
The third person I interviewed is Nie Peng, a professor at Xi’an Jiaotong University in Xi’an, China. He and two co-authors studied Chinese women ages 18 to 49. They found that an increase in the fertility of peers reduced the probability that women would want only one child and increased the probability that they would want three, four or more children. The research “provides support for the role of social norms in the fertility choices of reproductive-aged Chinese women,” they wrote.
我采访的第三个人是在中国西安交通大学教授聂鹏。他和两位合著者研究了18至49岁的中国女性。他们发现，同群者生育数量的增加会降低女性只想要一个孩子的可能性，并增加她们想要三个、四个或更多孩子的可能性。他们写道，这项研究“为社会规范在中国育龄女性生育选择中起到的作用提供了支持”。
Nie told me he read Angrist’s paper years ago and kept its caveats in mind as he designed his team’s research. In any case, he said, peer effects aren’t the whole story. Even though Chinese authorities desperately want to raise the nation’s birthrate, it’s still hard for young families to raise children, he said. He said he and his wife have one child, a 5-year-old girl. When he picks her up from kindergarten, he said, there’s a long line — of grandparents, because all the other parents have to work.
聂鹏告诉我，他多年前读过安格里斯特的论文，并在为他的团队设计研究时牢记其中给出的注意事项。他说，无论如何，同群效应并不能解释一切。他说，尽管中国当局迫切希望提高国家的出生率，但年轻家庭养育子女仍然很困难。他说他和他的妻子有一个孩子——一个五岁的女孩。他说，去幼儿园接孩子时，排着长队的是祖父母，因为父母们都必须工作。