Britain has long favoured an odd school system whereby well-to-do parents buy an education at the better state schools by giving money to homeowners who live near those schools, rather than by giving the money to the schools themselves. This is not very satisfactory, and there are two logical responses. One is to let the parents give the money to the schools. The other is to prevent people from buying a place at a good school through the housing market, and instead assign places from a much wider area using a lottery¹. This bold new experiment is about to be tried in Brighton and Hove.

一些家长对此怒不可遏是可以理解的：他们为服务付费（尽管是通过间接的方式），却突然发现这种服务要以类似抓阄的方法来进行分配。他们房子可能将失去价值。或许小杰里米(Little Jeremy)甚至根本就没法去那所很好的学校读书了。但与全国各地的家长一样，布赖顿市那些失去择校权利的家长们也担心同样一个问题：如果学校允许过多的"坏"孩子入校，那么小杰里米的成绩将受到影响。

Some parents are understandably livid: they paid for a service (albeit indirectly) and suddenly discover it's being handed out like a raffle² prize. Their houses will probably lose value. Little Jeremy may not even go to that wonderful school at all. But Brighton's dispossessed parents are also worried by the same thing that worries parents all over the country: that if their school allows too many of the "wrong" type of children in the door, Little Jeremy's performance will suffer.

家长们担心的问题，正是经济学家所说的"同伴效应"(peer effect)。同伴效应是当你整天与一个坏同伴待在一起时所出现的结果。不过，表明这种效应存在的证据，不像英国家长们所想象的那么多。

What these parents are worrying about is what an economist³ would call a "peer effect". Peer effects are what happen when you hang around in the wrong company. Yet the evidence for their existence is slimmer than the nation's parents assume.

难点正在于此。如果杰里米整天与"好"孩子一起玩，他的行为举止就会好，为什么呢？显而易见的解释是，他之所以表现好，是因为他的同伴对他产生了积极的影响，但这就好像假设他选择那些同伴，或是使那些同伴被选中，是因为他也是一个"好"孩子，这两种假设都同样可信。约翰o特里(John Terry)球踢得很棒，是因为他周围都是优秀球员，还是他身边都是优秀球员，是因为他足球踢得棒呢？

The difficulty is this. If Jeremy hangs around with the "right" kids and does well, why? The obvious explanation is that he did well because his peers were a good influence on him, but it is just as plausible⁴ to suggest that he chose those peers, or had those peers chosen, because he was one of the "right" kids, too. Does John Terry play great football because he is surrounded by great footballers, or is he surrounded by great footballers because he plays great football?

聪明的研究者能够理清其中一些效应。经济学家布鲁斯o萨塞尔多特(Bruce Sacerdote)曾使用与医学研究者用于检验某种头痛新疗法同样的方法：随机测试。他发现，达特茅斯学院(Dartmouth College)学生的室友基本上是随机分配的。学院使用的一些选择依据是性别、是否吸烟、作息时间--但多数情况下，宿舍分配是抽签的结果。

Clever researchers can disentangle some of these effects. The economist Bruce Sacerdote used the same technique that medical researchers would use to test a new headache remedy: a randomised trial. He realised that students at Dartmouth College had roommates assigned largely at random⁵. There was some selection at work based on sex, smoking and preferences for hours of work - but mostly, the assignments were the result of a lottery.

萨塞尔多特发现一种温和的（从统计数据上看是明显的）同伴效应。如果分配的室友平均积分点(GPA)比你高，那么你自己的成绩会有所提高。如果你室友的GPA成绩位于分布图顶端，你的成绩往往会比平均水平大约高5%。如果他们的成绩比平均水平低20%，那么你的成绩会比平均水平低1%。萨塞尔多特并不清楚其中的原因，但鉴于学生无权选择室友，因此这肯定是一种真正的同伴效应。

Sacerdote found a modest - and statistically⁶ robust⁷ - peer effect. Being assigned a roommate with a higher grade-point average improves your own. If your roommate is at the top of the grade- point distribution you'll tend to be about 5 per cent better than average. If they are 20 per cent below average you'll tend to be 1 per cent below average. Sacerdote doesn't know what the cause is, but since students did not choose their peers, it must be a genuine peer effect.

不过，多数有关同伴效应的研究并不是非常细致。托马斯o内希巴(Thomas Nechyba)和杰克o维格多(Jake Vigdor)对美国北卡罗莱纳州的公立学校进行研究，在此基础上撰写了一篇颇有见地的论文，其中他们着重强调了这些疏忽之处。他们提出了一些表明同伴效应的、表面上看似很有力的证据，但随后他们证明，这些明显的效应甚至在同伴出现之前就在起作用了。换言之，通过观察杰里米五年级的同学，你可以推断出杰里米与四年级其他同学在一起的表现。同样，约翰o特里队友的水准，表明特里在加入切尔西俱乐部(Chelsea)之前，就已经是一名出色的球员。

Most studies of peer effects are not so careful, however. In a clever paper based on studies of North Carolina's public schools, Thomas Nechyba and Jake Vigdor highlight the pitfalls⁸. They provide what appears to be strong evidence of peer effects - but then demonstrate that these apparent effects are at work before the peers ever appear. That is, by looking at Jeremy's fifth-grade classmates you can work out how Jeremy performed, with different classmates, in the fourth-grade. Similarly, the quality of John Terry's team-mates is a sign that Terry was a good footballer before he joined Chelsea.

内希巴和维格多还表示，一旦他们将教育质量因素纳入考虑范围，同伴效应就消失了。拥有聪明同伴的学生，同样也拥有优秀的老师。或许布赖顿市和霍伍市的人们，应该少去担心杰里米会遇上不合适的同学，而应该更担心他会遇上不合适的老师。

Nechyba and Vigdor also show that the peer effects evaporate once they consider the quality of teaching. Students with smart peers are also students with better teachers. Perhaps the good folk of Brighton and Hove should worry less about Jeremy falling in with the wrong sort of classmate, and more about him falling in with the wrong sort of teacher.