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.
一些父母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法），却忽然发现这种服务要以类似抓阄的办法来进行分配。他们房屋可能将失去价值。可能小杰里米甚至根本就没法去那所非常不错的学校念书了。但与全国各地的父母一样，布赖顿市那些失去择校权利的父母也担忧同样一个问题：假如学校允许过多的坏孩子入校，那样小杰里米的成绩将遭到影响。
Some parents are understandably livid: they paid for a service 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.
父母担忧的问题，正是经济学家所说的同伴效应。同伴效应是当你整天与一个坏同伴待在一块时所出现的结果。不过，表明这种效应存在的证据，不像英国父母所想象的那样多。
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.
难题正在于此。假如杰里米整天与好孩子一块玩，他的行为举止就会好，为何呢？显而易见的讲解是，他之所以表现好，是由于他的同伴对他产生了积极的影响，但这就仿佛假设他选择那些同伴，或是使那些同伴被选中，是由于他也是一个好孩子，这两种假设都同样可信。约翰o特里球踢得非常棒，是由于他周围都是出色球员，还是他身边都是出色球员，是由于他足球踢得棒呢？
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?
聪明的研究者可以理清其中一些效应。经济学家布鲁斯o萨塞尔多特曾用与医学研究者用于检验某种头痛新疗法同样的办法：随机测试。他发现，达特茅斯学院学生的室友基本上是随机分配的。学院用的一些选择依据是性别、是不是抽烟、作息时间--但多数状况下，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。假如分配的室友平均积分点比你高，那样你一个人的成绩会有所提升。假如你室友的GPA成绩坐落于分布图顶端，你的成绩总是会比平均水平大约高5%。假如他们的成绩比平均水平低20%，那样你的成绩会比平均水平低1%。萨塞尔多特并不了解其中是什么原因，但鉴于学生无权选择室友，因此这一定是一种真的的同伴效应。
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非很细致。托马斯o内希巴和杰克o维格多对美国北卡罗莱纳州的公立学校进行研究，在此基础上写作了一篇颇有见地的论文，其中他们着重强调了这类疏忽之处。他们提出了一些表明同伴效应的、表面上看上去非常有力的证据，但随后他们证明，这类明显的效应甚至在同伴出现之前就在起用途了。换言之，通过察看杰里米小学五年级的同学，你可以判断出杰里米与小学四年级其他同学在一块的表现。同样，约翰o特里队友的水准，表明特里在加入切尔西会所之前，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，一旦他们将教育水平原因纳入考虑范围，同伴效应就消失了。拥有聪明同伴的学生，同样也拥有出色的老师。可能布赖顿市和霍伍市的大家，应该少去担忧杰里米会遇上不适合的同学，而应该更担忧他会遇上不适合的老师。
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.