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.
一些父母對此怒不可遏是可以理解的：他們為服務付費（盡管是通過間接的方法），卻忽然發現這種服務要以類似抓鬮的辦法來進行分配。他們房屋可能將失去價值。可能小杰里米甚至根本就沒法去那所非常不錯的學校念書了。但與全國各地的父母一樣，布賴頓市那些失去擇校權利的父母也擔憂同樣一個問題：假如學校允許過多的壞孩子入校，那樣小杰里米的成績將遭到影響。
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.
父母擔憂的問題，正是經濟學家所說的同伴效應。同伴效應是當你整天與一個壞同伴待在一塊時所出現的結果。不過，表明這種效應存在的證據，不像英國父母所想象的那樣多。
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.
難題正在于此。假如杰里米整天與好孩子一塊玩，他的行為舉止就會好，為何呢？顯而易見的講解是，他之所以表現好，是由于他的同伴對他產生了積極的影響，但這就仿佛假設他選擇那些同伴，或是使那些同伴被選中，是由于他也是一個好孩子，這兩種假設都同樣可信。約翰o特里球踢得非常棒，是由于他周圍都是出色球員，還是他身邊都是出色球員，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?
聰明的研究者可以理清其中一些效應。經濟學家布魯斯o薩塞爾多特曾用與醫學研究者用于檢驗某種頭痛新療法同樣的辦法：隨機測試。他發現，達特茅斯學院學生的室友基本上是隨機分配的。學院用的一些選擇依據是性別、是不是抽煙、作息時間--但多數狀況下，宿舍分配是抽簽的結果。
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.
薩塞爾多特發現一種溫和的（從統計數據上看是明顯的）同伴效應。假如分配的室友平均積分點比你高，那樣你一個人的成績會有所提升。假如你室友的GPA成績坐落于分布圖頂端，你的成績總是會比平均水平大約高5%。假如他們的成績比平均水平低20%，那樣你的成績會比平均水平低1%。薩塞爾多特并不了解其中是什么原因，但鑒于學生無權選擇室友，因此這一定是一種真的的同伴效應。
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.
不過，多數有關同伴效應的研究并非很細致。托馬斯o內希巴和杰克o維格多對美國北卡羅萊納州的公立學校進行研究，在此基礎上寫作了一篇頗有見地的論文，其中他們著重強調了這類疏忽之處。他們提出了一些表明同伴效應的、表面上看上去非常有力的證據，但隨后他們證明，這類明顯的效應甚至在同伴出現之前就在起用途了。換言之，通過察看杰里米小學五年級的同學，你可以判斷出杰里米與小學四年級其他同學在一塊的表現。同樣，約翰o特里隊友的水準，表明特里在加入切爾西會所之前，就已經是一名出色的球員。
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.
內希巴和維格多還表示，一旦他們將教育水平原因納入考慮范圍，同伴效應就消失了。擁有聰明同伴的學生，同樣也擁有出色的老師。可能布賴頓市和霍伍市的大家，應該少去擔憂杰里米會遇上不適合的同學，而應該更擔憂他會遇上不適合的老師。
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.