Is David Cameron’s proposal to attract the “brightest people” into teaching based upon a logical error? I ask because of this:Finland, Singapore and South Korea have the most highly qualified teachers, and also some of the best education systems in the world, because they have deliberately made teaching a high prestige profession.Let us grant that he’s right on the facts: Finland and South Korea do score highly on the Pisa. But what’s the causality? One possibility is that highly qualified teachers cause pupils to become good. If this is the case, then Cameron’s policy might well be justified.But there’s another possibility. Maybe these countries have a culture in which education is especially highly valued. If so, bright people will want to become teachers, and pupils will do well at school, but there won’t be causality from the former to the latter. Instead, both will arise from that culture.If this is the case, then attracting brighter people into teaching might not raise standards.There’s some empirical evidence here. Research in Sweden has found that teachers with high measured cognitive skills are actually bad for lower-ability pupils. And researchers (pdf) in North Carolina have found that the effect of teachers’ qualifications upon pupil’s achievements are often statistically insignificant, or even negative, once they control for the fact that better-qualified teachers tend to teach better pupils anyway*.This evidence is roughly consistent with Claude’s claim that there’s a “total lack of correlation” between someone’s ability to pass exams at age 21 and their ability to inspire pupils years later. This raises the question. Why, then, does Cameron seem so confident that improving teachers’ qualifications is a good idea?One possibility is that he’s just ignorant about the evidence, and about the difficulties of distinguishing between correlation and causality in social research. Tories do, remember, have a lamentable tendency to display sheer pig-stupidity in interpreting social statistics.The other possibility is that Cameron does know these problems, but just doesn’t care. His policy is designed to win the approval of numpties, not to actually improve school standards. After all, only ghastly oiks go to state schools anyway.* This evidence is quite consistent with Cameron's claim that "good" teachers are very important. It's just that "good" is a matter of experience, inspirational ability and suchlike, rather than mere formal exam passes alone.