Wednesday, December 23, 2015

The Tucker-Drob Bates Meta-analysis

Elliot Tucker-Drob and Tim Bates have published a meta-analysis [fire-walled] of the Scarr-Rowe effect, the observation that the heritability of intelligence is lower in impoverished families.

The reaction to the analysis has been a little strange.  I read the meta-analysis as broadly supportive of the existence of the effect, more details on that below. To the extent I have some personal attachment to the finding, which I admit, I felt good about it.

But to my surprise, the reaction on Twitter sounded as though they had disconfirmed the finding.  I think the first tweet I saw was this one....

Boy did he get it wrong, I thought, and (as I end up regretting every time) shot off a reply:

To my surprise, Tim Bates said in fact, they were right....

What was going on?  Had I completely misread the paper?

It turns out that of this had started up in response to a more general blog post by Stuart Ritchie (I hope to find the time to comment on that whole argument), in which he described the effect size as an "outlier".

Upon further twittering, Stuart agreed that "extreme outlier" (by the other poster) was an overstatement, but still... what exactly did Tucker-Drob and Bates show?  The effect size we reported in 2003 was indeed the largest in the meta-analysis, however:

1) It certainly wasn't an "extreme outlier" and other than being the largest wasn't an outlier at all.  It was reasonably close to the high-side of the other estimates.  Plus, TD&B performed a quantitative test of the null hypothesis that all US effect sizes were drawn from the same population, and failed to reject it.

2) Not only was the average effect size non-zero in the US, it remained non-zero if Turkheimer (2003), or even every study I have had anything to do with, was removed from the analysis.

3)  Although the mean effect size was smaller than what we reported, but it was by no means trivial, in a p<.05 even though it's ridiculously tiny kind of way.  The TD&B graph of the effect looks almost identical to what we reported, with a somewhat less dramatic y-axis.  Still lots of variation in heritability across SES.

4) The whole process here, in which (quasi-, we weren't the first) discovery samples turn out to have unrealistically large sample sizes, is a perfectly normal part of science.  One discovers things because they are big; after that, when everyone is looking for the effect, big or small, they shrink.  The winner's curse.  All one can hope as an early adopter of a finding is that it remains (a) significantly non-zero, and (b) substantively non-trivial, and the Scarr-Rowe interaction accomplished this, at least in the US.

5)  The US vs. Europe part is interesting, but there is a perfectly reasonable environmental basis for speculation about why it would happen, which is that there is greater socioeconomic inequality in the US.  Speculation, yes, but perfectly good fodder for future studies.  The conclusion that it doesn't happen in Europe, by the way, is exactly what I have concluded in writing at least three times.

It doesn't seem to me that TD&B leaves much for everyone to be arguing about.  The interaction happens, with moderate ES, in the US but not Europe. Period.  Why exactly it happens is unknown.  I have already confessed to the somewhat unscientific feeling of commitment I have to the effect.  I think those on the other side, with more of a hereditarian bent than me, should be big enough to admit that they are actively rooting against it, because it threatens their global assumption that genes always predominate.  In fact I think the best thing about the TD & B paper is how even-handed it is.


MACKBOOK123456 said...

It's odd that Britain has some of the highest SES inequality in Europe and still has very little GxE interaction. Perhaps this is perilous to the notion of SES equality being the driver behind IQ differences between class.

Another scholar who writes about this is JayMan, what do you think about his ideas explaining this?

Timothy Bates said...

For those who would like to play with the underlying data, they are all online here:

For those not inside the academic firewall, the paper might be available here

The National Collaborative Perinatal Project PIQ and meta-analytic US estimates are .236 and .076 respectively.

Figure here

Sara De Mulder said...

I am a Belgian phd student and it surprises me that in Europe sessie doesn't seems to created inequality in school results.for example in Belgium massieve inequality in school resultats for children with a migration background even 3th generatie is extreme problematic. We have been notified on this several Times by the OESO my argument is the profound different in socio economic status. And familie support would deal with that as proven by Heckman. This study is now used an argument we are ding well.

Sara De Mulder said...

Sorry for typo's using Phone and Dutch dictionary dominates :)