Sunday, October 18, 2015

Larger brains do not lead to high IQs

One has to laugh.  I have often noted that people reporting research results tend to conclude what they want to conclude rather than what the data concerned actually shows.  I had a lot of fun reporting such cases in my own years as an active researcher.  See here. The heading above, taken from MedicalXpress, is another case in point.

It is politically correct.  Leftists don't like to think that there is ANYTHING inborn or hard-wired in human beings.  And, as a consequence, many of them reject ANY physical basis for IQ. And the heading above is just such a rejection. One problem: The findings they were allegedly summarizing in fact showed a stable and statistically significant correlation between brain size and IQ!

So what is going on?  Before I comment further, I reproduce the underlying journal abstract.

Meta-Analysis of Associations Between Human Brain Volume And Intelligence Differences: How Strong Are They and What Do They Mean?

Jakob Pietschnig et al.


Positive associations between human intelligence and brain size have been suspected for more than 150 years. Nowadays, modern non-invasive measures of in vivo brain volume (Magnetic Resonance Imaging) make it possible to reliably assess associations with IQ. By means of a systematic review of published studies and unpublished results obtained by personal communications with researchers, we identified 88 studies examining effect sizes of 148 healthy and clinical mixed-sex samples (> 8,000 individuals). Our results showed significant positive associations of brain volume and IQ (r = .24, R² = .06) that generalize over age (children vs. adults), IQ domain (full-scale, performance, and verbal IQ), sample type (clinical vs. healthy sample), and sex. Application of a number of methods for detection of publication bias indicates that strong and positive correlation coefficients have been reported frequently in the literature whilst small and non-significant associations appear to have been often omitted from reports. We show that although the positive association of brain volume and IQ seems to be robust, its strength has been overestimated in the literature. While it is tempting to interpret this association in the context of human cognitive evolution and species differences in brain size and cognitive ability, we show that it is not warranted to interpret brain size as a necessary cause or isomorphic proxy of human intelligence differences.


Got that?  They found that there was an association between the two variables but it was weak.  In other words, brain size was only one of the factors influencing IQ -- a conclusion which would surprise no-one familiar with the field.  Cortical complexity would obviously be a much more major influence. And cortical size considered independently of overall brain volume would also be a very promising variable for study.

The correlation (.24) IS weaker than we would expect from prior studies of head size.  Head size normally correlates over .3 with IQ.  One of the authors of this study was however the ultra-cautious Dutch psychologist Jelke Wicherts.  You can't put anything past him!  So he has ensured extensive allowance for confounding factors etc. So the figure reported above might reasonably be treated as a lower bound.  ALL correlations seem to shrink and all IQs seem to rise when the distinguished Prof. Wicherts gets hold of them! Wicherts is an Associate Professor in Psychological Methods specializing in errors with statistics and the measurement of intelligence (IQ).

The much-published Jelte Wicherts

And as further context, let me note that in psychological research generally, a correlation of .24 would be greeted with frabjous joy. My recollection is that a majority of correlations in psychological research are around that magnitude.  To have detected ANY effect of one variable on another is normally felt worthy of congratulation. And I am not entirely being mocking in saying that.  Any item of human behaviour is bound to be multi-causal so any one influence on the behaviour concerned MUST usually be small.  The behaviour will be the product of many influences, not one.

A final note:  The heading of the MedicalXpress article is not the only bit of political correctness in it.  They also state that average male and female IQs are the same. The wicked Richard Lynn's very extensive study of the data showed that women are in fact down by a couple of points. 

That's not the important point about female IQ, however. The smaller variability (SD) of female IQ has long been known and is well-accepted.  And that restricts female achievement at the top of the range.  I had better not spell that out further in case I get prosecuted for hate speech.  You can get away with saying things in academic language that would land you in trouble if you said  them in plain English -- JR

No comments: