A theory of IQ: Cognitive profiles and cortical structures

In the last post we presented three mysteries, the relatively small brains of the highly intelligent Ashkenazi Jews, the scientific and economic underperformance of North-East Asian countries relative to their mean IQ (around 105), and the Flynn effect of rising IQ scores during the 20th century.

To arrive at a theory capable of resolving them, we will discuss first the Ashkenazi cognitive profile and then a simplified model of the neocortex.

The Ashkenazi cognitive profile

Freud, Einstein, Marx.

Often most impressive about famous Jewish thinkers is their ability to put a lot of different and seemingly unrelated facts into a big conceptual framework. Sometimes that results in nonsense, sometimes in pure genius, and sometimes it changes the world.

It also seems to be the driver behind the crazy percentage of Nobel prizes in economics collected be Ashkenazim. A slightly uncharitable description would be that Ashkenazim seem to have a unique ability to create just-so stories. Examples for this ability are easy to come by [1],[2],[3].

The IQ profile of Ashkenazim is unusual as well. They excel at mathematical and especially verbal tasks, though they seem to lag slightly behind other Europeans when it comes to visuo-spatial IQ [4].

The biological underpinning of these extraordinary abilities are suggested by the two big genetic disease clusters typical for Ashkenazim, one increases dendritic growth, i.e. the ability of neurons to form synapses with other neurons, the other hampers DNA repair, which is conjectured to also boost the number of synapses created in the brain [5].

This implies that the fast creation of new connections is the driver behind the high Ashkenazi IQ.

A cartoon model of the human neocortex

For the purpose of this blogpost I introduce a very simplified model of the human neocortex, the seat of human intelligence.

The cortex can be viewed as consisting of pattern recognizers, which have three kinds of input connections [6]: The feedforward input that contains the actual pattern uses only 20 percent of the dendritic connections of this pattern recognizer. The remaining 80% are distributed between lateral connections, which provide temporal context and therefore facilitate sequence learning, and feedback connections, which provide overall context.

A theory of IQ

My hypothesis is that the faster creation of lateral synapse boosts the verbal IQ of Ashkenazim, while easier creation of feedback connections allows for the conceptual and analytic excellence that distinguishes Jewish thinkers.

If this hypothesis is correct, the slightly lower verbal IQ of NE-Asians suggests a slightly lower number of synapses being created. An overall lower creation of feedback synapses would also lead to a lower ability to come up with creative conceptual theories. This ability is difficult to test and therefore doesn’t show up in the “lab”. But it is more plausible than verbal IQ to be the essential ingredient for economic and scientific success.

The full correspondence I suggest is that the number of pattern recognizers determines visuo-spatial and mathematical IQ. The number of lateral synapses corresponds to verbal ability and the top down connectivity to the ability to create new high level concepts.

But wait a second, don’t the Ashkenazim have very high mathematical IQ as well? Quite right, and they achieve that by re-purposing part of the visual cortex for higher level patterns. Basically evolution just shuffled IQ points from spatial to mathematical, by reassigning pattern recognizers.

With that last little flourish everything falls nicely into place, the IQ profiles, the brain sizes, the underperformance relative to IQ.

The Flynn effect can now be explained by brain size increase via better nutrition and fewer childhood diseases [7],[8]. This fits perfectly to a puzzling aspect of the Flynn effect, namely that the gains have been enormous on pure pattern recognition tests, like the Raven’s, while IQ tests based on sequence learning, like verbal IQ or digit span have shown little or no Flynn effect.

If the half a standard deviation bigger NE-Asian brains do a standard deviation better on spatial reasoning/maths than European brains, it is not unreasonable to assume that the increase in brain size by more than a standard deviation [7],[8] during the 20th century might account for the whole Flynn effect on the Raven’s which is more than 30 points.

Of course, the brain is lot more complicated than my simple model suggests. The hippocampus, the basal ganglia, the thalamus, etc. all likely play a essential role in human intelligence. The Flynn effect might have several additional causes, especially education seems to be a candidate that shouldn’t be neglected. However, as a very rough overview, this seems to be a useful framework.

[1] Guns, Germs and Steel

[2] Legal systems very different from ours

[3] Unsong

[4] Cognitive Style of Eastern European Jewish Males

[5] Natural History of Ashkenazi Intelligence

[6] Cortical Learning Algorithms

[7] More evidence for increasing brain size over the 20th century

[8] Human brain growth in the 19th and 20th century.

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