An anonymous commentator on the post “Bigger brains cause Flynn effect” said as follows:
So we are supposed to believe that digit span is plummeting, color perception is getting worse, writing is being dumbed down for everyone, our reaction times will soon be so bad we will be unable to play Doom - and also that brain sizes are increasing and this explains the Flynn effect and the effect is genuine rather than hollow?
Michael Woodley of Menie replies:
Bigger brains are not necessarily more efficient.
There is a considerable amount of evidence indicating that brain size and general intelligence (g) are actually quite independent from one another. In humans it has been found that the evolutionary lability of brain volume is low, relative to what would be expected of g (Miller & Penke, 2007). This suggests that different selection pressures operate on each trait. The best evidence for this comes from the observation that there has been a general tendency towards decreasing brain volume in human populations over the last 30,000 years (Beals et al., 1984), despite molecular indications of increases in g over the same time period (Rindermann et al., 2012). This finding generalizes in cross-species comparisons involving primates – brain size appears to be relatively phylogenetically conserved across taxa, whereas g (or G as we are talking about species differences) shows signs of having been subject to accelerating directional selection (Fernandes & Woodley of Menie, 2014).
Selection for g therefore seems to be associated with increases in the efficiency of neural substrates, which does not require an increase in overall brain mass, and can even occur in tandem with reductions in brain mass. Several causes of this have been proposed, to my mind however the most plausible explanation is that it is a simple matter of thermodynamics – the boosting of g via a relatively low investment into streamlining the brain’s neural architecture rather than via a high investment increasing of its bulk is the path of least resistance in terms of energetics.
It is important to note also that brain structure does not exhibit the property of measurement invariance across individual humans – large brains are not simply scaled up versions of smaller ones (e.g. Semendeferi et al., 2011). This finding is consistent with the observation that g-loading is only a weak to modest moderator of the strength of the association between brain volume and measures of cognitive ability (Woodley of Menie et al., in preparation), as it indicates that volume-related changes in the structural organization of the brain have substantial effects on specialized cognitive abilities, not just g. It also helps explain the recent ‘downgrading’ of the magnitude of the brain volume-IQ correlation to .24 (Pietschnig et al., 2015), as much of the mass of the brain is associated with functions that are completely unrelated to cognition, but can nonetheless vary considerably from individual to individual.
The relative independence of brain size from g can therefore plausibly account for the co-occurrent secular trends towards decreasing neurological efficiency (suggesting decreasing g) on the one hand coupled with indications of increasing brain mass. I do not anticipate that the larger brains of modern populations are structured in the same way as the relatively smaller brains of a century ago. I predict that most of the mass increase will have been driven by disproportionate investments of grey matter into features of the brain responsible for memory and learning, such as the right hippocampal formation, the right dorsolateral prefrontal cortex, the cerebellum and the septum pellucidum. The net of this will be modern brains that, whilst less generally efficient, are nonetheless better able to develop the sorts of g-independent specialized abilities latent in the Flynn effect
Beals, K.L., Smith, C.L., & Dodd, S.M. (1984). Brain size, cranial morphology, climate, and time machines. Current Anthropology, 25, 301–330.
Fernandes, H. B. F., & Woodley of Menie, M. A. (2014, June). Differences in
evolutionary patterns and rates of general cognitive ability compared to
neuroanatomical indicators in the primate phylogeny. Oral presentation, Evolution
2014, Raleigh, NC.
Miller, G.F., & Penke, L. (2007). The evolution of human intelligence and the
coefficient of additive genetic variance in human brain size. Intelligence, 35, 97–114.
Pietschnig, J., Penke, L., Wicherts, J.M., Zeller, M., & Voracek, M. (2015). Meta-analysis of associations between human brain volume and intelligence differences: How strong are they and what do they mean? Neuroscience and Biobehavioral Reviews, 57, 411–432.
Rindermann, H., Woodley, M.A., & Stratford, J. (2012). Haplogroups as evolutionary markers of cognitive ability. Intelligence, 40, 362–375.
Semendeferi, K., Teffer, K., Buxhoeveden, D.P., et al. (2011). Spatial organization of
neurons in the frontal pole sets humans apart from great apes. Cerebral Cortex, 21,
Woodley of Menie, M.A., Fernandes, H.B.F., te Nijenhuis, J., & Metzen, D. (In preparation). Modest support for the processing volume theory of general intelligence: A meta-analysis of the Jensen effect on brain volume (under review).