Four errors and a fallacy: pitfalls for the unwary in comparative brain analyses

Abstract

Comparative analyses are the backbone of evolutionary analysis. However, their record in producing a consensus has not always been good. This is especially true of attempts to understand the factors responsible for the evolution of large brains, which have been embroiled in an increasingly polarised debate over the past three decades. We argue that most of these disputes arise from a number of conceptual errors and associated logical fallacies that are the result of a failure to adopt a biological systems‐based approach to hypothesis‐testing. We identify four principal classes of error: a failure to heed Tinbergen's Four Questions when testing biological hypotheses, misapplying Dobzhansky's Dictum when testing hypotheses of evolutionary adaptation, poorly chosen behavioural proxies for underlying hypotheses, and the use of inappropriate statistical methods. In the interests of progress, we urge a more careful and considered approach to comparative analyses, and the adoption of a broader, rather than a narrower, taxonomic perspective.

Evolutionary Lags in the Primate Brain Size/Body Size Relationship

“…For present purposes, and to be conservative, I followed Deaner & Nunn in assigning a group size of N=1 to Daubentonia. However, there are good grounds for believing that this species actually lives in dispersed communities of N»8 (Dunbar & Shultz 2023). If their correct group size is anywhere close to 8, this would place this contrast very close to the star (Homo-Pan contrast), yielding an improved overall linear fit (r 2 =0.622, p=0.004).…”

“…Second, it was not widely appreciated at the time that the social brain data do not in fact form an homogenous set; instead, it consists of four separate, and very distinctive, socio-cognitive (as opposed to taxonomic) grades (Dunbar 1993;Dunbar & Shultz 2021, 2023. Setting conventional regressions through data that have grades falls foul of the Simpson-Yule Paradox (a version of the Ecological Fallacy) (Simpson 1951;Dunbar & Shultz 2023). Because grades increase the error variance in the data, this has the effect of dramatically lowering the slope of conventional least squares regressiona problem that was discussed at some length during the early days of comparative analyses (Mace et al 1981;Harvey & Clutton-Brock 1985;Harvey & Pagel 1988, 1991.…”

“…Because grades increase the error variance in the data, this has the effect of dramatically lowering the slope of conventional least squares regressiona problem that was discussed at some length during the early days of comparative analyses (Mace et al 1981;Harvey & Clutton-Brock 1985;Harvey & Pagel 1988, 1991. Partitioning the data by grades may thus yield very different results (Dunbar & Shultz 2023).…”

“…(Note: the few cases claiming that brain size and group size are uncorrelated all turn out to be statistically flawed: they inadvertently tested hypotheses about constraints, not hypotheses about selection effects. For details see Dunbar & Shultz 2023. ) Here, I reanalyse the Deaner & Nunn data to determine whether a brain lag is evident if we take grades into account.…”

Coleoid Cephalopods Demonstrate Asocial Path to the Evolution of Big Brains

“…Muthukrishna and colleagues attempted to reconcile these disparate and sometimes contradictory 5,23,34 results under a unified formal theoretical framework, the “Cultural Brain Hypothesis” (CBH) 35 . The CBH, following previous work on cultural intelligence 11,36,37 shifted the focus from sociality to learning, both socially and asocially, and development with a particular focus on ecological and social constraints and payoffs to information.…”