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The Cultural Brain Hypothesis and the transmission and evolution of culture Muthukrishna, Michael


Humans are an undeniably remarkable species with massive brains, amazing technology, and large, well-connected social networks. The co-occurrence of these traits is no accident. Here, I introduce the Cultural Brain Hypothesis and Cumulative Cultural Brain Hypothesis. The Cultural Brain Hypothesis is a single theory that explains the increase in brain size across many taxa. In doing so, it makes predictions about the relationships between brain size, adaptive knowledge, group size, social learning, and the length of the juvenile period. These predictions are consistent with existing empirical literature, tying together these otherwise disparate measured relationships. The Cumulative Cultural Brain Hypothesis makes predictions about the conditions under which these processes lead to a positive feedback loop between brain size and adaptive knowledge, ratchetting both upward in a co-evolutionary duet. I argue that these conditions, which include a rich environment, low reproductive skew, and high transmission fidelity, are the key to the uniquely human pathway, explaining our large brains and various aspects of our psychology that led to and sustain those large brains. The predictions of these two hypotheses are consistent with other theories within a Dual Inheritance Theory framework – the idea that natural selection led our species to develop a line of cultural inheritance in addition to the line of genetic inheritance shared by all species on Earth. I test many of these theories against competing theories across 4 experiments with human subjects. These experiments include cultural transmission experiments, which support a causal relationship between sociality and cultural complexity, and two social learning experiments, which test the environmental and individual predictors of biased social learning. The overall findings support Dual Inheritance Theory and the Cumulative Cultural Brain Hypothesis. Finally, I lay the groundwork for improving the way in which these theories account for human social structures by proposing a theory to help explain the social structures unique to our species. I then demonstrate the utility of this model by using it to make predictions about the implications of population-level differences in personality and social influence for the transmission and evolution of culture.

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