Correlates the sizes between an species' brain and their social group, indicating  a maximum size for how large a social group an individual can maintain. (For humans, it is 150, which is consistent with many functional hunter-gatherer groups and military units). 

The Social Brain Hypothesis

• A large brain aids in the managing of complex social relationships between conspecifics ("the social brain hypothesis"). 
• A large brain is fostered by the development of new skills and innovation that increase survivability (namely: innovations in behavior, use of tools, and social learning). 

Villanova University

Environmental demands that influence an organism's survivability. Can be positive (beneficial) or negative (detrimental). 

Selection Pressures 

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

• Developing a large brain requires a long, arduous gestation period followed by a laborious birthing, complicated by the incompatibility of an infant's large head and a woman's narrow pelvis region (which is necessary to stand on two feet). 

• Brain development continues years after birth, extending a human infant's vulnerability and dependency on their parents. 

• The brain demands a proportionately larger chunk of our energy, even when at rest, than other parts of our body. 

• The brain is maintained by more than half our genes, which are particularly vulnerable to errors, and can result in behavioral disorders. 

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