Scott Alexander in Slate Star Codex:
Elephants have bigger brains than humans, so why aren’t they smarter than we are?
The classic answer has been to play down absolute brain size in favor of brain size relative to body. Sometimes people justify this as “it takes a big brain to control a body that size”. But it really doesn’t. Elephants have the same number of limbs as mice, operating on about the same mechanical principles. Also, dinosaurs had brains the size of walnuts and did fine. Also, the animal with the highest brain-relative-to-body size is a shrew.
The classic answer to that has been to look at a statistic called “encephalization quotient”, which compares an animal’s brain size to its predicted brain size given an equation that fits most animals. Sometimes people use brain weight = constant x (body weight)^0.66, where the constant varies depending on what kind of animal you’re talking about. The encephalization quotient mostly works, but it’s kind of a hack. Also, capuchin monkeys have higher EQ than chimps, but are not as smart. Also, some birds have lower encephalization quotients than small mammals, but are much smarter.
So although EQ usually does a good job predicting intelligence, it’s definitely not perfect, and it doesn’t tell us what intelligence is.
A new AI Impacts report on animal intelligence, partly based on research by Suzana Herculano-Houzel, starts off here. If we knew what made some animals smarter than others, it might help us figure out what intelligence is in a physiological sense, and that might help us predict the growth of intelligence in future AIs.