The encephalization quotient (EQ) is a number that describes how much bigger or smaller an animal’s brain is compared to what you’d expect for its body size. A human EQ of about 7 means our brains are seven times larger than predicted for a mammal of our weight. The concept was developed by neuroscientist Harry Jerison, who argued that raw brain size alone is meaningless without accounting for body size, since larger animals naturally need larger brains just to manage basic body functions like movement and organ regulation.
How EQ Is Calculated
The core idea behind EQ is that brain size doesn’t scale one-to-one with body size. A whale isn’t thousands of times smarter than a mouse just because its brain is thousands of times heavier. Instead, brain size follows a predictable curve: as body size increases, brain size increases too, but at a slower rate. This pattern is called allometric scaling.
Jerison’s formula captures this by comparing an animal’s actual brain mass to the brain mass predicted by the curve:
EQ = brain mass / (0.12 × body mass0.67)
The 0.12 is a scaling constant derived from averaging across mammals, and the 0.67 exponent reflects the rate at which brain size increases relative to body size. An EQ of 1.0 means a species has exactly the brain size you’d predict. Above 1.0 means a bigger brain than expected; below 1.0 means smaller. A cat, for instance, lands right at 1.0. Dogs come in at 1.2, elephants at 1.3, and bottlenose dolphins at 5.3.
EQ Values Across Species
Humans sit at the top of the EQ scale among mammals, with values around 7.4 to 7.8. That enormous number reflects the fact that our roughly 1,400-gram brains are far larger than the 200 or so grams you’d predict for a primate of our body weight. Dolphins and some other toothed whales also rank high, with bottlenose dolphins at 5.3 and capuchin monkeys reaching as high as 4.8.
The numbers get interesting further down the list. Chimpanzees score 2.2 to 2.5, while gorillas land at only 1.5 to 1.8, despite being our close relatives. Gorillas’ large body size pushes their expected brain mass up, which lowers their EQ even though their brains are physically large. Whales as a group score around 1.8, and elephants sit at 1.3. Rats come in at just 0.3, mice at 0.5, and opossums at a low 0.2.
What EQ Tells Us About Evolution
Jerison originally developed EQ to track cognitive evolution, particularly in the human lineage. His measure suggests a long, gradual increase in relative brain size starting around 4 million years ago with early hominins like Australopithecus, continuing through Homo erectus, and accelerating sharply in the last 300,000 years with modern Homo sapiens. Alternative measures paint a slightly different picture: cognitive stasis between 4 and 2 million years ago, followed by a steady climb. Either way, the archaeological record roughly lines up. Hominin tool use didn’t exceed what modern great apes can do until around 2 million years ago, when material culture started becoming more diverse and complex.
Why EQ Doesn’t Equal Intelligence
EQ is a useful shorthand, but it has real limitations as a proxy for intelligence or cognitive ability. The most obvious problem is at the extremes of body size. Shrews have brains that make up 10% or more of their body volume, which would give them extraordinarily high EQ values. Blue whales, by contrast, have brains that occupy less than 0.005% of their bodies. Nobody considers shrews smarter than whales.
A deeper problem is that EQ treats all brain tissue as equal. It doesn’t account for differences in how brains are organized, how densely packed neurons are, or which brain regions are enlarged. This matters enormously. Cortical neuron counts, for example, tell a very different story than EQ alone. Humans have roughly 11.5 billion cortical neurons. African elephants have about 11 billion, and false killer whales have 10.5 billion, yet their EQ values (1.3 and 1.8 respectively) are far below ours. Gorillas have 4.3 billion cortical neurons with an EQ under 2, while chimpanzees pack 6.2 billion into a smaller body and score around 2.2 to 2.5.
Dogs illustrate the disconnect well. With an EQ of 1.2, dogs score higher than cats at 1.0. But cats have roughly 300 million cortical neurons compared to dogs’ 160 million. EQ ranks dogs above cats; neuron count ranks cats above dogs.
Problems With the Scaling Formula
Even the math behind EQ is debated. Jerison used a scaling exponent of 0.67 (two-thirds), but different analyses of larger datasets have produced different exponents. One analysis of over 1,500 mammal species found the exponent to be 0.75 using standard methods, and as low as 0.57 when correcting for evolutionary relatedness between species. The choice of exponent changes the predicted brain mass for any given body size, which shifts every species’ EQ value.
There’s also the issue of what body size actually reflects. Changes in body size can result from dietary shifts, habitat pressures, or locomotion adaptations that have nothing to do with cognition. A species that evolves a smaller body due to limited food resources will appear more encephalized even if its brain hasn’t changed at all. Researchers have pointed out that it can be difficult to separate selection for brain size from ecological pressures acting on body shape, making EQ a blunt instrument for understanding why brains evolved the way they did.
Modern Alternatives to EQ
Neuroscientists increasingly look beyond EQ when comparing cognitive capacity across species. Total cortical neuron count is one popular alternative, since it more directly measures the raw computational hardware available for complex thought. By this measure, humans, elephants, and some whale species are surprisingly close, all hovering around 10 to 11.5 billion cortical neurons, while the behavioral gap between these species is enormous. That gap likely comes down to how neurons are connected and organized rather than sheer number.
Other researchers focus on the size of specific brain regions relative to the rest of the brain, particularly the prefrontal areas involved in planning and decision-making. Some have proposed entirely new scaling approaches that use primate-specific or order-specific regression lines rather than a single mammal-wide average, since a mouse and a dolphin share a common ancestor so far back that a single scaling rule may not capture meaningful variation.
EQ remains widely cited because it’s simple, intuitive, and useful for broad comparisons. It captures a real biological pattern: species we consider cognitively sophisticated, like primates, dolphins, and corvids among birds, consistently score above average. But it works best as a rough first pass rather than a definitive measure of what any species can think or do.