"In primates, having a bigger brain means you have a disproportionately larger frontal cortex," said Eliot Bush, a PhD candidate at Caltech who worked on the study.

While the study does not offer any deep insight into what separates humans from other mammals, it does show that much of what makes people and our cousins the apes different may lie in our primate heritage.

A comparison to carnivores - the order that includes lions, tigers and dogs - shows they do not have the same disproportionately large frontal cortex.

This may explain why a 5-pound (2-kg) house cat is every bit as intelligent as a 100-pound (40-kg) lion.

Bush said the finding, published in this week's issue of the Proceedings of the National Academy of Sciences, disputed theories that suggested human behavior could be traced in part to a disproportionately large frontal cortex compared to the rest of the brain.

It turns out that smaller primates such as lemurs and bush babies actually devote a larger proportion of their brains to the frontal cortex. But because they are small overall, this area is small.

Humans and apes are big and have big brains, so while the proportion is not as big, the frontal cortex ends up being huge, comparatively.

This could explain some behavioral differences that make humans, apes and monkeys - the large primates - unique.

"Primates are able to understand abstractions - maybe understanding those two monkeys over there are a mother and a daughter, whereas maybe other mammals don't have that ability," Bush said in a telephone interview.

"In humans, the frontal cortex is involved in a lot of interesting things such as social relationships. I think that quite possibly is what it is about." For their study, Bush and his professor, John Allman, compared 43 mammals, including 25 primates and 15 carnivores.

They found that in primates, the ratio of frontal cortex to the rest of the cortex was about three times higher in a large primate than in a small one.

The ratio does not change in carnivores.

They did not look at other notably big-brained animals such as cetaceans - the group that includes whales and dolphins.