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The Loom

June 30, 2004

Machiavellian Monkeys

Our brains are huge, particularly if you take into consideration the relative size of our bodies. Generally, the proportion of brain to body is pretty tight among mammals. But the human brain is seven times bigger than what you'd predict from the size of our body. Six million years ago, hominid brains were about a third the size they are today, comparable to a chimp's. So what accounts for the big boom? It would be flattering ourselves to say that the cause was something we are proud of--our ability to talk, or our gifts with tools. Certainly, our brains show signs of being adapted for these sorts of things (consider the language gene FOXP2). But those adaptations probably were little more than tinkerings with a brain that was already expanding thanks to other factors. And one of those factors may have been tricking our fellow hominid.

In the 1980s, some primatologists noticed that monkeys and apes--unlike other mammals--sometimes deceived members of their own species, in order to trick them out of food or sneak off for some furtive courtships. The primatologists got to thinking that deception involved some pretty sophisticated brain power. A primate needed to understand something about the mental state of other primates and have the ability to predict how a change in that mental state might change the way other primates behaved.

The primatologists then considered the fact that humans aren't the only primates with oversized brains. In fact, monkeys and apes, on average, have brains twice the size you'd predict for mammals of their body size. Chimpanzees and other great apes have particularly big brains, and they seemed to be particularly adept at tricking each other. What's more, primates don't simply have magnified brains. Instead, certain regions of the brain have expanded, such as the neocortex, the outer husk of the brain which handles abstract associations. Activity in the neocortex is exactly the sort of thinking necessary for tricking your fellow ape.

Taking all this into consideration, the primatologists made a pretty gutsy hypothesis: that the challenges of social life--including deception--actually drive the expansion of the primate brain. Sometimes called the Machiavellian Intelligence hypothesis, it has now been put to its most rigorous test so far, and passed quite well. Richard Byrne and Nadia Corp of the University of St. Andrews in Scotland published a study today in the Proceedings of the Royal Society of London. (The link's not up yet, but here's a New Scientist piece.) They found that in 18 species from all the major branches of primates, the size of the neocortex predicts how much deception the species practices. Bigger brains mean more trickery. They were able to statistically rule out a number of other factors that might have created a link where none existed. And they were able to show that deception is not just a side-effect of having a big brain or something that opportunistically emerges more often in big groups. Deception is probably just a good indicator of something bigger going on here--something psychologists sometimes call "social intelligence." Primates don't just deceive one another; they also cooperate and form alliances and bonds, which they can keep track of for years.

While deception isn't just an opportunistic result of being in big groups, big groups may well be the ultimate source of deception (and by extension big brains). That's the hypothesis of Robin Dunbar of Liverpool, as he detailed last fall in the Annual Review of Anthropology. Deception and other sorts of social intelligence can give a primate a reproductive edge in many different ways. It can trick its way to getting more food, for example; a female chimp can ward off an infanticidal male from her kids with the help of alliances. Certain factors make this social intelligence more demanding. If primates live under threat of a lot of predators, for example, they may get huddled up into big groups. Bigger groups mean more individuals to keep track of, which means more demands on the brain. Which, in turn, may lead to a bigger brain.

If that's true, then the human brain may have begun to emerge as our ancestors huddled in bigger groups. It's possible, for example, that early hominids living as bipeds in patchy forests became easier targets for leopards and other predators. Brain size increased modestly until about two million years ago. It may not have been able to grow any faster because of the diet of early hominids. They probably dined on nuts, fruits, and the occasional bit of meat, like chimpanzees do today. That may not have been enough fuel to support a really big brain; brain tissue is incredibly hungry, demanding 16 times more energy than muscle, pound for pound. It was only after hominids began making butchering tools out of stones and got a steady supply of meat from carcasses that the brain began to expand. And it was probably around this time (between 2 and 1.5 million years ago) that hominids began evolving the extraordinary powers of deception (and other sorts of social intelligence) that humans have. We don't just learn how other people act--we develop a powerful instinct about what's going on in their minds. (I wrote about the neuroscience behind this "mentalizing" last year in an article for Science.)

So next time you get played, temper your anger with a little evolutionary perspective. You've just come face to face with a force at work in our evolution for over 50 million years.

UPDATE 7/3/04: A skeptical reader doubted some of my statements about the brain and the energy it requires. Those who crave more information should check out Northwestern University anthropologist William Leonard's article "Food for Thought" in Scientific American.
Posted by Carl at 11:45 AM
  Comments and Trackbacks

Aha! So Carlo Lorenzini had it all wrong ...

Posted by blake on June 30, 2004 01:50 PM | Permalink to Comment
Go read more about conniving primates...

Excerpt: Hey, I might have made a throw-away comment about Machiavellian monkeys in the hodge-podge below, but Carl Zimmer has made some meatier comments on the subject. Check it out!

Read the rest...

Trackback from Pharyngula, Jun 30, 2004 1:54 PM

Primate brains will be keeping cogscientists busy for some time. One aspect of Carl's "Soul Made Flesh" that was missing was the work of Michael Persinger at Laurentian University. He can create "religious visions" [or something similar] with magnetic stimulation of the temporal lobes.

Is it possible that our belief in a "soul" is due to differences in the lobes between us and chimpanzees?

the bunyip
stephen in Ottawa

Posted by the bunyip on July 1, 2004 10:06 AM | Permalink to Comment

Hmm, so the bigger the group, the bigger the stimulus for the bigger brain. Is that it?

Explains what is critically missing in the "red" states, I guess.

Posted by roger on July 2, 2004 02:28 PM | Permalink to Comment

Nah. It's more a matter of having evolved to the point that we can function for ourselves without needing to huddle in bigger groups like the blue-staters.

(Offered in a mood as light-hearted as I trust Roger's was.)

Posted by triticale on July 3, 2004 09:23 AM | Permalink to Comment
Tooth and Claw

Excerpt: Many years ago, at a company Christmas party, the wife of one of the engineering staff was trying to explain to me why we humans shouldn't eat other animals. She claimed we hadn't evolved to eat meat, and offered my...

Read the rest...

Trackback from triticale - the wheat / rye guy, Jul 3, 2004 11:27 PM
Two on the Monkey-Mind

Excerpt: Ah, the monkey-mind, that primal and social part of our brains that evolved long before the human species emerged. Carl Zimmer has an interesting post, Machiavellian Monkeys, suggesting that neocortex size of primates increases with the propensity for ...

Read the rest...

Trackback from Many-to-Many, Jul 6, 2004 11:55 AM

I have two questions to discuss:
1. Why do other animals that live in large groups (ants, bees, fish that live in swarms, sheep, bisons, etc.) not have giant brains? Is it because they live in groups, but are not "complex" enough to act as individuals?
2. Why does the ability to deceive make more sense (evolutionarily) for females (who's brains are better prepared for social intelligence than male brains)?

herrlothar

Posted by herrlothar on July 6, 2004 08:34 PM | Permalink to Comment
Suomi on hyvä jalkapallossa!

Excerpt: Noniin, se oli valhe. Ei ehkä maailman paras huijaus, mutta parempi mihin moni muu nisäkäs pystyisi, koska ihmisellä on niin suuret aivot.

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Trackback from Peikkokin blogaa, Jul 7, 2004 5:58 AM
Links for 15th of July

Excerpt: Global Rich List - you're on it To PhD or not to PhD? at plasticbag.org. "She's only Two Cats Mad on the Spinster Eccentricity Index" Steveberlinjohnson.com on Fahrenheit 9/11 Corante.com/loom on Machiavellian Monkeys, social intelligence and cortex s...

Read the rest...

Trackback from idiolect.org.uk, Jul 14, 2004 7:10 PM

Nobody wants constructive criticism. It's all we can do to put up with
constructive praise.
prozac online "All this modern technology just makes people try to do everything at once."
-Hobbes
prozac

Posted by Florence Anne on September 5, 2004 05:45 AM | Permalink to Comment

When I use the term 'repugnant' I do so in my own opinion: I do not use non-free
software on machines I control. This licence is non-free, and masquerading it as free
is offensive. I have contributed lots to the Free Software community myself, and I
would be completely outraged if any of my contributions were being shipped in a
non-free product. Contributions are contributions to public software, not private
profits.

Posted by michael on September 7, 2004 02:22 PM | Permalink to Comment

Maria sharapova is a great Rusian tennis player!

Posted by maria sharapova on September 21, 2004 12:32 PM | Permalink to Comment

Parents need more kid safe web sites! We cant let our kids surf the web without
being directed to some inappropriate site. Dont believe me? Do a search on
Pocahontas and watch how many inappropriate sites come up. Even major kids
search engines only block certain words, there are ways around that. The only one I
have found so far was at http://www.netsx.org , I dont know about most parents
but we dont have time to monitor our kids every second when they want to go on
the computer. Anyway rant off.

Posted by Safesite on September 25, 2004 11:50 PM | Permalink to Comment

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