What Makes Our “Social Brain” Special?

Walking along the busy New York City streets, it’s easy to get lost in the commotion. You call your mom on your phone, grab a beer with your work buddies, make small talk with the barista at your favorite coffee shop — you are constantly interacting with the people around you. From a biological perspective, our behavior is strange. No other species on the planet interacts with others members of its species as much as we do. Other species rarely live in groups larger than their immediate family, let alone cities exceeding 8.5 million people. So what makes us so social?

It’s easy to see what we’ve done and see that we’re different. However, trying to identify why we’re different and just how different we are has been extremely difficult. We have evolved and continue to evolve following the same rules as everything else living on this planet. Our biological goal is to survive and reproduce — the same as everything from fruit flies to blue whales. So why aren’t savanna baboons the ones plotting a course for Mars? Why aren’t dolphins creating supercomputers, artificial intelligence, and super-dolphin robots? Why haven’t sea otters invented nuclear weapons capable of destroying the world 10 times over? Identifying what makes us unique is a question that has perplexed scientists for centuries — why us?


If you ask anyone that learned anything in their high school science class, they’d tell you humans are special because we have big brains. While that answer isn’t exactly wrong, the explanation is a little more complicated than that. Suzana Herculano-Houzel explores what makes us “special” using comparative neuroscience in her Laboratory of Comparative Neuroanatomy at Vanderbilt University. She explains brain evolution by reconstructing how brains vary between different species, identifying the rules to how brains are built.

In her 2013 Ted Talk, Herculano-Houzel says, “Our brain weighs between 1.2 and 1.5 kilos [2.6 and 3.3 pounds], but elephant brains weigh between 4 and 5 kilos [8.8 and 11 pounds], and whale brains can weigh up to 9 kilos [19.8 pounds] … theirs may be bigger, but ours is better.” Not all brains are created equally, and size doesn’t necessarily mean more processing power. Thus, in order

Even more, 16 billion of those neurons are located in the cerebral cortex, the most of any cortex. Considering that the cerebral cortex is the area of the brain responsible for higher cognitive tasks like thinking, language, and consciousness, it’s easy to argue that our abnormally large cerebral cortex is what makes us special. We now know the underlying mechanisms that make us special, but how did a big brain and cerebral cortex come to be in the first place?


Evolution is a process of necessity and adaptation, an economic process in which everything comes at a cost. That being said, if we have a large brain that weighs only 2 percent of our body weight, but takes up nearly 25 percent of the calories we eat, its cost must be offset with some tremendous evolutionary benefit. In the Social and Evolutionary Neuroscience Research Group (SENRG) at Oxford, evolutionary psychologist and anthropologist Robin Dunbar explores the evolution of human society and the underlying biological mechanisms.

One of Dunbar’s many discoveries is what he calls the “social brain hypothesis.” He suggests that animals living in stable social groups face cognitive demands that animals living alone do not. By researching primate sociality, he has been able to observe and compare how different social interactions impact our brains. In primates, he’s found that species with bigger brains, tend to have more complex social behaviors and interact with larger social groups. Also, brain size is impacted not just by the number of people around you — but how you interact with them. Think about the brain sizes of a school of fish, a flock of birds, or the chambers of most political buildings. Larger brains didn’t evolve for us to just live around more people, they evolved so we could cooperate and work together better.

However, even humans have a neural limit to the number of people we’re able to maintain stable relationships with. Creatively dubbed “Dunbar’s number,” Robin Dunbar found that the number of social relationships a primate can maintain is limited by the brain’s neocortex size — for humans, it’s about 150. He first observed the relationship between neocortex size and the number of stable relationships in monkeys, who groom each other to maintain social relationships and even maneuver social ranks.

Now we might not eat bugs off of each other’s back, but people maintain strong social relationships through face-to-face interactions and shared experiences too. Above 150 however, primate groups tend to fall apart due to social conflict. Yet, one would note people today clearly interact with more than 150 people in their lives — just check out any teenager’s Instagram. While our brains have evolved to support our social capacities, they can only do so much. Beyond our neural limits, we see the emergence of other social beliefs that have allowed us to cooperate beyond our biological capacity.


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