brain mass, body mass, and compound interest

Here’s something I find interesting: the mass of an animal’s brain correlates very well with the mass of the rest of it. That is, most of what the brain does seems to relate to sheer embodiment: moving around, tracking the various sensory inputs. Keeping the lights on, as it were. The longer your string of lights – say, if you’re an elephant – then the more processing power you need.  And the correlation is surprisingly linear.

Here’s a version of that relationship as I found it online, in a chart prepared by Harry J. Jerison of UCLA:


The red line in the middle is the typical ratio, with variation for particular species represented by the dots on either side.

Now, ranking animal intelligence is problematic: the different species need different aptitudes depending on their niche. So a cat is very smart at being a cat, but dumb at being a dog. But it’s interesting to me that the animals we consider brainy, like wolves, do in fact end up above the red line. Those below, not so much.

Putting it another way, if I’m locked in a room and need a bird to bring me the key, I’d rather count on the crow than the ostrich.

So the first thing about this chart is there seems to be something to it.

The second thing is that those of us above the red line tend to be social. Cooperation with others requires a whole new tier of mental stamina, as you may know from staff meetings. L’enfer, c’est les autres.

And the third thing, the ringer: after you control for body mass, there’s not a lot distance from smart to stupid.  I think of that as analogous to the way a modest pay raise can feel like more than it is. Once you’ve covered the nervous system’s nut, the rest is pin money. Now you can hunt in packs, or echo-locate your way back to the cave.

porpoisesThe relatively small difference between us and porpoises may indicate the limits of this chart. There are probably things that matter for brain power more than mere mass, such as the number of folds, or the ratio of synapses to neurons. Intelligence itself is so hard to define that it’s hard for us to say what physical traits engender it.

But still, I think our proximity to the porpoise is thought-provoking. I look at that and wonder what made that slight advantage so significant? I mean, how come at Sea World, we’re not the ones performing for them?

I think it’s because we took our slight advantage in intelligence and made it accumulate over time.

Most who obsess over such questions will tell you that humans aren’t the only animals with language and culture, that you can see symbolic communication among bees for example, or local habits of conduct shared and passed down within groups of baboons.

But we’re certainly the first to take such full advantage of them. By learning stuff and passing it along – across time and space with language and culture – we spare each generation the need to start from scratch. Over the long haul, this turned a miniscule edge into an astonishing lead.

It’s like other small changes that accumulate over time, like bacteria growth or asset appreciation, or the observation Jared Diamond makes in Guns, Germs and Steel, about the relative nutritional value of some grasses from one continent to the other. Give one side a little advantage and a lot of time, and the differences become stark.

And I’ll admit it: this chart makes me proud to work in education. It reminds me that the raw material wasn’t nearly as crucial as our ability to share and transfer what we learn.  We took that marginal extra distance from the axis, and with teaching and learning, we compounded it over time like interest.  It’s essentially human.  It’s magic.


2 thoughts on “brain mass, body mass, and compound interest

  1. Ken, that is just so beautiful in it’s simplicity, while at the same time being so complex. Just one more reason you’re my FN. Kudo’s….

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