Behave: The Biology of Humans at Our Best and Worst

by Robert M. Sapolsky

Arbitrary boundaries on continua can be helpful. But never forget that they are arbitrary.

Genes aren’t about inevitabilities; they’re about potentials and vulnerabilities. And they don’t determine anything on their own. Gene/environment interactions are everywhere. Evolution is most consequential when altering regulation of genes, rather than genes themselves.

The homunculus has no clothes.

Be dubious about someone who suggests that other types of people are like little crawly, infectious things.

We are constantly being shaped by seemingly irrelevant stimuli, subliminal information, and internal forces we don’t know a thing about.

If you had to boil this book down to a single phrase, it would be “It’s complicated.” Nothing seems to cause anything; instead everything just modulates something else. Scientists keep saying, “We used to think X, but now we realize that . . .” Fixing one thing often messes up ten more, as the law of unintended consequences reigns. On any big, important issue it seems like 51 percent of the scientific studies conclude one thing, and 49 percent conclude the opposite. And so on. Eventually it can seem hopeless that you can actually fix something, can make things better. But we have no choice but to try. And if you are reading this, you are probably ideally suited to do so. You’ve amply proven you have intellectual tenacity. You probably also have running water, a home, adequate calories, and low odds of festering with a bad parasitic disease. You probably don’t have to worry about Ebola virus, warlords, or being invisible in your world. And you’ve been educated. In other words, you’re one of the lucky humans. So try.

There are spinal cord neurons in blue whales that are half the length of a basketball court.

From an informational standpoint, a neuron has two different types of signaling systems. From the dendritic spines to the starts of the axon hillock, it’s an analogue signal, with gradations of signals that dissipate over space and time. And from the axon hillock to the axon terminals, it’s a digital system with all-or-none signaling that regenerates down the length of the axon.

the more neurons that neuron A projects to, by definition, the more neurons it can influence; however, the more neurons it projects to, the smaller its average influence will be at each of those target neurons. There’s a trade-off.

the threshold of the axon hillock can change over time, thus changing the neuron’s excitability. What things change thresholds? Hormones, nutritional state, experience, and other factors filling this book’s pages.

different neurotransmitters depolarize to different extents—in other words, some have more excitatory effects than others—and for different durations. This allows for a lot more complexity in information being passed from one neuron to the next.

they “hyperpolarize” the dendritic spine, opening different types of channels that make the resting potential even more negative (e.g., shifting from –70 mV to –80 mV). In other words, there are such things as inhibitory neurotransmitters.

Fun example: Acetylcholine stimulates your diaphragm to contract. Curare, the poison used in darts by Amazonian tribes, blocks acetylcholine receptors. You stop breathing.

Bruh

Back to the brain loving contrasts, energetically screaming its head off when it has something to say, and energetically being silent otherwise.

Me

Or the ears telling you it was definitely an A, not an A-sharp or A-flat.*

So cool

What can neuron 1 learn? How to recognize that particular guy. Neuron 5 is equally specialized. But what can neuron 3 learn? How Victorian gentlemen dressed. It’s the neuron that will help you identify the Victorian in the quartet below: Neuron 3’s knowledge is general and comes from the overlap of the first layer’s projections. Neurons 2 and 4 are also generalist neurons, but they’re less accurate because they have only two exemplars each. So neuron 3 is at the convergent center of this network. And the fanciest parts of the brain are wired up in a way that resembles this fairy-tale circuit, writ large—at the same time, neuron 3 is a more peripheral element in some other circuit sending projections to it (say, a circuitry that would be drawn perpendicular to this page), neuron 1 is at the very center of some other network in the fourth dimension, and so on. All of these neurons are embedded within multiple networks.

What might we call the consequence of some types of atypically wide associative nets of neurons? Creativity.