Behave: The Biology of Humans at Our Best and Worst

What happens in the brain to determine a behavior one second before that behavior occurs? Answering this question forms one of the longest and most difficult chapters in the book. In it, Sapolsky provides an overview of the basic neuroscience of human behavior, which draws on work spanning the last 100 years of scientific examination of the structure of the brain in both animal and human subjects. This chapter deals primarily with the anatomical layout of the brain, particularly the functions of the Limbic System, Frontal Cortex, and Dopaminergic system.

Though challenging, Sapolsky argues that the brain is the all-important topic in the study of behavior because “the brain is the final common pathway, the conduit that mediates the influences of all the distal factors to be covered in the chapters to come” (22). In other words, if we are looking for a place to unite varied sciences of behavior, we should look at the brain. As such, forms and methods of brain research discussed in this chapter, like fMRI—imaging of active brains to see how they respond to different stimuli—and lesion studies—destruction of parts of (animal) brains to see what functions are lost—will be returned to throughout this book. The fact that the brain is the final common pathway also means this chapter is a very important one to understand. For those completely new to brain science, Sapolsky suggests reading “Appendix 1: Neuroscience 101.” For those reading this guide alone, consult Index of Terms.

The Limbic System

The limbic system (LS) is situated in the mid-brain and is the part of the brain involved in our emotional responses to sensory stimulus. The limbic system communicates emotion to the parts of layer 1 of the brain (i.e., “lower” brain) that control automatic functions, called the autonomic nervous system (ANS). It does so particularly through one structure at the border between Layer 1 and Layer 2, the hypothalamus. Essentially, the hypothalamus receives signals from the LS and sends them to the ANS, and “this is how emotions change bodily functions” (26), such as causing you to empty your bladder when you are scared (to help you run faster).

The LS also communicates to the main layer 3 structure, the cortex, which decodes sensory information, does cognition, and processes language and logic. Importantly, due to its LS connections, cognition is influenced by emotional states and vice versa. The gatekeeper between these two regions of Layer 2 and Layer 3 is a section of the cortex known as the prefrontal cortex (PFC).

The picture of top-to-bottom and bottom-to-top styles of communication that Sapolsky describes here (the LS is in the mid-brain and communicates to lower layer 1 and upper layer 3) is a functional simplification. It relies on a model of the human brain conceptualized in the early 20th century known as the Triune Brain. This concept held that the brain has three parts, the lowest and oldest for automatic functions, the middle for emotional and sensory functions, and the top and newest for cognition. Although roughly true, the brain is much more complex than this, with all of its parts constantly evolving.

Another structure within the LS is the amygdala, which mediates aggression and emotions associated with triggering aggression: anxiety, disgust, and innate and learned fear. It is the part of the brain that allows us to learn to be afraid of new stimulus through repeated conditioning exposures. Alongside fear, the amygdala is a key structure in learning social distrust, such as when individuals are anxious over whether they are being treated fairly by others.

Several regions of the brain have informational inputs to the amygdala that trigger its fear, anxiety, and aggression. These include information from the senses, pain receptors, and projection from the insular cortex (within the prefrontal cortex), which processes gustatory, olfactory, and emotional/moral disgust. The amygdala can also control our behavior in reflexive ways, as it projects to motor outputs, allowing some fear responses to trigger automatic actions before information can make it up to Level 3 for processing in the cortex. This can lead to potentially deadly mistakes. To illustrate how important understanding the amygdala’s shortcut past the cortex is, Sapolsky draws on a contemporary story designed to pull on our heart-strings: the shooting of Stephon Clark by a policeman when the policeman mistook Stephon’s cell phone for a gun. Importantly, as later chapters will show, if our culture was more invested in breaking down some of the automatic reactions we have when we see people of different races, the amygdala’s fear response might not have been activated in the first place, saving Stephon from this policeman’s automatic reaction.

The Frontal Cortex

The frontal cortex (FC) is the last brain region to fully mature. Processes of the FC include working memory, planning, strategy, decision making, delaying gratification, and reining in impulsivity. In short, “the frontal cortex makes you do the harder thing when it’s the right thing to do” (45). As such, it is central to acts we deem moral or respectable, such as reining in our reflexive gluttony or greed.

The FC also sports a special type of neuron called von Economo, or spindle neurons, that are only present in socially complex species. This neuron controlling social behavior is present in the FC because reining in our impulses is crucial to social success. Indeed, when the FC is damaged, behavior becomes socially inappropriate and disinhibited. To illustrate this, Sapolsky draws on the famous psychological case study of Phineas Gage, a rail foreman who had a section of his PFC destroyed in an accident and became unruly and violent. Note, this is an example of a real-life lesion experiment, as mentioned above, which can usually only be performed on animals for ethical reasons. This shows us how important studying history can be to getting a better picture of biology.

The FC is also key in categorical thinking, or the mental organization of information. In essence, the FC is about creating and following rules based on inhibition and effort. This is an example of how doing the harder thing when it is the right thing is not necessarily related to moral decision. It can be expressly remembering a new rule instead of an old habit or remembering an abstract concept and applying it in a new situation.

These inhibitory and executive functions of the brain are metabolically expensive. “Cognitive load” refers to the concept that the frontal cortex can only work so hard for so long before its effectiveness decreases. However, rules learned by the FC—such as motions associated with playing the piano—eventually become automatic and do not require cognitive effort. This is why practice makes any task easier and why learning to treat people like equals can actually make us have to work less hard cognitively to do so. This theme becomes important several times later in the text, most prominently in discussions of how people act morally: they are trained to do so to the point that our PFC does them automatically.

The newest part of the FC is the prefrontal cortex (PFC). The PFC is the “decider” (46), the region of the FC responsible for choosing between sets of two options such as “act” and “don’t act.” Two important subparts of the PFC are the dorsolateral prefrontal cortex (dlPFC) and ventromedial prefrontal cortex (vmPFC). The dlPFC is the most “rational, cognitive, utilitarian, unsentimental” (54) region of the FC, activated in such instances as answering yes to the classic runaway trolley dilemma: is it “okay to kill one person to save five” (55). The vmPFC is the opposite; it is about the impact of emotion on decision making, such that when this region is damaged, a patient is more likely to be willing to kill a family member to save five strangers because rationally one life is less of a loss than 5, even if it is your own child: “It’s Mr. Spock, running on only the dlPFC” (56). To make effective decisions, both of these sections of the PFC need the other: good decisions are not only cognitive, but they are also emotional and intuitive.

The PFC also projects to the amygdala, the brain region for feeling and learning fears, as discussed above. This projection of the PFC into the amygdala is activated when we suppress rage or desire to flee. In other words, the PFC allocates executive control to emotional responses: doing the harder thing when it is the right thing to do. Here, Sapolsky reminds us of the complex meaning of the word “right”: sometimes it means morally correct, at other times it means “best thing for me to do at the moment.” The PFC does the same activity in both cases: suppressing one response to promote another. This comes back to Sapolsky’s core theme: biology can’t tell us if an act is right or wrong by looking at the act in a vacuum.

The Mesolimbic/Mesocortical Dopamine System

The neurotransmitter dopamine is synthesized in multiple brain regions. One is the ventral tegmental area or “tegmentum,” which sends dopaminergic neurons to the amygdala as well as the hippocampus, PFC, and nucleus accumbens, collectively referred to as the “dopaminergic system.” This system is associated with producing feelings of reward and happiness when it receives stimuli we deem pleasurable: sex, drugs, food, aesthetic pleasure, and in some cases cooperation with others including in violent punishment of norm violators.

Though dopamine is a powerful motivator, we also habituate to it quickly. As level of reward stimulus stabilizes, dopamine levels also stabilize or even decrease. This makes pleasure more difficult to induce with repeated exposure to the same previously pleasurable stimulus. As Sapolsky writes in a highly philosophical passage pulling on this field of science, reduced dopamine signals for stable rewards is what makes human beings so increasingly hard to satisfy. This passage leaves out some of the emerging science, however, of how we can reduce our constant sense of craving.

Dopamine “is about […] rapidly habituating reward” (70). However, “once reward contingencies are learned, dopamine is less about the reward than its anticipation” (70): our brain releases dopamine not when we are rewarded but when we are consciously pursuing a reward. Two variables control the amount of anticipatory dopamine release: the size of the anticipated reward and the presence of uncertainty, with the greatest degree of uncertainty (i.e., 50% chance of reward) releasing the most dopamine. Put into layman’s terms, this explains why rewards we are uncertain we will receive feel the most rewarding to us once we get them, which explains such addictions as gambling. Why is the dopaminergic system so highly stimulated by anticipation? Its projections into the PFC, the brain’s decision-making center, explain: dopamine is about motivating us to pursue reward, a highly useful brain system in the reward-scarce contexts of our evolution.

To sum up this chapter, behavior is immediately related to “messages bouncing around in the brain” (81) between regions with different specializations. These include the limbic system, specialized in emotion, which projects to the ANS through the hypothalamus, triggering reflexive behavioral responses to emotional stimulus. Inside the LS is also the amygdala, the brain’s center for fear, aggression, anxiety, and arousal. The cortex is the realm of cognition, with its crown jewel the PFC associated with decision making and delaying gratification, which can work to suppress the amygdala. The dopaminergic system is the center for motivation and anticipating reward. It projects into the PFC, encouraging us to choose difficult options for higher rewards.