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Sapolsky’s Behave: The Biology of Humans at Our Best and Worst is a sweeping exploration of human behavior, blending neuroscience, psychology, anthropology, and sociology. One of the book’s most compelling themes is the “Us vs. Them” dynamic—a psychological and biological phenomenon that shapes how individuals and groups interact, cooperate, and conflict. Sapolsky’s analysis of this topic is rooted in evolutionary biology, brain chemistry, and social context, offering a nuanced understanding of why humans so readily divide the world into in-groups and out-groups.

The Evolutionary Roots of “Us vs. Them”

Sapolsky argues that the tendency to categorize people into “Us” (the in-group) and “Them” (the out-group) is deeply embedded in human biology. From an evolutionary perspective, this division served a survival function. Early humans who cooperated within their groups were more likely to survive and reproduce, while those who trusted outsiders indiscriminately risked exploitation or harm. The brain’s reward systems reinforce in-group loyalty: oxytocin, often called the “love hormone,” strengthens bonds within groups but can also amplify aggression toward outsiders. This duality highlights how biology can both unite and divide us.

Neurological Underpinnings

The brain’s amygdala, prefrontal cortex, and insula play critical roles in processing social identity and threat perception. When we encounter someone from an out-group, the amygdala—associated with fear and aggression—can become hyperactive, triggering defensive or hostile reactions. Meanwhile, the prefrontal cortex, responsible for rational thought, may downregulate, making it harder to empathize with outsiders. Sapolsky emphasizes that these neural mechanisms are not fixed; they are malleable and shaped by context, culture, and individual experience.

The Role of Oxytocin

Oxytocin, often celebrated for promoting trust and social bonding, has a darker side. Sapolsky cites studies showing that oxytocin increases cooperation and generosity within groups but can also fuel prejudice and aggression toward out-groups. This hormone doesn’t just make us more social—it makes us more selectively social, reinforcing the boundaries between “Us” and “Them.”

The Psychology of In-Group Favoritism

Sapolsky explores how quickly and arbitrarily humans form in-groups. Experiments like the minimal group paradigm demonstrate that even trivial distinctions (e.g., preferring one color over another) can lead people to favor their own group and discriminate against others. This tendency is not just a product of culture; it is a cognitive shortcut that conserves mental energy. The brain categorizes people automatically, often unconsciously, which can lead to bias and stereotyping.

Dehumanization and Moral Exclusion

One of the most dangerous consequences of the “Us vs. Them” dynamic is dehumanization. Sapolsky discusses how out-groups are often stripped of their humanity in the minds of in-group members, making harm against them feel justified. Historical examples, from wartime atrocities to everyday discrimination, illustrate how dehumanization enables cruelty. The brain’s capacity for empathy shrinks when faced with someone perceived as fundamentally different.

The Power of Context

Sapolsky stresses that context matters. The same person who is generous and kind to their in-group can become hostile or indifferent to out-groups. Stress, competition for resources, and perceived threats amplify these divisions. Conversely, positive interactions, shared goals, and exposure to out-group members can reduce prejudice. This plasticity suggests that while the “Us vs. Them” impulse is innate, it is not inevitable.

Overcoming the Divide

Sapolsky is cautiously optimistic about our ability to mitigate the worst effects of in-group/out-group dynamics. He points to several strategies:

Shared Identities: Creating superordinate identities (e.g., national or global citizenship) can expand the definition of “Us” and reduce conflict.
Empathy and Perspective-Taking: Encouraging people to see the world through the eyes of out-group members can bridge divides. Neuroscientific evidence shows that empathy activates brain regions associated with self-other overlap, making out-group members feel more like “Us.”
Institutional Design: Policies and social structures that promote cooperation—such as mixed schools, diverse workplaces, and inclusive media representation—can weaken the “Us vs. Them” reflex over time.
Education and Exposure: Familiarity reduces fear. Sapolsky cites studies showing that contact with out-group members, under the right conditions, decreases prejudice.

The Paradox of Human Nature

Sapolsky’s analysis reveals a paradox: humans are capable of both extraordinary cooperation and horrifying violence, often driven by the same biological and psychological mechanisms. The “Us vs. Them” dynamic is not a flaw to be eradicated but a feature to be understood and managed. By recognizing its roots in our biology and psychology, we can design interventions that foster inclusivity and reduce conflict.

A Call for Self-Awareness

Behave challenges readers to confront the uncomfortable reality that our noblest and most destructive impulses often stem from the same sources. The “Us vs. Them” dynamic is a part of being human, but it is not our destiny. Sapolsky’s work suggests that by understanding the science behind our social instincts, we can build a world where the circle of “Us” grows ever wider, and the line between “Us” and “Them” becomes increasingly blurred.

Susan Fiske’s work and Sapolsky’s historical examples add depth and urgency to his analysis of “Us vs. Them” dynamics. Let’s weave these elements into the discussion, as they illuminate both the psychological mechanisms and real-world consequences of in-group/out-group divisions.

Susan Fiske’s Contribution: The Stereotype Content Model

Sapolsky draws on Susan Fiske’s Stereotype Content Model (SCM) to explain how we perceive and categorize others. Fiske’s research identifies two fundamental dimensions along which we judge people:

Warmth: How friendly, trustworthy, or intentioned a group is perceived to be.
Competence: How capable, skilled, or intelligent a group is perceived to be.

These dimensions create four quadrants of stereotypes:

High warmth, high competence: Admired groups (e.g., “our” in-group, close allies).
High warmth, low competence: Pitied groups (e.g., the elderly, people with disabilities).
Low warmth, high competence: Envied or resented groups (e.g., wealthy elites, rival nations).
Low warmth, low competence: Contemptible groups (e.g., marginalized or stigmatized out-groups).

Fiske’s model helps explain why some out-groups face pity, others envy, and still others contempt or dehumanization. For example, immigrants or refugees might be pitied as helpless but also resented as a burden, while rival ethnic or political groups might be seen as both threatening and incompetent, justifying exclusion or aggression.

Sapolsky highlights how these stereotypes are not just cognitive shortcuts but emotional triggers. The brain’s response to out-groups—especially those perceived as low in warmth—can activate the insula, a region associated with disgust. This neural reaction makes it easier to justify harm or neglect toward those groups, as they are subconsciously stripped of their humanity.

Historical Examples: The Consequences of “Us vs. Them”

Sapolsky uses history to show how the “Us vs. Them” dynamic, when unchecked, can lead to atrocities. Here are some of his most striking examples:

1. The Rwanda Genocide (1994)

Sapolsky points to the Rwandan genocide, where Hutu extremists systematically murdered an estimated 800,000 Tutsi and moderate Hutu in just 100 days. The propaganda leading up to the genocide framed Tutsis as cockroaches—a classic dehumanization tactic that made mass violence psychologically easier for perpetrators. This echoes Fiske’s findings: when a group is perceived as both cold (low warmth) and incompetent (low competence), they become targets for extreme violence.

2. The Stanford Prison Experiment (1971)

While controversial, this study illustrates how quickly arbitrary group divisions can escalate into abuse. College students randomly assigned to roles of “prisoners” or “guards” began to treat each other with cruelty or submission within days. The experiment demonstrates how context and perceived power can override individual morality, turning ordinary people into oppressors or victims based on group identity.

3. World War II and the Holocaust

Sapolsky discusses how Nazi propaganda portrayed Jews as vermin or diseased, tapping into deep-seated fears of contamination and threat. This dehumanization allowed ordinary Germans to participate in or ignore the Holocaust. The brain’s amygdala—responsible for threat detection—was hijacked by ideology, overriding empathy.

4. American Slavery and Jim Crow

The justification for slavery and segregation relied on pseudoscientific claims of racial inferiority, placing Black Americans in the low warmth, low competence quadrant. Sapolsky notes how these stereotypes persisted long after slavery ended, shaping policies and social norms that enforced segregation and discrimination.

Why These Examples Matter

These cases show that the “Us vs. Them” dynamic is not just theoretical—it has real, devastating consequences. They also reveal a pattern:

Dehumanization precedes violence.
Authority figures (politicians, media, religious leaders) can amplify or mitigate these divisions.
Stress and scarcity (economic downturns, wars) make people more susceptible to in-group/out-group thinking.

Sapolsky’s point is clear: the line between “Us” and “Them” is not fixed. It is drawn and redrawn by leaders, cultures, and circumstances. Understanding this should make us vigilant about how language, policies, and social norms shape our perceptions of others.

Fiske and Sapolsky: A Path Forward

Fiske’s work suggests that changing perceptions of warmth and competence can reduce prejudice. For example:

Humanizing narratives can increase perceived warmth.
Highlighting shared goals can shift focus from competition to cooperation.
Institutional accountability can disrupt the cycle of dehumanization.

Sapolsky adds that oxytocin’s dual nature means we must be intentional about fostering inclusive forms of trust, not just in-group loyalty. He cites programs like contact theory—where positive interactions between groups reduce prejudice—as evidence that change is possible.