LE 21: Biology and Evolutionary Functions of Aggression: Critique of the Seville Statement

• Remaining Assessments and Schedule:

  • There is one more midterm scheduled for the final day of class.

  • A final exam will take place during finals week.

  • The lowest of the four exam scores (three midterms plus one final) can be dropped.

  • The instructor cannot guarantee that the third midterm will be graded before the final exam, meaning students may have to decide whether to take the final without knowing their third midterm score.

  • Multiple-choice questions based on the lectures from Monday and Wednesday (the first aggression lecture) are due tonight.

• Holiday Notice:

  • There is no class this coming Monday due to Memorial Day. Students are encouraged to take a moment for relaxation and meaningful reflection.

Biological and Evolutionary Foundations of Aggression

• Lecture Objectives:

  • To explore how aggression is produced in the human mind and body.

  • To understand the evolutionary functions of aggression.

  • To examine phylogenetic research, archaeological evidence, and naturalistic observations of primates.

  • To discuss the neural circuits involved in aggression across species.

  • To evaluate the historical context of aggression, including topics like retaliation, honor, Thomas Hobbes's Leviathan, and dueling.

  • To address the debated effects of video games on aggression and how internal states stimulate aggressive responses.

The Seville Statement on Violence (1986)

• Background and Reach:

  • Originally authored in $1986$ by $20$ biologists and social scientists.

  • It was published in over $150$ outlets (scientific journals and magazines) and translated into $20$ languages.

  • It was endorsed by $75$ organizations, including the American Psychological Association (APA) and the United Nations (UN).

• The Naturalistic Fallacy:

  • The Seville Statement is a primary example of the naturalistic fallacy: the error of assuming that "the way things are is the way they should be."

  • This fallacy suggests we can derive ethics, duties, or moral obligations simply by observing natural facts. The authors of the statement wanted to claim that because aggression is "not" part of human nature (a factual claim), we have a moral obligation to be peaceful.

• Core Assertions (The Five "Scientifically Incorrect" Statements):

  1. It is scientifically incorrect to say we inherited a tendency to make war from animal ancestors.

  2. It is scientifically incorrect to say war or violent behavior is genetically programmed into human nature.

  3. It is scientifically incorrect to say that during human evolution, there has been selection for aggressive behavior more than other behaviors.

  4. It is scientifically incorrect to say humans have a "violent brain."

  5. It is scientifically incorrect to say war is caused by "instinct" or any single motivation.

Amending the Seville Statement: A Modern Scientific Critique

• Correcting Assertion 1 (Inheritance):

  • It is scientifically correct to say humans inherited a tendency for war from animal ancestors. Molecular and phylogenetic data support this.

• Correcting Assertion 2 (Genetic Programming):

  • It is scientifically nonsensical to use the term "genetically programmed." Genes do not program behaviors directly; they produce proteins and regulate other genes to build physical structures and neural systems.

• Correcting Assertion 3 (Selection):

  • It is almost certainly true that there has been selection for aggressive behavior. Selection favors traits that solve adaptive problems (e.g., resource acquisition, defense).

• Correcting Assertion 4 (Brain Systems):

  • Humans do not have a "violent brain," but we possess neural systems designed to regulate aggression. These systems dial aggression up or down based on context.

• Correcting Assertion 5 (Motivation):

  • While war is not caused by a single "instinct," it is simplistic to dismiss the biological motivations behind it. We must nuance the understanding of why groups engage in conflict.

Phylogenetic Data and Lethal Aggression

• The Genetic Clock:

  • Scientists use the known rate of genetic mutation to trace species back to common ancestors (speciation events).

  • By examining $1,000$ mammalian species, researchers can infer the amount of "homicidal" violence (intraspecies lethal aggression) throughout history.

• The Intensification of Violence in Mammalian Lineages:

  • The common ancestor of all mammals was not particularly violent.

  • Marsupials: Split off early and followed their own evolutionary path.

  • Rodents: A later split showed an intensification of aggression.

  • Primates: The fork sending tree shrews one way and primates another marked a significant increase in aggression.

  • Great Apes: Another intensification event occurred in the branch leading to great apes (orangutans, gorillas, chimpanzees, bonobos, and humans).

  • Humans and Chimpanzees: Approximately $6 \times 10^6$ to $8 \times 10^6$ years ago, humans and chimpanzees split from a common ancestor. Both species show a higher potential for lethal intraspecies aggression compared to other great apes like bonobos or gorillas.

Naturalistic Observations of Chimpanzees

• Key Literature:

  • Demonic Males by Richard Wrangham and Dale Peterson (published in the mid-$1990s$).

  • Chimpanzee Politics by Frans de Waal.

• Fission-Fusion Societies and Territoriality:

  • Chimpanzee groups (often around $100$ individuals) may split (fission) when resources are scarce.

  • Once split, former friends or relatives become hostile strangers.

  • Boundary Patrols: Males form groups (typically $3$ to $5$ individuals) to patrol the perimeter of their territory.

• Lethal Intergroup Conflict:

  • In a famous study in the Congo Basin, researchers documented over $20$ killings in a $10$-year period.

  • Killings almost always occurred at the territory boundaries, not in the center.

  • Aggressors use overwhelming force (e.g., $5$ against $1$) to neutralize members of rival groups.

  • The payoff for this violence is expanded territory, which provides more food and better reproductive success for the victorious group.

Archaeological Evidence (CSI: Paleolithic)

• Pre-Agricultural Evidence:

  • Studies of human skulls between $10,040$ and $40,000$ years old show a high prevalence of violence.

  • Approximately $25\%$ of well-preserved skulls from this era show evidence of fatal head trauma.

• The Neanderthal Example:

  • A $30,000$-year-old Neanderthal skull shows clear evidence of a fracture caused by a blunt instrument, such as a hammer or bat.

• The Nataruk Site (Kenya):

  • Anthropologists found $12$ skeletons in a dry lakebed in Kenya dating back $10,000$ years (pre-agriculture).

  • The skeletons showed horrific injuries: broken arms, legs, and hips, and signs of being bound (hands behind backs).

  • This suggests a single, highly violent intergroup encounter, proving that systematic warfare predates agriculture and permanent settlements.

Evolutionary Functions of Aggression

• Competition for Resources: Aggression is used to fight for mates, territories, and goods (e.g., hermit crabs fighting for shells).

• Establishing Hierarchies: Many species use aggression to maintain a "pecking order," which actually reduces overall conflict by ensuring everyone knows their place (status negotiation).

• Deterrence and Incapacitation: Aggression can stop a rival from attacking or physically prevent them from competing.

• Self-Defense and Protection of Assets: Defending one's own body, offspring, mates, and food supplies.

• Reputation Management: Intentionally broadcasting aggressive potential to third parties to prevent future challenges. An individual may fight not just to get stuff back, but to show others what will happen if they try the same thing.

• Supporting Kin and Allies: Using violence to aid genetic relatives (offspring, nieces, nephews) or social partners.

The Biology of the Aggressive Response

• Neural Systems in Vertebrates: All vertebrates share two primary brain systems for regulating aggression: one more reflective/deliberative and one more reactive/reflexive.

• The Decision-Making Process:

  • External Cues: Species-specific sensory information (e.g., the presence of a rival, an intruder in the house).

  • Internal States: Assessments of one's own reproductive status (e.g., is a gestating mother better off fighting or fleeing?), energy balance (hunger), stress levels, sleep, and prior experience/skill with violence.

• Species-Specific Motor Outputs:

  • Rodents: Use olfactory cues; behaviors include biting, lunging, and tail-rattling.

  • Song Sparrows: Use auditory and visual cues.

  • Humans: Integrate multisensory information and assign complex meaning (e.g., interpreting a "menacing face" or a history of interaction); behaviors include punching, stabbing, using weapons, or forming social alliances to gang up on others.

Case Studies in Aggressive Signaling

• Song Sparrows (Duke University Research):

  • Researcher Rindy Anderson studied "wing-waving" in sparrows.

  • Wing-waving is a visual signal given before or during an attack, functioning much like a human "flipping the bird" or putting their "dukes up."

  • While physical attacks are rare because they are dangerous, these signals allow birds to resolve territorial disputes without potentially lethal combat.

• Richard Wrangham and Sarkar's Model:

  • They distinguish between two types of aggression in primates:

    • Reactive Aggression: "Hot," System 1 behavior. It is emotional, spontaneous, reflexive, and triggered by a perceived threat (e.g., someone stealing your property). It usually involves one-on-one conflict.

    • Proactive Aggression: Deliberate and planned (to be discussed in the next lecture).