4. Behavioral Immune System

Behavioral immune system definition

Psychological system to avoid pathogens.

Parasite threat uniqueness

Persistent, invisible, and internally harmful.

Functional invisibility

Parasites cannot be easily detected.

Hurt-from-within threat

Pathogens damage the body internally.

Immunological cost

Physiological immunity is energetically costly.

Proactive defense

Avoidance prevents immune activation.

Disgust function

Avoid infection and contamination.

Fear function

Avoid external physical threats.

Smoke Detector Principle

Overreacting is less costly than underreacting.

Functional Flexibility Principle

Response adjusts to vulnerability context.

COVID coughing reactions

Increased moral judgment under disease threat.

Disease and dating

Avoidance reduces social and mating behavior.

Disease and xenophobia

Ingroup preference increases under pathogen threat.

Collectivism link

High pathogen prevalence correlates with collectivism.

Health-attractiveness link

Attractiveness used as proxy for health.

Why parasites are a unique threat

They are persistent, invisible, and harm the body from within

Hurt-from-within threats

Pathogens that damage the body internally

Hurt-from-without threats

Predators and physical dangers causing external harm

Why immune responses are costly

They consume energy and impair normal functioning

Fear vs disgust

Fear avoids external threats, disgust avoids infection

Ecological fallacy / Simpson’s paradox

Aggregate-level effects can reverse at the individual level

Parasite-Stress Theory of Sociality

High pathogen prevalence leads to more collectivism, conformity, and xenophobia

Tight Cultures

Strict norms, low tolerance for deviance.

Loose cultures

Flexible norms, higher tolerance.

Galton’s Problem

Cultures may share traits because they are neighbors or related (not independent)