Behavioral Immunity

Loss of Heterogeneity in Humans #

• humans are very similar genetically
• Population thinking: Crises lower population → smaller gene pool
  • greater influence on founder effects
    • at some point in history, only 20,000 humans were alive

Impact of disease on human evolution #

• Until WWII, more soldiers died of disease than wounds
• Decimation of native population: 50% of Aztecs died of smallpox, 95% of Indigenous North American populations

Why did Europeans carry diseases, but others did not? → A: agriculture

• caused humans to become sedentary
• high density living
• domesticated cattle, sheep, goats living in the same area
  • diseases moved easily from animals to humans

modern society: cities, global transportation…

Behavioral Immunity #

• ability for humans to detect presence of pathogens
  • example: bad odor, blemishes…
  • tuned to be oversensitive (false positives)

functional flexibility: responses can be expensive; tradeoff in safe behavior

In other species: #

• dedicated latrine spots: stool contains pathogens
• grooming: remove parasites, build relationships
• medications: nest fumigation, plants…

Sexual reproduction as a defense against disease #

• by increasing heterogeneity of offspring via sexual reproduction, population is better protected against disease
• good genes hypothesis: showy displays on birds, etc. are an honest signal of low disease (immunocompetence)
  • → human response to pathogens → polygyny

Experiments in behavioral immunity #

• individuals injected w/ immunotoxin judged less desirable
• cultures w/ lower disease level → increased sexuality, more extraverted, open to new experience

(Schaller, Murray 2008)

• Xenophobia often used as rationalization for removing possible pathogens
• Parochialism: treating strangers differently
  • strangers may have different disease background: after being primed by disease, people less likely to support unfamiliar cultures (greater xenophobia) — Schaller, Park 2011
• Collectivism: group-oriented behaviors
  • higher historical disease ⇔ more collectivism
  • may be more adaptive w/ pathogen presence

Disgust: a mechanism for behavioral immunity #

Biological disgust #

• facial expression, nausea, vomiting — protect body from dangerous substances
• curled lip, wrinkled nose, lower heart rate

Hypotheses:

• elicitor of disgust correlated with disease (body products, bad food, animals, body violations) — death, injury, infection
• women more sensitive to elicitors: especially during pregnancy

Cognitive disgust #

• learned behavior: e.g. incest
• used to enforce cultural norms, possibly based on biological reasons

Moral disgust #

• protect society

• norm violations

• mental model (“magic thinking”) — don’t take chances

• disgust decreases with familiarity

  • → own body odors
  • → changing baby’s diapers

Human: