The evolution of human behavior ZOL 313 June 19, 2008

The evolution of human behavior ZOL 313 June 19, 2008 Objectives 1. Understand the benefits and limitations of studying human behavior from an evolutionary perspective. 2. Understand how we an study human behaviors such as altruism, parental care, and competition from an adaptationist perspective. 3. Generate adaptive hypotheses and predictions about human behavior.

Caveats to studying human behavior from an evolutionary perspective: 1. Humans do not consciously do things to increase their inclusive fitness. (Neither do non-human animals.) Those organisms that tended to behave in a way that maximized their fitness tended to leave more offspring who also behaved that way. 2.Saying a behavior is adaptive does not mean it is morally/ethically right. 3.Saying a behavior is adaptive does not mean it is genetically determined/developmentally inflexible. 4.Not all human behavior is adaptive in modern society. However, it could have been adaptive for our ancestors in the recent past (EEA).

Environment of Evolutionary Adaptedness (EEA) Natural selection has designed our minds for the kind of life our hunter- gatherer ancestors experienced for thousands of years. Modern society is very different from what our ancestors in the recent past experienced.

Hypothesis 1: Inclusive fitness Prediction: Hypothesis 2: Direct reciprocity Prediction: Hypothesis 3: Indirect reciprocity: Those who perform charitable acts acquire a reputation for generosity, which increases their fitness because others are more likely to help them. Prediction: Why do humans give blood?

Why are humans altruistic? Prediction: Players will be more generous toward other players Hypothesis: Those who perform charitable acts acquire a reputation for generosity, which increases their fitness because

Friends smile (Duchenne smiles) when performing a sharing task. Hypothesis: Prediction: Those who smiled more in the sharing task will be more generous in a different altruistic situation. Smiles as an honest signal of altruism

Importance of reputation Hypothesis: Detection and punishment of non-altruistic humans is adaptively important. Prediction:

Mothers’ diets at conception influences the sex of their baby: Skip breakfast/eat less= Eat breakfast/more nutrients (cereal)= Adaptive sex determination in humans Likely due to male embryos being less likely to implant at low nutrient levels. How could this be adaptive? In many other animals, more males are produced in times of more resources. If males are more energetically expensive and/or more risky, then it makes adaptive sense to

Prenatal parent/offspring conflict Parents have higher fitness if they provide Offspring have higher fitness if the can get Parent/offspring conflict during pregnancy: 1.The fetus secretes the hormone hGC which delivers more blood (nutrients) to the fetus (benefit) but 2.The placenta secretes hormones that decrease the mother’s sensitivity to insulin, which results in more blood glucose for the fetus (benefit), but 3. Higher quality embryos can produce more human chorionic gonadotropin, which makes them less likely to be spontaneously aborted but mothers more likely to Prediction: Morning sickness is negatively correlated with first-trimester spontaneous abortions.

Parent/offspring conflict and abortion/infanticide Hypothesis: If investing in a fetus/newborn will reduce the lifetime reproductive success of a woman, ending her investment in that offspring may increase her lifetime fitness. Prediction: Prediction: Very young or very old women will be more likely to commit infanticide.

Hypothesis: Because men lack certainty of paternity, they should have evolved psychological mechanisms to prevent them from caring for the children of another man. Prediction: Prediction: Relatives of new mothers should be more likely to say a baby looks like its father when Certainty of paternity and adaptive divorce

Adaptive parental care by stepparents Hypothesis: Humans have evolved psychological mechanisms that encourage us to bias our parental care toward Prediction: Stepfathers will be more likely to give money to attend college to Prediction: Stepparents will be more likely to maltreat children who are not their own.

Adaptive benefits of adoption Hypothesis 1: Adoption is adaptive because it increases inclusive fitness. Prediction: Hypothesis 2: Adopted children can help increase the family workforce, benefiting genetic children. Prediction: At one time, 30% of all children became adoptees in Oceania.

Adaptive benefits of adoption Prediction: Infertile couples or couples who have just lost an only child will be more likely to adopt. Prediction: Hypothesis 3: Adoption is the maladaptive byproduct of otherwise adaptive proximate mechanisms causing humans to desire children and family.

Adaptive favoritism in helping children marry Hypothesis: Bridewealths/dowries are arbitrary cultural traditions with no adaptive significance. Prediction: These two forms of marriage payment should be Bridewealth: Dowry:

Prediction: Inheritance rules that favor sons should be more common in Adaptive favoritism in helping children marry Prediction: Bridewealths (men competing for women) should be more common in Prediction: In modern societies, very wealthy parents should leave more inheritance to their

Why do human women experience menopause? Hypothesis: “The Grandmother Effect” Once they reach a certain age, women can increase their fitness more by helping to raise grandchildren than Prediction: Women whose mothers are alive will Prediction: Grandmothers who live longer will have

Violence as an adaptive strategy? Hypothesis: Prediction: Homicide rate in Chicago neighborhoods should be negatively correlated with life expectancy.

Frequency dependent selection for left handedness Left-handed people have lower fitness than right-handed people. Hypothesis: Left handedness has persisted because when lefties are rare Prediction: More violent societies will have a

Hypothesis: Red color is important to men because in close animal relatives red frequently acts as an honest signal of Prediction: Men should be more distracted by red than other colors and should be more distracted by it than women are. Olympic contestants randomly assigned red uniforms The adaptive significance of red

Adaptive “tribal tendencies” Hypothesis: Humans (especially men) tend to have strong allegiances to local sports teams because we have evolved psychological mechanisms to promote a group-centered, “us vs. them” sense of morality. Chimpanzees and humans are the only known species in which groups of males band together with the intent of

Adaptive voting based on facial structure? Hypothesis: Humans adaptively use facial structure as a signal of leadership qualities. Prediction: Bush: dominance more masculine Kerry: likeability/intelligence more feminine