Download Human behavior I

The evolution of human behavior
ZOL 313
June 19, 2008
The evolution of human behavior
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.
ZOL 313
June 19, 2008
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)
Modern society is very different from what our ancestors in the recent past
experienced.
Natural selection has designed our
minds for the kind of life our huntergatherer ancestors experienced for
thousands of years.
Why do humans give blood?
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 are humans altruistic?
Hypothesis: Those who perform charitable acts
acquire a reputation for generosity, which
increases their fitness because
Prediction: Players will be more
generous toward other players
Smiles as an honest signal of altruism
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.
Importance of reputation
Hypothesis: Detection and punishment of non-altruistic
humans is adaptively important.
Prediction:
Adaptive sex determination in humans
Mothers’ diets at conception influences the sex of their baby:
Skip breakfast/eat less=
Eat breakfast/more nutrients (cereal)=
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.
Certainty of paternity and adaptive divorce
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
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:
At one time,
30% of all
children
became
adoptees in
Oceania.
Hypothesis 2: Adopted children can help increase the family workforce,
benefiting genetic children.
Prediction:
Adaptive benefits of adoption
Hypothesis 3: Adoption is the maladaptive byproduct of
otherwise adaptive proximate mechanisms causing
humans to desire children and family.
Prediction: Infertile couples or couples who have just lost an only child
will be more likely to adopt.
Prediction:
Adaptive favoritism in helping children marry
Bridewealth:
Hypothesis: Bridewealths/dowries
are arbitrary cultural traditions with
no adaptive significance.
Prediction: These two forms of
marriage payment should be
Dowry:
Adaptive favoritism in helping children marry
Prediction: Bridewealths (men competing for women) should be more common
in
Prediction: Inheritance rules that favor sons 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
The adaptive significance of red
Olympic contestants randomly assigned red uniforms
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.
Adaptive “tribal tendencies”
Chimpanzees and humans are the only known
species in which groups of males band together
with the intent of
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.
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