Social Psychology II
Class 1: evolutionary psychology
Altruism and natural selection
1) Natural selection and behavior
a. Evolutionary psychology investigates the evolutionary origin of
behavior and consequences for current psychological mechanisms.
i. Darwin (1859): “The Origin of Species”
1. Natural selection was mentioned in this book for
the first time
2. Theory of natural selection: Selection in nature occurs
because some organisms survive better and reproduce
better in a certain environment à some animals had
different beaks and now look different
a. survival of the fittest: better adaptation to
environment
b. better fitness (= transferring next- generation
genes) à higher fitness is higher chance to
transfer your genes to next generations
3. Primary interest of Darwin: evolution of physical
features of animals
ii. Modern Darwinians:
Primary interest: evolution
of behavior (e.g., altruistic
behavior)
1. Evolutionary
perspective on
human nature:
Universal
characteristics à
These successful characteristics provide such an
evolutionary advantage, that they spread throughout
the population and become typical for all mankind
1. Bipedalism (we started to walk on
two feet, straight back)
2. Universal psychological
features/characteristics
2) Altruistic behavior
a. = helping others although it may be detrimental to your own fitness
i. Meanwhile natural selection = seemingly selfish process, so
how are we so altruistic?
1
, 1. How to explain this?
a. The universal need to belong (every human
has this)
In the past if you were excluded from a group
your chance of survival were low. (evolutionary
explanation). Altruistic behavior is benefitial.
b. Baumeister & Tyce (1990): “social anxiety” as an
adaptation to prevent exclusion from the group
3) Inclusive fitness and kinship (Hamilton, 1964)
a. = your personal reproductive success (amount of children you have)
+ the effects you have on the reproduction of your genetic relatives,
weighted by the degree of genetic relatedness
i. identical twin: genetic relatedness = 100%
ii. parents - child: 50%
iii. brother/sister: 50%
iv. aunt/uncle - niece/nephew: 25%
v. cousins: 12.5%
b. = better fitness (of next generation) by helping (genetic) relatives à
giving kidney to sibling instead of stranger that you are not related
with increases the chance of his genes surviving (brother is 50%,
brother gets a child; child is 25%)
i. This implies...
1. One takes risks for genetic relatives
2. The higher the genetic relatedness, the higher the risk
one takes
a. Daly & Wilson, 1988
i. less care by step-parents than biological
parents
ii. more child abuse by step-fathers (100 x
more than biological fathers)
b. Burnstein et al., 1994
i. more help towards genetic relatives in
trouble
1. Conclusion:
a. Who we help in different
situations is predictable from
an evolutionary
perspective
b. Strong evidence for the
inclusive fitness theory
c. We tend to help people
younger that are dependant
on others vs older people
that are independent à why?:
2
, younger people have
reproductive value
4) Reciprocal altruism
a. Why do we help unrelated individuals? à “I help you when you help
me”
b. only possible if
i. the person being helped can be recognized later
ii. deceivers can be punished: cheaters are excluded
iii. only in intelligent species (primates, humans)
iv. only with acquaintances and small groups / tribes (guarantees
reciprocity)
c. In the end, helping increases fitness!
Sexual selection and sex differences in behavior
1) Sexual selection and parental investment
a. Sexual selection = the selection of –and different access to sexual
partners
b. Forms of sexual selection/competition:
i. Intrasexual competition: the winner passes on more genes
(male competition) (competition between members of the
same sex)
ii. Intersexual competition: choosing a mate based on their
preferences (female choice)
c. Parental investment theory (Trivers, 72): Sexes differ in terms
of time and effort spent in raising offspring (pregnancy, feeding,
protecting,...)
i. women invest more in raising offspring; 9 month pregnancy,
breastfeeding
ii. men invest only in their seed
à variation in reproduction between the sexes (female
record: 69 children/ male record: over a 1000)
à this imbalance will result in several sex differences!
d. Prediction: The sex that invests less in raising offspring will prefer
having more
(sexual) partners
1. Undergraduates – What’s the desired number of sexual
partners? (Buss & Schmitt (1993)
Women: 1 during the next month / 4 to 5 in their whole
life
Men: 2 during the next month / 8 during the next year /
on average 18 in their whole life
3
, e. Prediction: The sex that invests less in raising offspring will be less
selective in
choosing partners
i. Undergraduates – “I’ve noticed you around...I find you very
attractive”:
(Clark & Hatfield, 1989)
• Would you go out with me tonight? (Yes à F: 55% / M: 50%)
• Would you come over to my apartment tonight? (Yes à F:
6% / M: 69%)
• Would you go to bed with me tonight? (Yes à F: 0% / M:
75%)
Replication in Austria ~ identical findings (Voracek et al.,
2005)
Attitude towards casual sex: men more positive
• Oliver & Hyde (1993): d = .81
• Petersen & Hyde (2010): d = .45
1. Conclusion: men are less selective in choosing
partners.
2) Mating preferences
a. Prediction: men are looking for partner who is young and
physically
attractive (~ sign of fertility) & women are looking for a partner who
can provide material support (good financial prospect)
i. Confirmed in 37 cultures (Buss, 1989); Replicated in 2011
(Russock)
‘Structural powerlessness hypothesis’: Alternative
explanation for the difference/ Eagly & Wood (1999): men are
associated with the role of breadwinner / ‘money maker’;
women are financially dependent on their man (gender roles)
↔ When women have a higher income/SES (“money is not an
issue”), they place even higher importance to financial
means/SES in men
1. women long for a partner who provides financial
perspectives
2. women long for a partner who is ambitious and
industrious
3. men long for a partner who is younger, women long for
a partner who is older
4. men long for a partner who is physically attractive
a. Both sexes a physically attractive partner
• waist-to-hip ratio:
• optimal .90 for men
• optimal .67-.80 for women
4
Class 1: evolutionary psychology
Altruism and natural selection
1) Natural selection and behavior
a. Evolutionary psychology investigates the evolutionary origin of
behavior and consequences for current psychological mechanisms.
i. Darwin (1859): “The Origin of Species”
1. Natural selection was mentioned in this book for
the first time
2. Theory of natural selection: Selection in nature occurs
because some organisms survive better and reproduce
better in a certain environment à some animals had
different beaks and now look different
a. survival of the fittest: better adaptation to
environment
b. better fitness (= transferring next- generation
genes) à higher fitness is higher chance to
transfer your genes to next generations
3. Primary interest of Darwin: evolution of physical
features of animals
ii. Modern Darwinians:
Primary interest: evolution
of behavior (e.g., altruistic
behavior)
1. Evolutionary
perspective on
human nature:
Universal
characteristics à
These successful characteristics provide such an
evolutionary advantage, that they spread throughout
the population and become typical for all mankind
1. Bipedalism (we started to walk on
two feet, straight back)
2. Universal psychological
features/characteristics
2) Altruistic behavior
a. = helping others although it may be detrimental to your own fitness
i. Meanwhile natural selection = seemingly selfish process, so
how are we so altruistic?
1
, 1. How to explain this?
a. The universal need to belong (every human
has this)
In the past if you were excluded from a group
your chance of survival were low. (evolutionary
explanation). Altruistic behavior is benefitial.
b. Baumeister & Tyce (1990): “social anxiety” as an
adaptation to prevent exclusion from the group
3) Inclusive fitness and kinship (Hamilton, 1964)
a. = your personal reproductive success (amount of children you have)
+ the effects you have on the reproduction of your genetic relatives,
weighted by the degree of genetic relatedness
i. identical twin: genetic relatedness = 100%
ii. parents - child: 50%
iii. brother/sister: 50%
iv. aunt/uncle - niece/nephew: 25%
v. cousins: 12.5%
b. = better fitness (of next generation) by helping (genetic) relatives à
giving kidney to sibling instead of stranger that you are not related
with increases the chance of his genes surviving (brother is 50%,
brother gets a child; child is 25%)
i. This implies...
1. One takes risks for genetic relatives
2. The higher the genetic relatedness, the higher the risk
one takes
a. Daly & Wilson, 1988
i. less care by step-parents than biological
parents
ii. more child abuse by step-fathers (100 x
more than biological fathers)
b. Burnstein et al., 1994
i. more help towards genetic relatives in
trouble
1. Conclusion:
a. Who we help in different
situations is predictable from
an evolutionary
perspective
b. Strong evidence for the
inclusive fitness theory
c. We tend to help people
younger that are dependant
on others vs older people
that are independent à why?:
2
, younger people have
reproductive value
4) Reciprocal altruism
a. Why do we help unrelated individuals? à “I help you when you help
me”
b. only possible if
i. the person being helped can be recognized later
ii. deceivers can be punished: cheaters are excluded
iii. only in intelligent species (primates, humans)
iv. only with acquaintances and small groups / tribes (guarantees
reciprocity)
c. In the end, helping increases fitness!
Sexual selection and sex differences in behavior
1) Sexual selection and parental investment
a. Sexual selection = the selection of –and different access to sexual
partners
b. Forms of sexual selection/competition:
i. Intrasexual competition: the winner passes on more genes
(male competition) (competition between members of the
same sex)
ii. Intersexual competition: choosing a mate based on their
preferences (female choice)
c. Parental investment theory (Trivers, 72): Sexes differ in terms
of time and effort spent in raising offspring (pregnancy, feeding,
protecting,...)
i. women invest more in raising offspring; 9 month pregnancy,
breastfeeding
ii. men invest only in their seed
à variation in reproduction between the sexes (female
record: 69 children/ male record: over a 1000)
à this imbalance will result in several sex differences!
d. Prediction: The sex that invests less in raising offspring will prefer
having more
(sexual) partners
1. Undergraduates – What’s the desired number of sexual
partners? (Buss & Schmitt (1993)
Women: 1 during the next month / 4 to 5 in their whole
life
Men: 2 during the next month / 8 during the next year /
on average 18 in their whole life
3
, e. Prediction: The sex that invests less in raising offspring will be less
selective in
choosing partners
i. Undergraduates – “I’ve noticed you around...I find you very
attractive”:
(Clark & Hatfield, 1989)
• Would you go out with me tonight? (Yes à F: 55% / M: 50%)
• Would you come over to my apartment tonight? (Yes à F:
6% / M: 69%)
• Would you go to bed with me tonight? (Yes à F: 0% / M:
75%)
Replication in Austria ~ identical findings (Voracek et al.,
2005)
Attitude towards casual sex: men more positive
• Oliver & Hyde (1993): d = .81
• Petersen & Hyde (2010): d = .45
1. Conclusion: men are less selective in choosing
partners.
2) Mating preferences
a. Prediction: men are looking for partner who is young and
physically
attractive (~ sign of fertility) & women are looking for a partner who
can provide material support (good financial prospect)
i. Confirmed in 37 cultures (Buss, 1989); Replicated in 2011
(Russock)
‘Structural powerlessness hypothesis’: Alternative
explanation for the difference/ Eagly & Wood (1999): men are
associated with the role of breadwinner / ‘money maker’;
women are financially dependent on their man (gender roles)
↔ When women have a higher income/SES (“money is not an
issue”), they place even higher importance to financial
means/SES in men
1. women long for a partner who provides financial
perspectives
2. women long for a partner who is ambitious and
industrious
3. men long for a partner who is younger, women long for
a partner who is older
4. men long for a partner who is physically attractive
a. Both sexes a physically attractive partner
• waist-to-hip ratio:
• optimal .90 for men
• optimal .67-.80 for women
4
