Summary of:
Buss, D. M. (2007). Evolutionary Psychology: The New Science of the Mind. (3rd Ed.).
Boston: Allyn and Bacon.
Summary by Jessica Nunnally, Monica Ramos, and Alex Kraszewski
For Dr. Mills’ Psychology 452 class, Spring, 2009
BUSS CHAPTER 7 OUTLINE/SUMMARY
PROBLEMS OF PARENTING
In evolutionary terms, offspring are a vehicle for their parents, through which their
parent’s genes may be passed on to later generations, without which ones genes may
cease to exist. Consequently, natural selection should and does favor strong parental
mechanisms that help ensure the survival and future reproduction of one’s offspring. If
one’s offspring failed to survive, then one’s mating efforts would be wasted and
reproductively meaningless. Thus, evolution should result in the creation of adaptations
for caring for one’s children and helping them survive, despite potential risk to one’s self.
However, despite the importance of offspring and their survival for the
continuation of one’s genes, many species do not engage in parental care at all. This is
due in part to the amount of cost involved for a parent to invest in their child. Indeed,
caring for a child requires the expenditure of important resources that could otherwise be
used for the purposes of finding additional mates or increasing one’s reproductive output.
Additionally, parents who protect their offspring put themselves and their own survival at
risk. Consequently, if a species engages in parental care it is reasonable to assume that
the reproductive benefits must outweigh the reproductive costs.
WHY DO MOTHERS PROVIDE MORE PARENTAL CARE THAN FATHERS?
Throughout the animal kingdom, females are significantly more likely than males to
engage in parental care for their children. Cross-cultural human data has shown that
women care more for their children than men specifically with respect to time spent in
the child’s vicinity, touching the child, and teaching the child. There are two evolutionary
based hypotheses related to the predominance of female parental care relevant to this
discussion; the paternity uncertainty hypothesis and the mating opportunity costs
hypothesis.
The Paternity Uncertainty Hypothesis
Throughout the animal kingdom, mothers are typically completely certain that they
contributed genetically to their offspring. In contrast, males can never be 100 percent
certain that a child is theirs; this is the problem of paternity uncertainty. Paternity
uncertainty means that from a male perspective, there is always some possibility that
another male has fertilized the female’s egg and thus, the male’s own genetic contribution
to the female’s offspring is in question. For example a female may have already mated
with another male or could mate with another male at any time during the courtship.
Consequently, if a male invests in another’s child, he suffers large resource costs to
himself and his own legitimate offspring. Thus, in the context of paternity uncertainty
parental investment has far greater pay offs and benefits for mothers than for fathers,
which may explain the tendency for greater female investment in children.
The Mating Opportunity Cost Hypothesis
Mating opportunity costs are experienced by both sexes and are defined as “missed
additional matings as a direct result of effort devoted to offspring”. However, despite
being experienced by both sexes, males suffer greater costs as a result of missed mating
opportunities than their female counterparts. Consequently, because these costs are
greater for males than for females, since males can increase their offspring by mating
with a variety of women and do not have to endure pregnancy in order to reproduce,
males will be less likely to exercise parental care. Thus, this hypothesis would predict
that in situations in which there is a surplus of males but limited access to females, males
would be more likely to exercise parental care in order to maintain their ability to
reproduce with that female and insure the propagation of their genes.
AN EVOLUTIONARY PERSPECTIVE ON PARENTAL CARE
“Selection will favor the evolution of mechanisms in parents that favor offspring who are
likely to reproduce.” Consequently evolved parental mechanisms of care should take into
account three things: (1) Genetic relatedness of the offspring. (2) Ability of the offspring
to convert parental care into fitness. And (3) Alternative uses of the resources that might
be available to invest in offspring.
Genetic Relatedness of the Offspring
“Substitute parents will generally tend to care less profoundly for children than natural
parents”. Studies of parental feelings among stepparents for their stepchildren support
this Darwinian view, with only 53% of stepfathers and 25% of stepmothers claiming to
have any parental feelings for their stepchildren. Consequently, mothers seek to insure
their mate (the father of their child) of their genetic relatedness to the child by (1)
reassuring him of her sexual fidelity during the period in which the child was conceived
and (2) Influencing his perception of the child’s resemblance to him (inferring genetic
relatedness). Research supports these efforts, confirming that mothers (and their side of
the family) are more likely to say a child resembles the father. Additionally, research
indicates that perceived physical relatedness may predict how much parental investment
the father gives.
Parent’s Investment in Children
Anthropologists assessing the effects of male paternity uncertainty on their investment in
children’s college education made three predictions rooted in evolutionary theory: (1)
Men will allocate more resources to their genetic children than to their stepchildren, (2)
men who are uncertain about whether a child is genetically theirs will invest less than
those who are certain, and (3) men will invest more in children when the child’s mother
is their current mate than they will in children from former mateships. Their results
supported all three predictions. Genetic relatedness influenced greatly the amount of
investment with genetic children being 5.5 times more likely to receive money for
college. Additionally, children for whom there was a large degree of paternity uncertainty
were only 13% as likely to receive money for college compared to their high paternity
certain counterparts. Finally, a child was approximately three times as likely to receive
money from their father if the child’s mother was the respondent’s current mate at the
time the child entered college.
Child Abuse and Other Risks of Not Living with Both Parents
According to the inclusive fitness theory, genetic relatedness should be a predictor of
infanticide, the less relatedness the greater the likelihood of infanticide. Daly and
Wilson’s study confirmed this prediction. Their data showed that children living with one
genetic parent and one stepparent are approximately 40 times more likely to be physically
abused than children living with both biological parents. This increased risk is still
present even when poverty and SES are accounted for.
Child Homicide as a Function of Genetic Relatedness
In addition to child abuse risk, Daly and Wilson have explored the relationship between
genetic relatedness and child homicide. Their results showed that rates of child murder
are significantly higher for stepparents than for genetic parents, with the highest risk for
children age 2 or younger. Additionally, they found that the risk of a preschool-aged child
for being killed ranged from 40 to 100% higher for stepchildren than for children living
with both genetic parents.
Sex Differences in Parenting Adaptations
The “primary caretaker hypothesis” asserts that women will have evolved adaptations
that increase the odds of their children’s survival. Consequently, female interest in infants
should emerge early on and remain elevated to insure enough parenting experience and
motivation to successfully raise their own child. Research has confirmed this idea,
showing that women are better than men at recognizing infant facial expressions.
However, it is important to note that although sex differences exist, this does not imply
that men do not provide for or protect their children. In fact, humans are one of the few
species which exhibit relatively high levels of paternal investment in offspring.
Offspring’s Ability to Convert Parental Investment into Reproductive Success
When considering this point, it is important to note that it does not imply that parents will
only invest in children who are incredibly healthy; rather it focuses on the child’s ability
to convert parental investment into fitness and reproductive success. Research shows that
parental investment significantly impacts a child’s physical and social well-being
although causal connections cannot be made. Daly and Wilson contend there are two
reasonable candidates for assessing which children should be invested in most: (1)
whether the child was born with an anomaly, and (2) the age of the child.
Parental Neglect and Abuse of Children with Congenital Abnormalities
Studies show that a significant portion of such children are institutionalized and
never or rarely visited. Additionally, the rates of child abuse and neglect for these
children are approximately 7.5 to 60%, a substantially higher rate then that of
their developmentally typical counterparts (approximately 1.5%).
Maternal Care Based on the Health of the Child
Maternal interactions with pairs of twins in which one twin was healthier than the
other showed that every mother by the infants’ age of 8 months directed more
positive maternal behaviors toward the healthier infant. Consequently, the health
status of the baby affected the degree of positive maternal behavior.
Age of the Child
Reproductive value increases from birth to puberty. Consequently, parents should be less
likely to harm a child as the child ages. Research shows that infants are at much higher
risk of being killed by their genetic parents than any other age group. However, a
possible alternative explanation for these findings is that children become increasingly
capable of defending themselves as they age.
Investment in Sons versus Daughters: The Trivers-Willard Hypothesis
This hypothesis contends that parents will produce more sons and invest more in sons
when the parents are in good health and have a strong chance of producing a son who
will be successful in mating. In contrast, parents who are in poor condition or have few
resources to invest will invest more in daughters. This hypothesis is based on the insight
that the condition of the son or daughter might make it more likely that one or the other
would be more capable of utilizing parental care. However, research related to this
hypothesis has been inconclusive and future studies are needed.
ALTERNATIVE USES OF RESOURCES AVAILABLE FOR INVESTMENT IN
CHILDREN
Women’s Age and Infanticide
Younger women have many years to reproduce thus passing on an opportunity to bear
one child may involve minimal cost. However, older women may not have as many
reproductive years left, thus passing on an opportunity may entail much larger cost to
them. Consequently, younger women should be more likely than older women to commit
infanticide. This hypothesis is strongly supported by research on the Ayoreo Indians,
which showed that infanticide is lowest among the oldest age group of women.
Additionally, it has been shown that young Canadian women commit infanticide more
often than older Canadian women.
Women’s Marital Status and Infanticide
Daly and Wilson propose that martial status will affect the likelihood that a woman will
commit infanticide. Research has shown that unwed mothers are more likely to commit
infanticide than their wed counterparts regardless of age of the mother.
Parental Effort versus Mating Effort
Effort put toward parenting is effort and resources that cannot be directed toward
securing additional mates.
THE THEORY OF PARENT-OFFSPRING CONFLICT
Evolution predicts that parents and children will have conflicts. These conflicts are over
the allocation of parental resources to the child. Because the parent only shares 50% of
their genes with their child, the parent should desire to allocate resources evenly to each
child. However, because the child has the interest of their own genes at heart, they will
inevitable desire greater allocation of their parents resources, thus, resulting in parentchild conflict.
Mother-offspring Conflict in Utero
Conflict between a mother and her offspring begins in utero with the battle over whether
or not a fetus will be spontaneously aborted. Consequently, if the fetus is viable and not
aborted, it will demand resources from the mother for its survival that may harm the
mother and deplete her own resources, thus, taking more than the mother wants to give.
The Oedipal Conflict Revisited
The Oedipus Complex as put forth by Freud asserts that their will be conflict between
children and their same-sex parent over the opposite sex parent due to a sexual desire for
that parent. However, this theory is not supported as evolution would predict that sons
would rarely have any sexual desire for their mothers particularly because of adaptations
to avoid incest such as male preferences for younger women with more reproductive
years.
BUSS CHAPTER 8 OUTLINE/SUMMARY
PROBLEMS OF KINSHIP
When viewed through the lens of the inclusive fitness theory, all family members
function as “vehicles” of fitness for the spreading of one’s genes, but various members
differ in value for the purposes of the individual’s genes. All else being equal, selection
should favor adaptations for helping kin according to their genetic relatedness.
Consequently, when it comes to altruism, selection will favor helping a close relative
over a distant relative, and helping a distant relative over a stranger.
THEORY AND IMPLICATIONS OF INCLUSIVE FITNESS
Altruism is defined as including two components: (1) incurring a cost to the self, and (2)
providing a benefit to the other person.
Hamilton’s Rule – Under what conditions will a person act altruistically?
Although upon initially considering it, altruism does not seem to fit with evolutionary
perspectives, Hamilton concluded that altruism could in fact evolve if the costs to the self
were outweighed by the benefits to the recipient of the altruistic act. Thus, he proposed
this formula c<rb, where c = the cost to the self, r = the degree of genetic relatedness, and
b = the benefit to the recipient. Consequently, this formula defines the conditions under
which adaptations in favor of aiding kin can evolve. Thus, Hamilton’s theory of inclusive
fitness is the most important theoretical revision to Darwin’s theory of natural selection
this century.
Theoretical Implications of Hamilton’s Rule
Psychological adaptations are anticipated to have evolved for different types of
relatedness and kin relations.
Sibs versus Half Sibs
Full sibs have greater genetic relatedness than do half sibs. Consequently, one
would anticipate this to be very important when the decision to cooperate or
compete is a close call for survival. Additionally, one would expect greater
conflicts within stepfamilies.
Grandparents and Grandchildren
Grandparents are related to their grandchildren by an r of .25. The “grandmother
hypothesis” contends that menopause may have developed as a means of
redirecting one’s investment to their children and grandchildren instead of
focusing on one’s own reproductive potential. Consequently, adaptations for
allocating grandparents investment may have evolved.
Hypotheses about Universal Aspects of Kin
Daly, Salmon and Wilson proposed a set of hypotheses regarding the universal
aspects of the psychology of kinship: (1) Ego-centered kin terminology will be
universal, (2) all kinship systems will make critical distinction along the lines of
sex because of its reproductive implications, (3) Generation is critical, (4) kin
relationships will be universally array on a dimension of closeness (linked to
genetic relatedness), (5) the degree of cooperation and solidarity between kin will
be a function of degree of relatedness. (6) eleder members of an extended kin
family will encourage the younger members to behave more altruistically and
cooperatively toward collateral kin, (7) one’s position within an extended kin
network will be core components of the self-concept (beliefs about who you are
will include kin linkages), (8) people everywhere will be aware of who their
“real” kin are, (9) kinship terms will be used to persuade and influence other
people.
EMPIRICAL FINDINGS THAT SUPPORT THE IMPLICATIONS OF
INCLUSIVE FITNESS THEORY
Alarm Calling in Ground Squirrels
Squirrels will sometimes emit a high-pitched whistle that acts as an alarm warning
to the other squirrels of a potential threat. Those who are warned benefit, but
those who act as the alarm caller often suffer and are more likely to be killed by
the predator as a result. Three hypotheses have been proposed to explain this
squirrel altruism: (1) The predator confusion hypothesis-call might create
confusion and help all squirrels escape, (2) Parental investment hypothesis –
alarm callers children may be more likely to survive as a result, (3) Inclusive
fitness hypothesis – callers kin have increased likelihood of survival. The 2nd and
3rd hypothesis are supported, but the first is not.
Kin Recognition and Kin Classification in Humans
Exposure to kin in infancy is used as a cue to avoid incest. Odor is also used to
detect one’s own kin and is particularly strong in mothers. Kin terminology and
classification systems are used to distinguish kin based on genealogical distance,
social rank, and group membership which helps identify different kinship values.
Patterns of Helping in the Lives of Los Angeles Women
A Study of 300 women from Los Angeles found that helping exchanges were
more likely to occur with close kin than with distant kin. Additionally, researchers
found that helping among kin will be preferentially channeled to those with higher
reproductive potential. However, findings are not generalizeable to men or to
women outside of L.A.
Life or Death Helping among Humans
Researchers differentiated between two types of helping: (1) substantial helping,
and (2) relatively trivial helping. They hypothesized that patterns of altruism
should be stronger for the first type of helping than for the second. Subsequently,
researchers found that helping in the proposed hypothetical situations decreased
as the degree of genetic relatedness decreased and helping in life-or-death
situations declined as the potential recipients age increased.
Genetic Relatedness and Emotional Closeness: Is Blood Thicker Than Water?
Theorists have proposed emotional closeness as a psychological mediator for
helping motivations. Research found that individuals are more likely to be
emotionally close to their family members who have the greatest genetic
relatedness and that emotional closeness mediated altruistic behavior toward kin.
Additionally, frequency of contact, doing favors, and the amount of psychological
grief experienced when a child dies are all indicator of emotional closeness.
Consequently, emotional closeness may act as a mediator for altruistic acts.
Vigilance over Kin’s Romantic Relationships
The degree to which individuals maintain vigilance over their kin’s romantic
relationships is affected by genetic relatedness and the sex of the target (more
vigilant with females and closer kin).
Kinship and Stress
The stress hormone cortisol inhibits growth and reproductive function. Close kin
relationships are important for minimizing the presence of stress hormones and
their impact on a child (the more close kin present, the better).
Kinship and Survival
Studies on Mayflower and Donner Party survivors found that those who were
most likely to die had the fewest relatives around them.
Patterns of Inheritance
Smith, Kish, and Crawford found that in wills genetic relatives are bequeathed
more than nonrelatives, close kin receive more than distant kin, and direct
descendants receive more than collateral kin.
Investment by Grandparents
With respect to the child, the mother’s mother invests most, the father’s father
invests least and the mother’s father and father’s mother show intermediate
investment. This is speculated to be due to the fact that the mother’s mom is
100% certain that she is genetically related to the child and to the fact that there is
a great amount of paternity uncertainty among the father’s father.
A Broader Perspective on the Evolution of the Family
Evolutionary biologist Stephen Emlen defines family as “those cases where
offspring continue to interact regularly, into adulthood, with their parents”. He
then subdivides families into two categories: (1) simple families – a single parent
or conjugal pair in which only one female reproduces, and (2) extended families –
groups in which two or more relatives of the same sex may reproduce. If male is
present the family is biparental. If the male is absent, the family is matrilineal.
Only three percent of all birds and mammals form families. This is due to the
costs for the offspring including (1) delayed reproduction and suppression of
reproduction, (2) competition for resources. So, why have families? According to
the ecological constraints model families emerge because there are insufficient
reproductive vacancies that might be available to the sexually mature offspring.
Another theory, the familial benefits model contends that families form because
of the great benefits they provide for the offspring, including: (1) enhanced
survival as a result of aid and protection of the family (2) enhanced ability to
compete subsequently, (3) the possibility of inheriting family resources or
territory and (4) inclusive fitness benefits and being helped by relatives.
Emlen combines the two theories and posits several predictions related to
family dynamics of kinship and cooperation: (1) Families will form when there is
a shortage of reproductive vacancies but will break up when the vacancies
become available, (2) Families that control many resources will be more stable
and enduring than families that lack resources, (3) Help with rearing the young
will be more prevalent among families than among comparable groups lacking
kin relatives, (4) Sexual aggression will tend to be low in families compared with
groups of nonrelatives because relatives will evolve to avoid risks of inbreeding.
Additionally, Emlen poses several predictions related to changes in family
dynamics as a result of the loss or disruption of an existing breeder: (5) When a
breeder is lost, families will get into a conflict over who will fill the breeding
vacancy, and (6) the loss of an existing breeder and replacement by a breeder who
is genetically unrelated to family members already present will produce an
increase in sexual aggression.
Critique of Emlen’s Theory of the Family
According to Davis and Daly, there are three considerations that should be taken
into account when assessing Emlen’s predictions for human families: (1) Human
families might remain together because of competition from other groups, such
that remaining in a large kin-based coalition is advantageous, (2) humans engage
in extensive social exchanges based on reciprocal altruism with non-kin and (3)
nonreproductive helpers have little incentive to encourage their offspring to
disperse, which may help stabilize families.
The Dark Side of Families
There are three primary sources of familial conflict: (1) sibling conflict- over
access to parental resources, (2) parent-offspring conflict- over optimal allocation
of resources, and (3) parental conflict- over resource allocation.
BUSS CHAPTER 7: OUTLINE

Problems of Parenting
o Why do mothers provide more parental care than fathers?
 Paternity Uncertainty Hypothesis
 Mating Opportunity Cost Hypothesis
o An Evolutionary perspective on parental care
 Genetic Relatedness to Offspring
 Who are newborns said to resemble?
 Parent’s investment in children
 Child abuse and other risks of Not Living with both Parents
 Child homicide as a function of genetic relatedness
 Sex differences in parenting adaptations
 Offspring’s ability to convert parental care into reproductive success
 Parental neglect and abuse of children with congenital
abnormalities
 Maternal care based on health of the child
 Investments in sons versus daughters
 Alternative uses of resources available for investment in children
 Women’s age and infanticide
 Women’s marital status and infanticide
 Parental effort versus mating effort
o The Theory of Parent-Offspring Conflict
 Mother-offspring conflict in utero
 The Oedipal Complex
BUSS CHAPTER 8: OUTLINE

Problems of Kinship
o Theory and implications of inclusive fitness
 Hamilton’s rule
 Theoretical implications of Hamilton’s rule
 Sibships
 Sibs versus half sibs
 Grandparents and grandchildren
 Hypotheses about universal aspects of kinship
o Empirical findings that support the implications of Inclusive Fitness
Theory
 Alarm calling in ground squirrels
 Predator confusion hypothesis
 Parental investment hypothesis
 Inclusive fitness hypothesis
 Kin recognition and classification in humans
 Patterns of helping in the lives of Los Angeles women
 Life-or-death helping among humans
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Genetic relatedness and emotional closeness
Vigilance over kin’s romantic relationships
Kinship and stress
Kinship and survival
Patterns of inheritance
Investment by grandparents
A broader perspective on the evolution of the family
 Familial benefits model
 Ecological constraints model
 Emlen’s theory and predictions
o Critique of Emlen’s theory of the family
o The Dark side of Families
 Sibling conflict
 Parent-offspring conflict
 Parental conflict