Chapter 1: Clearing the Way for an Evolutionary Psychology
- Evolutionary psychology is an approach to psychology that takes seriously the idea that
our own species is a product of evolution. It is both old and new— old because it started
with Charles Darwin himself, who applied evolutionary thinking to human behavior in
his 1872 book The Expression of the Emotions in Man and Animals, and new because,
until the last decade or so, the theory of evolution has had relatively little impact on the
field of psychology.
- John Tooby and Leda Cosmides ( 1992) that many psychologists hold to a general set of
beliefs called the standard social science model, or SSSM, that has a pervasive
influence on their research and theories. The SSSM is a useful place to begin sketching
the landscape explored in this book precisely because evolutionary psychology disagrees
with the SSSM in several important ways..
o The SSSM has a long history that reaches back to the enlightenment philosophers,
long before the founding of psychology. It rests on three interrelated assumptions.
Assumption 1: The Blank Slate; Tabula Rasa
The blank slate embodies the idea that the human mind has no
inborn tendencies or inclinations.
Assumption 2: The Irrelevance of Biology
An equally important supporting pillar of the SSSM is the
assumption that biological constraints on human behavior are
minor and unimportant. With experience playing the leading role,
biological influences fade to the vanishing point. According to the
SSSM, humans have few if any “ instincts” and only a few basic,
biological drives, such as hunger, thirst, and sex.
Assumption 3: General- Purpose Learning Mechanisms.
According to the SSSM, experience exerts its effects through the
process of learning.
A general- purpose mechanism is one that can handle many
different kinds of input information and that can generate many
different kinds of output.
According to the SSSM, one or a very few learning mechanisms
account for all of human behavior.
- Critique of the Standard Social Science Model
o Tooby and Leda Cosmides ( 1992) have detailed the problems with the SSSM, we
discuss five interrelated issues: development, the nature/ nurture dichotomy,
learning, the unity of science, and design.
The SSSM Misunderstands the Nature of Development A truly blank slate could not respond to the environment, since it
has no rules for how to respond.
The SSSM Draws a False Dichotomy Between Nature and Nurture
It is a mistake to divide the causes of behavior between nature and
There are several genes that affect skin color by coding for the
production of pigments, notably melanin, that darken the skin.
These genes are active at some spontaneous level at birth. But their
subsequent level of activity, which determines how much melanin
is produced and deposited in the skin, depends critically on one
feature of the environment: the level of uvb radiation ( one
component of sunlight).
The pattern of response that the melanin genes encode, producing
more melanin when we experience high levels of uvb and
disassembling the melanin molecules when uvb levels drop. This is
the so- called norm of reaction for the melanin complex, the rule
that translates environmental variation into changes in the
organism. Traits like suntanning, where evolution has crafted a
particular norm of reaction, are called facultative traits
Nature and nurture work together in the development of every trait.
Environmental Effects Cannot Be Explained by General Laws of
o Learning mechanisms tend to be specialized for specific
kinds of problems.
The SSSM Drives a Wedge Between the Natural and Social Sciences
The wedge between the natural and social sciences can be
illustrated by the fact that psychologists and biologists for the most
part study behavior in isolation from each other, as indicated by the
way they approach differences between men and women. Both
psychologists and biologists agree that, on average, men desire to
mate more often and with a greater variety of partners than women
do. But they disagree about the causes of the sex difference.
the SSSM tend to attribute these differences to cultural influences,
whereas biologists have an evolutionary explanation
Exempting the behavior of organisms from the principles of
biology is like exempting the behavior of atoms from the principles
The SSSM Lacks an Overarching Theory of Design
Evolutionary psychology explains why people respond to their
environments in the ways they do. Traditional psychology is most concerned with mechanism, while
evolutionary psychology focuses on function.
o Tinbergen’s Levels of Explanation
Niko Tinbergen ; His major division is between proximate and ultimate
causes. Proximate questions ( explanations) have to do with
mechanisms. They are “ plumbing- and- wiring” explanations, and
generally answer “ how?” questions: How does the system work?
Ultimate questions ( explanations) concern the evolution of the trait.
Generally, they answer the question “ why?”: Why does this system exist,
and why does it have the form it does?
a. Developmental. Genes, gene- environment interactions; age and
sex- related variation
b. Physiological. Neuronal, hormonal, biochemical, and
c. Historical. Evolutionary origins; precursors.
c. Selective. Adaptive value; what problems does the trait solve?
o Margaret Mead and the Myth of Infinite Malleability
Mead is world famous for her writings on the culture of Samoa, a chain of
islands in the Southwest Pacific. In her classic book, Coming of Age in
Samoa, Mead ( 1928/ 1973) drew a picture of a carefree society in which
there was no selfishness or jealousy, and people avoided competition.
Mead claimed that children were reared communally without attachment
to their parents and so roamed freely in a sort of paradise where everyone
shared in their upbringing.
The only problem with this picture is that it is totally at odds with the facts
of Samoan life. Derek Freeman was an anthropologist who studied
Samoan culture during many field trips, stretching over several decades.
He found that Samoans are far from the carefree people of Mead’s
writings ( Freeman, 1996). They are strict in raising their children in
Margaret Mead and many others like her tried to show that human
behavior is almost infinitely malleable— that human behavior is, for all
interesting purposes, learned.
o Human Nature as Revealed by the Universal People
Donald Brown ( 1991) argues that anthropologists have vastly
underestimated the similarities among cultures simply because they are as
invisible to us as the air we breathe. He has described what he calls the
universal people— whose behavior is typical of every single human culture. This is the human nature that would be seen by the hypothetical
Martian who landed randomly anywhere in the world
Human nature is not infinitely variable.
The universal people live in families centered on mother, children, and a
man. Social etiquette, etc They recognize marriage, which defines socially
recognized sexual access to a fertile woman. They pay attention to
kinship, marking father and mother, son and daughter, aunt and uncle.
There is division of labor by age and sex. Women do more child care. Men
are more aggressive than women and dominate political affairs. They
show modesty about sexual behavior and bodily functions even if they go
about naked. They have supernatural beliefs and develop theories of
misfortune, disease, and death. They plan for the future, have rituals to
observe life passages, and mourn their dead.
o Human Nature and Human Variability
Genetically based traits are often highly responsive to local conditions.
The genetic fallacy is the mistaken idea that evolved traits are inflexible.
o Evolved Environmental Influences: The Example of Violence
Violence may be facultatively dependent on the frequency of violence in
There are three conventional explanations for the positive association
between violence viewing and violent behavior.
1. Violence viewing increases arousal that can spill over into
behavior ( e. g., Zillmann, 1989).
2. Violence viewing disinhibits violence; that is, it lowers our
inhibitions against violence ( e. g., Berkowitz, 1984).
3. Violence viewing evokes imitation.
Norms of reaction provide the key to understanding the influence of
environment on behavior.
o Let’s return to our model facultative trait, suntanning. We know that suntanning
has a genetic basis.
1. The fact that suntanning has a genetic basis does not imply that skin
color is fixed and invariant. Within a moderate range, skin color could be
darkened or lightened simply by adjusting uvb dosages up or down, that is,
by manipulating the environment in the appropriate way.
2. That suntanning has a genetic basis does not imply that individuals with
different skin colors necessarily have different genes governing their
melanin production. They could have the very same genes but differ in
skin color because of their different experience with uvb. 3. Family members, such as parents and offspring or brothers and sisters,
will resemble each other in skin color partly because they share many
genes and partly because they share much of their environment
o The naturalistic fallacy is the mistaken idea that an evolutionary approach
excuses many of the evils in society on the basis that they are “ natural.”
The naturalistic fallacy confuses is with ought. Something is not morally
acceptable simply because it is “ natural.”
1. Evolutionary psychology ( EP) is an approach to all of psychology, not simply
another specialty within psychology, such as learning or perception.
2. Darwin and some early psychologists such as William James and James
McDougall incorporated evolutionary thinking into the study of behavior, but
most contemporary psychologists ignore the implications of evolutionary theory.
3. Much of traditional psychology draws on the standard social science model (
SSSM), which rests on three assumptions: a. Human nature is a blank slate. b.
Biology plays a minor role in shaping human behavior. c. Learning operates by
one or a very few general- purpose mechanisms.
4. Evolutionary psychology levels five criticisms at the SSSM. a. The SSSM
ignores the fact that development depends on gene- environment interactions. b.
The SSSM reinforces a false dichotomy between nature and nurture. c. General-
purpose learning mechanisms cannot explain the wide array of known
environmental effects. Learning mechanisms are evolved, and like most evolved
traits, are shaped to serve particular narrow functions. d. The SSSM erects an
artificial wall between the natural and social sciences. e. The SSSM offers no
general unifying framework for all of psychology.
6. Tinbergen showed that it is possible and useful to explain behavior at multiple
levels of causation. 7. In this respect, evolutionary psychology asks ultimate or “
why” questions, whereas traditional psychology asks proximate or “ how”
questions. 8. The notion that human nature is infinitely malleable is empirically
false. 9. Donald Brown’s universal people demonstrate a large set of complex
behavior patterns common to all known societies. These commonalities suggest
that human behavior is far from infinitely flexible. 10. Evolved traits are not
necessarily fixed and inflexible; the belief that they always are is called the
genetic fallacy. 11. Many evolved traits are not fixed but exquisitely responsive to
environmental variation. These traits are said to be facultative, and suntanning
provides a useful example. 12. Violent tendencies may be facultatively responsive
to the perceived risks of violence. 13. Evolutionary psychology explains behavior;
it does not justify it. The naturalistic fallacy confuses is with ought. It is an error
to conclude that a situation is morally acceptable simply because it is a product of
evolution. Chapter 2: Evolution by Natural Selection
- Darwin’s Argument
o Natural populations ( of giraffes, daisies, or people) could grow exponentially.
o Despite this potential for exponential growth, natural populations normally are
o Many individuals do not leave as many offspring as they might.
Natural selection is the tendency for those best suited to the current
environment to produce more offspring.
o Those best suited to the prevailing environment leave the most offspring.
o Because of heredity, offspring are like their parents.
o Over many generations, natural selection builds individuals that are well adapted
to their environment.
“ Survival of the fittest” is a poor summary of natural selection.
- Evidence in favor of evolution
o The existence and pattern of the fossil record.
o The universality of genetic code.
o The analogy with animal breeding. Darwin noted how many different breeds of
dogs, sheep, and other domesticated species had been created simply by selective
o Direct observation. The theory of evolution predicts that due to environmental
pressures, some individuals will leave more offspring than others.
The work of Peter and Rosemary Grant ( e. g., Grant & Grant, 1986) and
their students on Darwin’s finches is an excellent example. They have
studied these birds for more than 30 years in the Galapagos islands. Each
climatic fluctuation has favored a particular class of birds ( e. g., those
with deeper beaks), and the favored type has consistently increased in the
- A Mistaken View of Evolution: The Great Chain of Being The Great Chain of Being is the
idea that life forms can be ordered in a ladder of complexity and sophistication.
The evolutionary tree is actually more of a bush. All organisms are the
result of the same amount of evolutionary fine tuning.
- Designs for reproduction are called adaptations. They are preserved by natural
selection because they aid reproduction in some way.
- Thus the pace of evolution is governed by the pace of reproduction, or what is called
generation time. The generation time of humans, the time from birth to reproduction, is
roughly 20 years.
- Selection works by choosing among existing alternatives, not by inventing entirely new
- Cumulative Selection o The vertebrate eye, discussed by Richard Dawkins in ( 1996), is a classic example
of how selection can add refinements over time to build complex adaptations.
o Adaptations are usually complex and integrated, and they show design for a
function that contributes to reproduction.
o Each organ is specialized for a narrow function.
o Adaptations are not free; they have developmental and maintenance costs.
- The environment of evolutionary adaptedness, or EEA ( Tooby & DeVore, 1987). The
EEA was the environment in which humankind evolved. Many of our adaptations are not
so much designed to meet today’s challenges as they are designed to solve the problems
that confronted our ancestors in the EEA.
- The disorder called prosopagnosia shows how specialized mental modules can be;
inability to recognize faces
- Psychological mechanisms are modular and task- specific.
- The mind is modular, not a single organ but a collection of many organs or modules,
each designed for a different task. Similarly, each module or organ is said to be domain-
specific ( Fodor, 1983). This phrase means that each module is designed to accept a
particular kind of input and to be more or less unresponsive to other sorts of stimuli.
- Darwin knew about traits like the peacock’s tail, and in 1871, some dozen years after the
publication of On the Origin of Species, he offered his theory of sexual selection to
- Unlike natural selection, sexual selection typically leads to the evolution of sex
- Sex differences in reproductive rates affect the availability of mates for males and
- Sexual selection favors traits that increase mate quantity for the fast sex and traits that
increase mate quality for the slow sex.
- Neighbours; selfish traits increase the fitness of the self but decrease the fitness of
neighbors. Altruistic traits do the opposite, decreasing the fitness of the self but adding to
the fitness of neighbors. To detail the remaining two, cooperative traits are good for both
the self and neighbors; spiteful traits are bad for both.
- Group selection; They believed that altruism would be perpetuated because it is good for
the group ( species or society) as a whole. Dissect the previous sentence carefully; it
contains two ideas. The first is that altruism is good for the group as a whole. The second
is that this good- for- the- group effect will cause altruism to spread. The first idea is
generally correct: You will do better in a group that includes many altruists.
o Advantages for the group are generally not sufficient to spread a trait.
- A model of group selection. Selfish individuals ( S’s) spread by individual selection.
Altruists ( A’s) persist because pure- S groups tend to go extinct. Migrants from pure- S
or mixed groups can “ infect” pure A groups. See text to understand the outcome.
- Summary 1. o Charles Darwin proposed the theory of evolution by natural selection to account
for species diversity and to explain why organisms seem to fit so well with their
o 2. Natural selection occurs because individuals who are better suited to their
local environments leave more offspring and because, when they do so, their
advantageous traits become more common in the population.
o 3. Many different lines of evidence support evolutionary theory, including the
fossil record, homology, the universality of the genetic code, the results of
artificial selection, and direct observations in natural populations.
o 4. The Great Chain of Being gives a false, linear picture of evolution.
o 5. Natural selection can spread even small advantages. By doing so over many
generations, it crafts designs for reproduction called adaptations.
o 6. Natural selection is powerful but often slow. It can build complex and efficient
adaptations, but it takes time to do so.
o 7. Selection is cumulative, layering new adaptations on old.
o 8. Adaptations are specialized, shaped for a narrow function, because
specialization allows better and more efficient performance.
o 9. Adaptations have costs; thus selection deletes useless traits and builds only
adaptations that justify their costs in terms of reproductive success.
o 10. The comparative method can be used to test hypotheses about the functions of
o 11. If the environment changes rapidly, adaptations may be out of date.
o 12. Much of human nature was shaped during the stone age; thus our
adaptations may not match the contemporary environment.
o 13. Natural selection builds psychological adaptations not by designing behavior
directly, but by shaping the psychological traits that produce behavior.
o 14. Like all adaptations, psychological adaptations are specialized ( modular).
o 15. Sexual selection is a kind of natural selection; it favors traits that help in
o 16. Sexual selection happens because the faster reproducing sex experiences a
shortage of mates.
o 17. When the sexes differ in reproductive rate, sexual selection favors different
traits in females and males.
o 18. Selection at the level of the population or species, called group selection, is
seldom powerful enough to overcome the force of natural selection operating at
the level of the individual.
Chapter 3: The Genetic Basis of Evolution
-Mendel discovered the basis of heredity
-Mendel’s first hybrid cross disproved the blending model. - Mendel’s Model of Heredity
1. The stuff of heredity consists of particles, not fluids.
2. Each normal adult carries a pair of genes for each trait. Heterozygous or homozygous
3. When breeding, each parent gives one, and only one, of its pair of genes for each trait.
As a result of each parent giving one gene, the offspring will end up with a pair. In this way the
alternation between reproduction and growth involves an alternation between states.
4. Although pairs of genes do not dilute or change each other permanently, they can
influence each other’s expression; dominant or recessive
- The Nature of Genes
- Genes are organized into chains called chromosomes
- Genes in variant forms are alleles, new alleles rise by mutation
- Polygenic traits are shaped by genes at several loci.
- Discrete traits, such as height in the pea plants studied by Mendel, are typically affected
by genes at a single locus. In such cases there are usually a small number of possible genotypes
and thus a small number of possible phenotypes.
- Natural Selection Shapes the Form of Gene- Environment Interactions
1. The environment is variable within the lifetimes of individuals ( e. g., the amount of
uvb varies from summer to winter or from one part of the habitat to another), and
2. The fittest alternative varies from one environment to the next ( e. g., the ideal level of
melanin production depends on the season).
- obligate traits “ expect” a certain range of environments. When that normal range is
exceeded, the results are unpredictable.
-Selection will prefer facultative or obligate traits depending on whether phenotypic
responsiveness would increase or decrease fitness respectively.
- Neo- Darwinism: Neo-Darwinism: The Union of Selection and Genetics
- Selection acting on phenotypes determines what alleles get passed to the next generation.
- Evolution is a struggle among alleles over genetic real estate called loci.
- Some alleles may be selectively neutral; the alternative alleles at a particular locus may
produce equally fit phenotypes. - Environmental variation is a second factor that can prevent one allele from completely
replacing its alternatives. Phenotypes produced by T might be favored for a few hundred years
and then, perhaps due to some climatic change, phenotypes produced by t would have an
-some loci continue to have multiple alleles is heterozygote superiority. We saw earlier that
some alleles are dominant to others, but that is not the only way that alleles can interact. In some
cases two different alleles work together in the heterozygote to produce a different phenotype
than either allele would in homozygotes; ex. Sickle-cell anemia
- Frequency- dependent selection is another mechanism that maintains more than one type of
individual in a population. In such cases the fitness of a phenotype depends on how common it is
in the population.
-For polygenic traits, the population will always include considerable scatter around the optimum
-here is the key point: Because of the genetic shuffling of sexual reproduction, even these
optimally anxious individuals would produce offspring with diverse combinations of anxiety
genes and who thus exhibited a range of no optimal anxiety levels.
- Heritability is a measure of the extent to which differences in a trait are caused by genetic
differences. Trail Marker: Heritability measures the extent to which individual differences are
caused by genetic differences.
- Evolutionary psychology offers an “ adaptationist” toolkit for discovering key environmental
triggers of behavior.
1. Using garden peas as a model, Gregor Mendel first discovered the rules of genetics.
2. Genes are particles, not fluids.
3. Diploid adults produce haploid sex cells.
4. These haploid cells combine to form new diploid individuals.
5. Recessive genes are expressed only in homozygotes.
6. Genes are organized into chromosomes, and genes for a particular trait are always
found at the same locus on the chromosome.
7. Most loci have only a single allele, but about one- third of human loci currently have
more than one. 8. Mutation is the ultimate source of all new alleles and hence of all genetic variation.
9. Genes are made out of DNA and, through the intermediary of RNA, they code for the
structure of proteins.
10. These proteins interact with each other and with the environment to produce
11. Genetic changes such as mutations can change the phenotype, but phenotypic changes
have no effect on the genotype.
12. Genes with obligate effects resist environmental interference.
13. Genes with facultative effects allow the phenotype to respond productively to
14. When a single phenotype has the highest fitness over a wide range of environments,
selection favors genes with obligate effects.
15. When environments are variable in time or space and the optimal phenotype varies
among environments, selection favors genes with facultative effects.
16. Behavioral adaptations are expected to be significantly facultative.
17. Genes are not directly evaluated by selection, but phenotypes are.
18. Phenotypes vary in fitness.
19. The genes that shape the fittest phenotypes are most likely to get passed to the next
20. For these reasons, only traits that have a genetic basis are subject to natural selection.
21. The genetic fallacy is the idea that genetically based traits cannot be modified; the
facultative model shows that this idea is false.
22. Evolutionary thinking offers a systematic approach for discovering norms of reaction
and thus for developing effect