Evolution, Genes, and
Behaviour
Chapter 3
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Chapter Outline
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Unlocking the secrets of genes
The genetics of similarity
Our human heritage: courtship and mating
Our human heritage: language
The genetics of difference
Our human diversity: the case of intelligence
Beyond nature versus nurture
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Evolution & Psychology
• Evolutionary Psychology
– A field of psychology emphasizing evolutionary
mechanisms that may help to explain human
commonalities in cognition, development, emotion,
social practices, and other areas of behaviour
• Behavioural Genetics
– An interdisciplinary field of study concerned with
the genetic bases of individual differences in
behaviour and personality
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Unlocking the Secrets of
Genes
• Genes and how they operate
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Genes
Chromosomes
DNA
Genome
• Studying genetic material
– Linkage studies
– Genetic markers
– The link between genetics and behaviour
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Unlocking the Secrets of Genes
• Genes
– functional units of heredity
which are composed of DNA
and specify the structure of
proteins
• Chromosomes
– rod-shaped structures within
cells that carry genes
• DNA (deoxyribonucleic acid)
– transfers genetic
characteristics by way of
coded instructions for the
structure of proteins
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Genome
• Genome
The full set of genes in each cell of an organism
(with the exception of sperm and egg cells)
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Studying Genetic Material
• Linkage studies
– Because genes lying close together on a
chromosome may be inherited together
across generations, researchers can look for
genetic markers in families
• Genetic markers
– A segment of DNA that varies among
individuals, has a known location on a
chromosome, and can function as a genetic
landmark for a gene
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The Link Between Genes and
Behaviour
• Even when researchers locate a gene
on a chromosome, they do not
automatically know its role in physical
or psychological functioning
• Most human traits are influenced by
more than one gene pair
– Examples include height and eye colour
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The Genetics of Similarity
• Evolution
• Natural selection
• Evolutionary biologists and
psychologists
• Innate human characteristics
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The Genetics of Similarity
• Evolution
– A change in gene frequencies within a
population over many generations
– A mechanism by which genetically influenced
characteristics of a population may change
– Changes may occur due to mutations or errors
occurring during copying of original DNA
sequence
– Changes may occur due to natural selection
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The Genetics of Similarity
• Natural Selection
– The evolutionary process in which
individuals with genetically influenced traits
that are adaptive in a particular
environment tend to:
• survive; and
• reproduce in greater numbers
– As a result, their traits become more
common in the population
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Evolutionary Biologists
• Start with an observation about a
characteristic and try to account for it in
evolutionary terms
– Plumage differences in male and female
peacocks
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Evolutionary Psychologists
• Ask what sorts of challenges human beings
might have faced and then infer which
behavioural tendencies might have been
selected to overcome these challenges
– Avoiding poisonous food and an innate dislike for
bitter tastes
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Sexual selection
• Intersexual selection: a member of one
sex chooses a mate from the other sex
on the basis of certain characteristics
• Intrasexual selection: members of the
same sex compete for a partner of the
other sex
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Innate Human Characteristics
• Infant reflexes
• Interest in novelty
• Desire to explore and manipulate
objects
• Impulse to play and fool around
• Basic cognitive skills
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Phenotype matching
• An individual’s ability to assess
relatedness to another individual based
on the phenotype, the expression of
gene-environment interactions
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Our Human Heritage:
Courtship and Mating
• Sociobiology
– Interdisciplinary field that emphasizes
evolutionary explanations of social
behaviour in animals, including humans
– We behave in ways that maximize our
chances of passing on our genes, and to
help our close biological relatives, with
whom we share genes, to do the same
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Evolution and Sexual Strategies
• Due to different kinds of survival and mating
problems, the sexes have evolved differently
in the areas of aggressiveness, physical
dominance, and sexual strategies
– Males compete with other males for access to
females, and try to inseminate as many females
as possible
– Females conceive and carry only a limited number
of pregnancies so they choose fewer, more
dominant males with good resources and high
status
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Differences in Sexual Strategies
Males
Females
• Want sex more often
• Are fickle and
promiscuous
• Drawn to sexual
novelty and even rape
• Are undiscriminating in
partner choice
• Concerned with
competition and
dominance
• Want sex less often
• Are devoted and
faithful
• Drawn to stability
and security
• Are cautious and
choosy in partners
• Less concerned with
competition and
dominance
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Evolutionary Psychologists and
the Question of Gender
• Evolutionary psychologists consider such analogies
of human and animal behaviours simplistic and
misleading
• Focus more on commonalities of human mating and
dating around the world
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Culture and the “Genetic Leash”
• Criticisms of sociobiological and evolutionary
explanations for sexual behaviour include:
– Evolutionary explanations of infidelity and
monogamy are based on stereotypes
– Actual behaviour of humans and other animals
fails to conform to images of promiscuous males
and coy females
– Human sexual behaviour is too varied to be
explained solely from an evolutionary perspective
– Historically, available mates for selection were
much fewer than they are today
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Our Human Heritage: Language
• The nature of language
• The innate capacity for language
• Learning and learning
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The Nature of Language
• Language
– A system that combines meaningless
elements such as sounds or gestures to
form structured utterances that convey
meaning
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Innate Capacity for Language
• Language too complex to be learned bit by bit
(Chomsky, 1957, 1980)
• Sentences have surface and deep structures.
– Surface structure
• the way a sentence is spoken
– Deep structure
• how a sentence is to be understood
• To transform surface sentence structures into
deep ones, children must apply rules of
grammar (syntax)
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Surface and Deep Structures
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Language Acquisition Device
• If we don’t teach syntax to toddlers, human
brains must contain a language acquisition
device (Chomsky, 1957;1980)
– An innate module that allows young children to
develop language if they are exposed to an adequate
sampling of conversation
• Children are born with universal grammar or a
sensitivity to the core features common to all
languages
– Examples include nouns and verbs, subjects and
objects, and negatives
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Evidence Supporting the LAD
• Children:
– in different cultures go through similar stages of
linguistic development
– combine words in ways adults never would
– learn to speak or sign correctly although adults do
not correct syntax
– If not exposed to adult language may invent a
language of their own
• Infants as young as 7 months can derive
simple linguistic rules from a string of sounds
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Evidence for Learning and Language
• Computerized neural networks can “learn”
aspects of language
• Children learn the probability that any given word
or syllable will follow another
• Although there are commonalities in language
acquisition, there are also many differences
• Parents respond to children’s errors by restating
or elaborating on the phrase
• Children imitate these adult recasts and
expansions
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The Genetics of Difference
• The meaning of heritability
– A statistical estimate of the proportion of the total
variance in some trait that is attributable to genetic
differences among individuals within a group
– Expressed as proportion (.60 or 60/100)
– Maximum value is 1.0
• Some variables such as height are highly
heritable, other variables such as musical
ability are moderately heritable
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Facts About Heritability
• An estimate of heritability
applies only to a particular
group living in a particular
environment
• Heritability estimates do not
apply to individuals, only to
variations within a group
• Even highly heritable traits
can be modified by the
environment
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Computing Heritability
• Studying adopted children allows researchers
to compare correlations between the traits of
adopted children and those of their biological
and adoptive relatives
• Results are used to compute heritability
estimate
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Computing Heritability
• If identical twins are
more alike than
fraternal twins, then
the increased
similarity must be
due to genetic
influences
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Computing Heritability
• Investigators have also studied identical
twins who were separated early in life
and reared apart
• Any similarities in traits between them
should be primarily genetic and should
permit a direct estimate of heritability
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Our Human Diversity:
The Case of Intelligence
• Genes and individual differences
• The question of group differences
• The environment and intelligence
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Our Human Diversity:
The Case of Intelligence
• Genes and individual differences
– Intelligence Quotient (IQ)
• measure of intelligences originally computed by dividing
a person’s mental age by his or her chronological age
and multiplying result by 100
• now derived from norms provided for standard
intelligence tests
– The kind of intelligence that produces high IQ
scores is highly heritable
• .50 for children and adolescents
• .60 -.80 for adults
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Twins and Intelligence
• Intelligence scores of identical twins are
always more highly correlated than those of
fraternal twins
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Adopted Children and Intelligence
• The scores of adopted children are
highly correlated with their biological
parents
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The Question of Group Differences
• Genetics are used to explain differences between
groups
• These differences are used to justify differential
treatment for these groups
– Example: Differences between average IQ scores for
African Americans and Caucasian Americans
• Genetic explanations have a flaw
– They use heritability estimates based on white
samples to estimate the role of heredity in group
differences
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The Question of Group Differences
• Those studies overcoming past methodological
problems fail to reveal genetic differences
between blacks and whites on IQ
• Examples:
– Children fathered by black and white American
soldiers in Germany after WWII and reared in similar
German communities did not differ significantly in IQ
– Black and white infants perform equally well on tests
for novelty
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The Environment and Intelligence
• Experiences that hinder
– Poor prenatal care
– Malnutrition
– Exposure to toxins
– Stressful family circumstances
• Experiences that help
– Good health care and nutrition
– Mental enrichment in home and child care or
school
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Beyond Nature Versus Nurture
• Heredity and environment always
interact to produce the unique mixture
of qualities that make a human
• Psychological diversity is adaptive
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