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Chapter 2: Genetic and Evolutionary Roots of Psychology
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Human Cloning
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Difference between genotype and phenotype
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Genotype: the complete genetic material of on organism
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Phenotype: the actual characteristics of an organism
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Phenotypic differences can be large, even with the same genes!
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Monozygotic (i.e., identical) twins reared in very different environments
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“The Unity of Life”
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The genotypes of humans are at least 99.9% identical.
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The .1% difference (and gene sequencing differences) leads to a lot of variability
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Humans share:
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“The Unity of Life”
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Post-Darwin: rise in animal research
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Dobzhansky: "Nothing in Biology Makes Sense Except in the Light of Evolution”
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The Biological Blueprint
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What are genes?
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Long molecules of DNA that codes for how and when to build a protein
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Roughly 25k of them in humans
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Located on structures in cells called chromosomes
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46 chromosomes in 23 pairs
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Pairs 1 – 22 = body and brain development
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Pair 23 = gender (males XY, females XX)
Genes can be “expressed” or lay dormant… gene expression depends on many factors (e.g.,
environmental stimulation).
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No trait is “purely genetic” (e.g., muscularity)
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Even those strongly genetic can be modified via environment (e.g., poor vision; PKU)
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Single gene determinants (e.g., dimples) vs. polygenic influences
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Dominant/Recessive alleles (alleles = 1 of 2 copies of a gene from a parent)
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Dominant alleles only require one to determine a trait
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Recessive alleles must be present in pairs to determine a trait
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The Biological Blueprint
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Dominant + Dominant = Dominant Phenotype
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Dominant + Recessive = Dominant Phenotype
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Recessive + Recessive = Recessive Phenotype
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This does not mean that dominant phenotypes are twice as common as recessive ones.
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Often recessive genes are much more common in a population (e.g., blue eyes in Scandinavia).
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Evolution
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Public Acceptance of Evolution
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Darwin’s Discovery
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Natural Selection
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Traits that increase survival and reproduction will most likely be passed on to
succeeding generations
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1. Substantial trait variation within species.
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2. Different traits confer different rates of “fitness”.
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3. Traits that increase fitness are most likely be passed on to succeeding
generations
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Examples: brown beetles; lactose tolerance; modern antibiotics
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Variation persists because 1) mutations; 2) environment is not a constant
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When environments change, so do the selective advantages
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Naturalistic fallacy - evolution is not “improving” organisms, just adapting them to their
current environments
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One species is not more evolved than another.
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Worms are just as adapted to their environment as we are to ours.
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Being “evolved” is measured by successful survival
Evolutionary Psychology
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Evolutionary Psychology
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The study of the evolution of behavior and the mind, using principles of natural selection.
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Sheds light on what is universal.
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Attempts to answer “why” questions, not just the “how”
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Generally, the answer to the “why” question is adaptation
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Evolutionary Psychology
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Natural selection favors genes that aid the survival and reproduction of our ancestors
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This includes psychological and behavioral traits
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Intelligence and language as adaptations
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Evolutionary psychology can help us explain a range of questions about human beings.
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“Selfish gene” theory: We are motivated by what protects and propagates our genes (e.g.,
sexual behavior; family protection)
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energetically costly to build & maintain
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makes the peacock much more vulnerable to predators
Why would evolution favor structures that make organisms less well adapted to their
environments?
Sexual Selection
Darwin argued that such traits evolved because of selection pressures arising from competition
among members of one sex for mating opportunities.
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Some structures couldn’t have evolved by natural selection, because they didn’t
promote adaptation to environment
Example: The peacock’s tail:
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Sexual Selection
Darwin noticed something interesting:
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Haldane: “Would I lay down my life to save my brother? No, but I would to save
two brothers or eight cousins.”
He called the selection that results specifically from mating competition “sexual
selection”
Behaviors that promote the replication and survival of genes will be favored
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Hence, those that increase the chance of reproduction get spread to future generations
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Gender Differences in Mate Selection
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Best way to spread genes is to maximize offspring
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Gender imbalance: why can’t females produce as many offspring as males? (67 vs 888)
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Female humans invest at least 9 months for pregnancy, and usually much more
(feeding, raising).
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A man’s minimum investment is…. not long at all.
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Sex Differences in Mate Selection
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What do men look for in a mate? What do women look for?
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Why from an evolutionary standpoint, might they favor for different characteristics?
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The parent who must invest the most in ensuring survival of offspring will be more choosy when
selecting a mate
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In most species, this is the female. (Seahorse as a counterexample)
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Clark and Hatfield Study
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“I have been noticing you around campus. I find you very attractive.”
This was followed by one of three invitations:
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What % of men said yes to each question?
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What % of women said yes to each question?
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Evolutionary Psychology & Gender
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37 cultures (10,000+ people): Men preferred attractive physical features suggesting youth and
health
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Women preferred resources and social status
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Evolutionary Psychology & Gender
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What are some things males and females do to be more attractive to potential romantic
partners?
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Women: makeup, exercise to be thin/shapely
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Men: symbols of status and wealth; exercise to be muscular
Female dilemma:
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The men that may make the best genetic parents may not make the best behavioral
parents.
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Evolutionary Psychology & Gender
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If it is all about maximizing offspring, why do men commit?
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Caveats about Evolutionary Psychology
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How much is real, and how much are “Just So Stories”?
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Evolution does not implies genetic determinism – we are not slaves to our genes
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Evolutionary theory does not mean that behavior cannot be changed.
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Socialization forces also very influential
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Behavior Genetics
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Environment
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every nongenetic influence, from prenatal nutrition to the people and things around us
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Behavior Genetics
Identical Twins
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study of the relative power and limits of genetic and environmental influences on
behavior
develop from a single fertilized egg that splits in two, creating two genetically identical
organisms
Fraternal Twins
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develop from separate eggs
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genetically no closer than brothers and sisters, but they share the fetal environment
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Twin Studies
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IQ correlated .86 for identical twins but .60 for fraternal (random groups of people correlate at
0)
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Studies of Extraversion and Neuroticism, identical twins are more similar than fraternal twins.
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Divorce Rate: Twin has divorced, will the other divorce?
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Identical – 5.5 times more likely to divorce
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Fraternal – 1.6 times more likely to divorce
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Criticisms
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Could these similarities between twins be explained by environmental similarities?
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Environment is more similar for identical twins than fraternal twins, and is more similar for
fraternal twins than random individuals
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Adoption studies seek to address this
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Adoptive children’s personality traits tend to be more similar to their biological parents than
their adopted parents.
But their values, manners, faith, politics etc. tend to align more with the adopted parents.
Heritability estimates the extent to which differences among populations are attributable to
genes.
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Heritability refers to the extent to which differences among populations are attributable to
genes.
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We cannot say what percentage an individual’s personality or intelligence is inherited.
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Heritability estimates vary depending on the range of populations and environments
studied
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Example: heredity for intelligence much higher in middle and upper class homes
than for the poor
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Nature vs. Nurture
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How much of who we are is determined by our biology (nature) compared to our environment
(nurture)?
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Genes and environment work together, with the environment reacting to and shaping what
nature predisposes.
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Eating disorders – genetic predisposition, but culture factors in as well
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Eating disorders are a contemporary Western cultural phenomenon.
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Interaction
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From conception on, we are a product of interactions between our genetic predispositions
and our surrounding environments.
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NATURE VIA NURTURE (more later)