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Behavior Genetics
BEHAVIOR GENETICS
 Behavior genetics challenges the behaviorist
perspective of John Watson who famously wrote:

"Give me a dozen healthy infants, well-formed, and
my own specific world to bring them up in and I'll
guarantee to take any one at random and train him
to become any type of specialist I might select — a
doctor, lawyer, artist, merchant-chief and, yes,
even into beggar-man and thief, regardless of his
talents, penchants, tendencies, abilities, vocations
and race of his ancestors."
BEHAVIOR GENETICS
 1.) BEHAVIOR GENETICS STUDIES IDIOGRAPHIC
DEVELOPMENT: What combination of genetic and
environmental on average affect where people are on the
normal curve.
BEHAVIOR GENETICS
 2.) BASIC CONCEPTS:
PHENOTYPE = OBSERVABLE OR MEASURABLE
CHARACTERISTICS

HAIR COLOR, IQ
 GENOTYPE = GENETIC COMPLEMENT OF
PERSON

HAVING A RECESSIVE GENE FOR COLOR
BLINDNESS
 POLYGENY: MANY GENES INFLUENCE A TRAIT
BUT NO ONE GENE HAS A MAJOR EFFECT


HUNDREDS OF GENES INFLUENCE IQ AND
PERSONALITY;
Few, if any cause more than 1% of the variation
GENOTYPE → ENVIRONMENT
INTERACTIONS: Active
 GENOTYPE → ENVIRONMENT INTERACTIONS: WAYS
THAT GENETIC TENDENCIES BECOME CORRELATED
WITH PARTICULAR ENVIRONMENTS.
 1.) ACTIVE GENOTYPE → ENVIRONMENT INTERACTION
(NICHE-PICKING):
 CHILD SEEKS OUT ENVIRONMENTS AS A RESULT OF
GENETIC INFLUENCES



CHILD WITH A SENSATION SEEKING TEMPERAMENT
The active genotype-environment interaction probably
increases in importance as the child grows older. Why?
GENOTYPE → ENVIRONMENT
INTERACTIONS: Evocative
 2.) EVOCATIVE GENOTYPE →
ENVIRONMENT INTERACTION: CHILD
EVOKES ENVIRONMENTS AS A RESULT
OF GENETIC INFLUENCES

CHILD WITH A DIFFICULT
TEMPERAMENT EVOKES NEGATIVE
RESPONSES IN CAREGIVERS; CHILD
WITH A SUNNY DISPOSITION GETS
POSITIVE RESPONSES.
GENOTYPE → ENVIRONMENT
INTERACTIONS: Evocative
 2.) EVOCATIVE GENOTYPE → ENVIRONMENT
INTERACTION: CHILD EVOKES ENVIRONMENTS AS A
RESULT OF GENETIC INFLUENCES
 Children with violent natural parents and children with
non-violent natural parents are adopted into separate
families, and the adoptive families are compared.



Children with violent natural parents have adoptive parents
who use harsh discipline.
Children with non-violent natural parents have adoptive
parents who use mild discipline.
This "environmental" effect is the result of an evocative
genotype → environment effect: Aggressive, difficult
children evoke harsh parenting.
GENOTYPE → ENVIRONMENT
INTERACTIONS: Passive
 3.) PASSIVE GENOTYPE ENVIRONMENT
INTERACTION:


CHILD IS PASSIVE RECIPIENT OF
ENVIRONMENTS WHICH FIT WITH
HIS/HER GENOTYPE.
INTELLIGENT PARENTS HAVE CHILD
WITH GENETIC POTENTIAL FOR
INTELLIGENCE;

PARENTS ALSO PROVIDE A GREAT DEAL OF
INTELLECTUAL STIMULATION WHICH
MESHES WITH THE CHILD'S GENETIC
POTENTIAL.
GENOTYPE → ENVIRONMENT
INTERACTIONS: Passive
 3.) PASSIVE GENOTYPE ENVIRONMENT
INTERACTION:
P=parent
C=Child
g=genes
e= environment
P
g
e
C
Shared and Unshared Environmental
Influences
 SHARED ENVIRONMENTAL INFLUENCES:
ENVIRONMENTAL INFLUENCES SHARED BY
CHILDREN IN THE SAME FAMILY.

EXAMPLE: CHILDREN IN SAME FAMILY GO TO
SAME SCHOOL, HAVE SAME ALCOHOLIC
MOTHER OR AFFECTIONATE MOTHER, ETC
 UNSHARED ENVIRONMENTAL INFLUENCES:
DIFFERENT CHILDREN RECEIVE DIFFERENT
ENVIRONMENTS; OR DIFFERENT CHILDREN
RESPOND TO THE SAME ENVIRONMENT
DIFFERENTLY;

EXAMPLE: DIFFERENT BIRTH ORDER, SEX
DIFFERENCES, PEER RELATIONSHIPS
Shared and Unshared Environmental
Influences
 UNSHARED ENVIRONMENTAL INFLUENCES: DIFFERENT
CHILDREN RECEIVE DIFFERENT ENVIRONMENTS; OR
DIFFERENT CHILDREN RESPOND TO THE SAME
ENVIRONMENT DIFFERENTLY;
 EXAMPLE: DIFFERENT BIRTH ORDER, SEX
DIFFERENCES
 According to Plomin et al. (2001), unshared environmental
influences are typically far more important than shared
environmental influences.
 Notice that the unshared environment idea is linked to the
active child concept (Why?) and is used to explain the fact
that adoptive siblings and even biologically related siblings
are typically not very similar.
 As the text says, 'Clearly, researchers in individual
differences can no longer assume a homogeneous home
environment for all siblings; be alert to this fact when you
read the reports and conclusions of such studies.'
Reaction Range: The range of phenotypic expression
depending on different environments of different quality.
Reaction Range: Different people have different reaction
ranges; they respond differently to the same
environments.
Reaction Range for IQ, including
abusive environments
IQ score
Genotype A
Genotype B
Genotype C
Genotype D
______________________________
Abusive
Poor
Average
Quality of Environment
Enriched
Canalization
 CANALIZATION: THE GENETIC RESTRICTION OF
A PHENOTYPE TO A SMALL NUMBER OF
DEVELOPMENTAL OUTCOMES, PERMITTING
ENVIRONMENTAL INFLUENCES TO PLAY ONLY
A SMALL ROLE IN THESE OUTCOMES;



Genes restrict the extent to which the environment
can influence the phenotype.
A highly canalized phenotype is not much
influenced by environmental influences.
A weakly canalized phenotype is open to
environmental influences.
Canalization
Behavior Genetics Methods: Adoption
Studies
 Adoption Studies: Comparison of adopted
children to natural and adoptive families.



Correlations with natural family indicate
genetic influence;
correlations with adoptive family indicate
environmental influences.
Assumption: environment before adoption
does not systematically affect the behavior
studied.
Behavior Genetics Methods: Adoption
Studies
 Findings: Adopted children's IQ scores are correlated with those
of their biological parents, indicating genetic influence.
 Their IQ scores are also correlated with their adoptive parents
at age 7, indicating environmental influence.
 However, at age 18, there is no correlation between adopted
children and their adoptive parents after 18 years of living
together!!!
 There is a distinction between beneficial effects of adoption on
the average IQ of adoptees and the correlation between
adoptees and their natural or adoptive parents. Adopted
children can benefit from adoption. For example, their IQ may
increase by an average of 5 points compared to a similar
sample that is not adopted. But the IQ’s of the adoptive
children may still be correlated to their natural parents but not
their adoptive parents.
 Correlations are independent of the mean.
Behavior Genetics Methods: Adoption
Studies
 The findings of adoption studies are
influenced by the range of the subject
population.


Researchers try to study children adopted
into a wide range of environments so that
they will not underestimate the effects of
environmental differences.
In general, a wider range, especially if it
included abusive, highly stressful
environments, would usually result in a
lower estimate of genetic influences. Why?
Behavior Genetics Methods: Twin
Studies
 Monozygotic (MZ) (identical) twins: Twins
that share the same sperm and egg.
 Dizygotic (DZ) (fraternal) twins: Twins that
have different sperm and egg.

Genetically they are no more alike than any
other two siblings.
Behavior Genetics Methods: Twin
Studies
 Environmental influences are indicated if MZ twins and DZ
twins have about the same correlations for a trait.
 Genetic influences are indicated if MZ twins are
substantially more similar than genetic twins.
 Assumption: Environmental influences do not tend to make
MZ twins more similar than DZ twins.
 This would occur if parents treat MZ twins more
similarly than DZ twins because, for example, it's cute to
have them dress alike and if this procedure actually
made their personalities or IQ more similar.
 One way to get around this is to study MZ twins reared
apart, especially if they are reared in radically different
environments.
Behavior Genetics Methods: Twin
Studies
 Table 2.5

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




Correlations for IQ scores
MZ twins reared together
.86
MZ twins reared apart
.79
DZ twins reared together
.60
Siblings reared together:
.47
Parent and Child:
.40
Foster parent and child at age 7: .31
 at age 18:
.00
Siblings reared apart:
.24
 Cousins:
.15
THE DEGREE OF GENETIC INFLUENCE INCREASES AS
CHILDREN APPROACH ADULTHOOD.
Genetic Influences are stronger in adulthood than among children.
 1.) CORRELATIONS OF MZ TWINS STAY
HIGH WHILE CORRELATIONS FOR DZ TWINS
DECLINE
 IQ CORRELATIONS FOR MZ AND DZ TWINS
AGE
MZ
DZ
6 MO
.75
.72
12 MO
.68
.63
24 MO
.81
.73
36 MO
.88
.79
4 YR
.83
.71
6 YR
.86
.59
8 YR
.83
.66
15 YR
.88
.54
THE DEGREE OF GENETIC INFLUENCE INCREASES AS
CHILDREN APPROACH ADULTHOOD.
Genetic Influences are stronger in adulthood than among children.
 2.) ADOPTION STUDIES
a.) CORRELATION BETWEEN ADOPTIVE PARENTS AND
ADOPTED CHILDREN DECLINES FROM .35 TO ZERO
 AGE 7: r=0.35
AGE 17: r=0.00
 b.) IQ OF ADOPTED CHILDREN DECLINES TO CLOSE TO
THE IQ OF NATURAL PARENTS
 AGE 7: IQ OF ADOPTED CHILDREN = 110
 AGE 17: IQ OF ADOPTED CHILDREN = 95
 EXPLANATION: ACTIVE GENOTYPE → ENVIRONMENT
INTERACTION (NICHE-PICKING) BECOMES MORE
IMPORTANT AS CHILDREN GET OLDER.
Abusive Environments Lower IQ
 CONCLUSION: DIFFERENT ENVIRONMENTS WITHIN THE
"NORMAL" OR "AVERAGE" RANGE DO NOT HAVE MUCH
INFLUENCE ON IQ.
 HOWEVER, ABUSIVE ENVIRONMENTS CAN AND DO AFFECT IQ.
AVERAGE VS ABUSIVE ENVIRONMENTS
Minor Gain from
Better Normal
Environments
IQ score
_____________________________________________
Abusive “Good enough” Average
Enriched
Abusive Environments
Dramatically lower IQ
Quality of Environment
Is heritability higher at the low end of
the socioeconomic status scale?
 A recent study by Turkheimer suggests less
heritability (genetic influence) at the lower end of
the socioeconomic scale, and much higher
heritability at the higher ends.
 The idea is that in better environments, the vast
majority of variation is caused by genetic
variation.
 But in poor environments, more variation is
caused by bad environments.
 However, other studies have not found this effect
except in clearly abusive environments.
Rank Order versus Average Effects
of Adoption






One classic adoption study showed that adopted children often averaged
20 or more IQ points higher than their biological mothers.
Because in this study the adoptive parents tended to be more highly
educated and more socially and economically advantaged than the
biological parents, this result was probably due to the more stimulating
home environment that the adoptive parents provided.
But note also that, despite this environmental influence on development,
individual differences seemed still to be substantially influenced by
genetic inheritance.
The rank ordering of the children's IQ scores more closely resembled that
of their biological mothers than that of their adoptive parents.
The children whose biological mothers had the lowest IQ scores were
likely to have lower IQ scores than the children whose biological mothers
scored higher.
Thus, although the absolute level of intellectual development was
apparently boosted by the environmental influences provided by the
adoptive parents, individual differences among the adopted children in
intellectual performance—that is, their relative standings in this regard—
appeared to stem more from their biological inheritance than from the
increased intellectual stimulation provided in their adoptive homes.
Rank Order versus Average Effects
of Adoption
CHILDREN ADOPTED NEAR BIRTH
120
111
115
129
125
Adoptive Parents: Average IQ= 120
114
112
110
108
106
Adopted Children: Average IQ = 110
94
92
90
88
86
Natural Parents: Average IQ = 90
On average, adopted children gain 20 IQ points, but individual
differences are affected by natural parents’ IQ
Rank Order versus Average Effects
of Adoption (not on test)
 TESTED AT AGE 7:
 ENVIRONMENTAL VARIATION IS IMPORTANT:


AVERAGE OF AC (110) > AVERAGE OF NP (90);
THIS IS AN AVERAGE EFFECT OF ADOPTION OF 20 IQ
POINTS.

STUDIES TEND TO SHOW THAT THIS AVERAGE EFFECT
WASHES OUT AS THE CHILDREN APPROACH
ADULTHOOD.
 GENETIC VARIATION IS IMPORTANT:
1.) AVERAGE OF AC (110) < AVERAGE OF AP (120)

2.) POSITIVE CORRELATION BETWEEN AC AND NP
Interactions between Genes and
Environments
 Interactions between genes and environments
modeled by Gottlieb's bi-directional model.



Note that the environmental effects on the genes
refer to turning the genes on or off, not to actually
modifying the genes.
In the mallard duck, a 'genetically governed
preference' for the sounds of other ducks is
modified by exposing them to different sounds
before they are born.
The idea would be that a different gene would be
turned on, not that the duck's genes are changed.
Interactions between Genes and
Environments
 Interactions between genes and environments
modeled by Gottlieb's bi-directional model.

Tennis and testosterone: Loss affects
 behavior (slumping posture)
 psychological state (low self-esteem)
 neural activity in certain parts of the brain;
 the genes for testosterone are turned off.

But the genes haven't been changed by these
events.
 When the person later wins a match, the genes may
turn on again.
Interactions between Genes and
Environments: Gottlieb’s model
Genes sometimes determine whether
environment has an influence
 The developmental stage of the child
affects whether the environment has an
influence



Example: Critical periods for the effects of
teratogens on the fetus.
Environmental influences are dependent
on timing.
Genetic systems determine when the
environment may have an influence.
Genes sometimes determine whether
environment has an influence
 The developmental stage of the child
affects whether the environment has an
influence
 PKU: Effects of dietary intervention on
PKU depend on when the intervention is
attempted. Earlier intervention is more
effective.
Heritability
 Heritability: A statistical estimate of the contribution of genetic
differences to phenotypic differences in traits, such as
intelligence or personality.
 Traits with high heritability are mainly influenced by genetic
differences:





Most of the differences we see among people are the result of
genetic differences.
Low heritability is the opposite: Most of the differences we see
among people are the result of environmental differences.
Heritability may be different for different groups and under
different environmental conditions.
Heritability may change with age. (In general, genetic influence
becomes stronger with age.)
What would happen to heritability if everyone was raised in the
same environment?


Since there are no environmental differences, all of the differences
in the population would be due to genetic differences.
This means the heritability would be 1.00—all of the variation
would be explainable by genetic differences between individuals.
Heritability
1.) There is phenotypic variation in a trait such as IQ or personality.
2,) SOME OF THIS PHENOTYPIC VARIATION IS DUE TO THE GENETIC
VARIATION (PEOPLE HAVE DIFFERENT GENES);
THIS IS Vg
3.) SOME OF THIS PHENOTYPIC VARIATION IS DUE TO ENVIRONMENTAL
VARIATION (PEOPLE EXPERIENCE DIFFERENT ENVIRONMENTS). THIS IS
Ve
4.) THE PHENOTYPIC VARIATION (Vp) THEN EQUALS THE SUM OF THE
GENETIC VARIATION (Vg) PLUS THE ENVIRONMENTAL VARIATION (Ve)
Vp= Vg + Ve
Heritability


HERITABILITY IS THE PROPORTION OF VARIATION IN A TRAIT
THAT IS CAUSED BY GENETIC VARIATION
H =Vg/Vp H = Vg/(Vg + Ve)
Like any proportion, it is a part over the whole—e.g., the proportion of females in
a class. In this case, it’s a part of the phenotypic variation (Vg) over the whole
variation (Vp).
Heritability
 Heritability is a proportion of the entire variation
that is due to genetic variation and therefore
ranges between 0 and 1. H = Vg/Vp = Vg/(Vg + Ve)
 A heritability of 1 would mean that all of the
variation is genetic, a heritability of 0 would mean
that all the differences are caused by different
environments.
 A heritability of .5 is common for many traits and
means that about half of the variation is the result
of people having different genes and about half is
the result of their being in different environments.
Heritability depends on the sample being studied: Historical
example
 1601: 400 YEARS AGO, ONLY RICH PEOPLE ATE
WELL.
RESULT: MOST OF THE PHENOTYPIC VARIATION
IN HEIGHT WAS
THE RESULT OF ENVIRONMENTAL VARIATION
 H1601 = Vg/(Vg + Ve) = 10/(10 + 90) = 0.100
 1995: ALMOST EVERYONE EATS ENOUGH TO
ACHIEVE THEIR MAXIMUM HEIGHT
RESULT: MOST OF THE PHENOTYPIC VARIATION
IN HEIGHT IS THE RESULT OF GENETIC
VARIATION
 H2001 = Vg/(Vg + Ve) = 10/(10 + 2) = 0.833
Heritability depends on the sample being studied: Island
example
 ISLAND X HAS A FAMINE THAT STRIKES
ALL;
ISLAND Y IS A PARADISE FOR ALL
 QUESTION: IS HERITABILITY FOR HEIGHT
HIGH ON BOTH ISLANDS, LOW ON BOTH
ISLANDS, OR HIGH ON ONE AND LOW ON
THE OTHER?

Heritability depends on the sample being studied: Island
example
 ISLAND X HAS A FAMINE THAT STRIKES ALL;
ISLAND Y IS A PARADISE FOR ALL
 QUESTION: IS HERITABILITY FOR HEIGHT HIGH ON BOTH
ISLANDS, LOW ON BOTH ISLANDS, OR HIGH ON ONE AND
LOW ON THE OTHER?
 ANSWER: HERITABILITY IS HIGH ON EACH ISLAND
BECAUSE Ve IS LOW ON EACH ISLAND. THIS IS BECAUSE
ON EACH ISLAND EVERYONE GETS THE SAME
ENVIRONMENT.
 THEREFORE, H CAN BE HIGH WITHIN EACH ISLAND EVEN
THOUGH THE DIFFERENCE BETWEEN THE ISLANDS IS
THE RESULT OF ENVIRONMENTAL DIFFERENCES.

Heritability depends on the sample being studied: Island
example
 THE BOTTOM LINE:
 1.) HERITABILITY CAN CHANGE AS A RESULT OF
CHANGES IN THE VARIATION DUE TO THE ENVIRONMENT
(i.e., CHANGES IN Ve)
 2.) CANNOT GENERALIZE RESULTS TO SAMPLES NOT
STUDIED BECAUSE EACH SAMPLE MAY HAVE DIFFERENT
GENETIC OR ENVIRONMENTAL SOURCES OF
PHENOTYPIC VARIATION
(i.e., DIFFERENT Ve OR DIFFERENT Vg).
 3.) AS ENVIRONMENTS BECOME MORE UNIFORM (LOWER
Ve), HERITABILITY IS HIGHER.
 4.) SUCCESSFUL SOCIAL PROGRAMS RAISE HERITABILITY
BECAUSE THEY DECREASE VARIATION DUE TO THE
ENVIRONMENT.
Partitioning Ve
 Ve can be partitioned into shared (Ve-s) and
unshared (Ve-ns) environmental influences.
 H = Vg/(Vg + Ve-s + Ve-ns)
 In general, Ve-ns >> Ve-s
Partitioning Vg into additive (Vga)
and non-additive (Vgna) components
Additive genetic influence:
Imagine a trait influenced by genes at 100 loci; each locus has 2
alleles. Each allele adds a small amount (+1) to the trait or adds
nothing (0). If you get all +1 alleles, your genetic potential for IQ
is 200; if you get all 0’s, then your IQ is zero. (This is
hypothetical!!) If people mate randomly, the average person
would have 100 +1’s and 100 0’s for a genetic potential of 100.
If it is an additive trait, then each gene has its effects independent
of every other gene that affects the trait. Each allele adds +1 or
0 no matter what other genes the person has. The gene has the
same in me as it has in you. The effect is independent of the
genetic background.
Partitioning Vg into additive (Vga)
and non-additive (Vgna) components
Non-Additive genetic influence (emergenesis):
If it is a non-additive trait, then the effect of each
gene depends on what other genes the
person has. A non-additive gene may add +2
for one person but -2 for another person
because they have different genetic
backgrounds.
Personality research indicates both Vga and
Vnga are important.
Partitioning Vg into additive (Vga)
and non-additive (Vgna) components
Vp = Vg + Ve = (Vga + Vgna) +(Ve-s + Ve-ns)
Hbroad = Vga + Vgna
Vp
=
Vga + Vgna
Vga + Vgna +Ves + Vens
Hnarrow = Vga
Vp
=
Vga
Vga + Vgna +Ve-s + Ve-ns
Environmentalists should like
Behavior Genetics
 Psychologists value behavior genetics not
just because it reveals genetic influences,
but because it also tells us lots about
environmental influences.
 Examples:


passive, evocative, and active (niche
picking) genotype → environment
interaction
shared versus unshared environmental
influences.
Some General Principles
 Strong genetic effects do not rule out environmental influences.


There may be strong correlations between biological relatives for,
say IQ (indicating relatively high heritability), but children may still
show a general rise in IQ levels as a result of adoption (indicating
environmental influence).
Genetic influences increase with age rather than decrease.
 Genes affect developmental change, and not all genes are turned
on at birth:

Many genes, like the genes for puberty and higher cognitive
processing, are not turned on until later in development.

Puberty, baby teeth, gray hair, and Piaget's stages are influenced
by genes turning on and off during development.