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

g e

C

P=parentC=Childg=genese= environment

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 environmentsIQ score

 

  ______________________________     Abusive            Poor            Average               Enriched

Quality of Environment

Genotype A

Genotype B

Genotype C

Genotype D

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                         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   

 IQ score                 _____________________________________________

   Abusive “Good enough” Average Enriched                                

Quality of Environment

Minor Gain from Better Normal Environments

Abusive EnvironmentsDramatically lower IQ

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 GENETICVARIATION (PEOPLE HAVE DIFFERENT GENES);THIS IS Vg

3.) SOME OF THIS PHENOTYPIC VARIATION IS DUE TO ENVIRONMENTALVARIATION (PEOPLE EXPERIENCE DIFFERENT ENVIRONMENTS). THIS IS Ve

4.) THE PHENOTYPIC VARIATION (Vp) THEN EQUALS THE SUM OF THEGENETIC VARIATION (Vg) PLUS THE ENVIRONMENTAL VARIATION (Ve)

Vp= Vg + Ve

Heritability

HERITABILITY IS THE PROPORTION OF VARIATION IN A TRAITTHAT 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 WASTHE RESULT OF ENVIRONMENTAL VARIATION

H1601 = Vg/(Vg + Ve) = 10/(10 + 90) = 0.100 1995: ALMOST EVERYONE EATS ENOUGH TO

ACHIEVE THEIR MAXIMUM HEIGHTRESULT: 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 HERITABILITYBECAUSE 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) componentsAdditive 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 = Vga + Vgna

Vp Vga + Vgna +Ves + Vens

Hnarrow = Vga = Vga

Vp 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.