Untangling Untangling Nature and Nature and

Nurture Nurture Behavior GeneticsBehavior Genetics: the study : the study of the interaction of genes of the interaction of genes

and environment on behaviorand environment on behavior

Nature vs. NurtureNature vs. Nurture“Nature” = the role

that genes have on a trait

“Nurture” = the role that life experience has on a trait.

Sometimes, it is surprising.

EpigeneticsEpigenetics• Often nature and nurture

interact• Overkalix study

– Famine in 1944– Grandpa exposed to famine as

preadolescent = low cardiovascular deaths of grandsons

– Grandpa NOT exposed to famine as preadolescent = high diabetes mortality in grandsons.

– Granddaughters of women who experience famine while in the womb died earlier.

Environmental factors can alter the expression of genes. These changes are called “epigenetic tags”.

Heritability Heritability • The percentage of trait variation in a

group that can be attributed to genetic differences

• Depends on the population

HeritabilityHeritability

Sara is very outgoing and agreeable

Suzy is timid and non-conforming

If this pair of IDENTICAL twins is my population, what percent of the personality differences is attributable to genetic differences between them?

The heritability is 0%.

HeritabilityHeritabilityJoe and Hank are not related. At birth, both were placed in identical environments that were maintained identical until they were 10 years old.

At 10, Joe is outgoing and friendly.

At 10, Hank is withdrawn and shy.

In this (very small) population, what is the heritability of their personality traits? In other words, what percentage of the difference is attributable to genes?

100%

How heritable is that trait?How heritable is that trait?Trait Population HeritabilityCreativity adolescents 25%Extraversion adults 30-50%Autism general 90-95%Depression general 40-55%Sleep quality general 35%TV viewing 3-5 year olds 10-45%Religious beliefs general 0%Religious conservatism women 40%Divorce general 55%Height women 92%

http://www.peele.net/lib/genes.html The Stanton Peele Addiction Website

Identical Twins v. Fraternal Twins

Same genes, Different genes, Same environment Same environment

Therefore, greater differences between the two groups indicate greater heritability (“Nature”). The trait might appear to have a large genetic component.

Twin Studies

Twin StudiesTwin StudiesCriticism: 1) Do fraternal twins

really have equally similar environments to identical twins? Differences in looks or inborn temperament may cause people to treat you differently.

2) Identical twins may share a placenta but fraternal twins do not. Thus, we can’t eliminate prenatal environments as the cause of the differences between the two groups.

Increased risk of schizophrenia if twin develops disease

0%

10%

20%

30%

40%

50%

60%

Fraternal twins Identical twins

% in

crea

se

Twin StudiesTwin StudiesIdentical Twins Reared Together v. Identical Twins Reared Apart

Same genes, Same genes, Same environment Different

environment

Greater difference between these two groups indicates greater role of the environment (“nurture”).

Twin StudiesTwin Studies

Criticisms: 1) Adoption agencies

try to place twins in similar families so the variation in environment may be small, and

2) There is a very limited sample.

Similarity of IQ Test Scores in Twins

0.5

0.6

0.7

0.8

0.9

Identical twins rearedtogether

Identical twins rearedapart

Cor

rela

tion

Adoption StudiesAdoption Studies

Is child more like adoptive parents… …or biological

parents?

Adoptive Parents - Nurture

Biological Parents - Nature

Adoption StudiesAdoption Studies

• Note: Two related siblings that grow up in the same family are strikingly different in personality (as I’m sure many of you can attest to).

Similarity of Values Between Adopted Children and their Parents

0

0.1

0.2

0.3

0.4

0.5

Biological parents Adoptive parents

Corr

elat

ion

Coef

ficie

nt

Similarity of Temperament Between Adopted Children and their Parents

0

0.1

0.2

0.3

0.4

0.5

0.6

Biological parents Adoptive parents

Cor

rela

tion

Coe

ffici

ent

HeritabilityHeritability

Your turnYour turnAnalyze the behavior genetics

research data to determine the role of nature and nurture in various traits.