Nature & Nurture in Psychology

Nature – genetics Nurture – environment

Nature and Nurture?

percentage of trait variation in a group/population that can be attributed to genetic differences

Heritability

Heritability

Sara is very outgoing and agreeable

Suzy is timid and non-conforming

If this pair of IDENTICAL twins is my population, what percent of their personality differences is

attributed to genetic differences?

Heritability is 0%

Identical twins are GENETICALLY identical

Heritability

Joe 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At 10, Joe is withdrawn & shy.

At 10, Hank is outgoing & friendly.

What percentage of their personality difference is attributable to genes?

Heritability is 100%

How is heritability determined?

Twin Studies & Adoption Studies

Identical Twins Fraternal Twins

same genes, different genes,same environment same

environment

Greater differences between the two groups indicate greater heritability (“nature”). The trait might have a large genetic component.

Twin Studies

v.

Twin StudiesCriticisms: (Don’t write this, just think

about it)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

cre

ase

Twin Studies

Identical Twins Raised Together

same genes,same environment

Greater difference between these two groups indicates greater role of the environment

(“nurture”).

Identical Twins Raised Apart

same genes, different environments

v.

Twin Studies

Criticisms: (Don’t write this, just think about it)

1) Adoption agencies try to place twins in similar families so the variation in environment may be small

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

Co

rrela

tio

n

Adoption StudiesIs the child more like adoptive parents…

…or biological parents?

Adoptive Parents - Nurture

Biological Parents - Nature

Adoption Studies

Note: Two related siblings that grow up in the same family are strikingly different in

personality.

Similarity of Values Between Adopted Children and their Parents

0

0.1

0.2

0.3

0.4

0.5

Biological parents Adoptive parents

Co

rrela

tio

n C

oeff

icie

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

Nature or Nurture?

NATURE NURTURE

Personality Divorce Risk Heart Rates Brain Waves Intelligence Levels

Values Attitudes Manners Political Beliefs Religious Beliefs Involvement in

charitable activities Age of death (7%

heritability)

Intelligence Levels – separated identical twins have similar intelligence levels, BUT adopted children score higher on

intelligence tests than their biological parents

Interests (50% heritability)

Trait Population Heritability

Creativity 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%Divorce general 55%Height women 92%

How heritable is that trait?

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

Heritability of Intelligence – most researchers believe it to be between 60-80%

“Nature deals the hand; nurture plays the cards.”

- Charles Brewer