Nature & Nurture in Psychology

Nature and Nurture? Nature – genetics Nurture – environment

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

Identical twins are GENETICALLY identical 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 Heritability is 100% 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 At 10, Joe is withdrawn & shy.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

Twin Studies v. same genes, different genes, Identical Twins Fraternal Twins same genes, different genes, same environment same environment v. Greater differences between the two groups indicate greater heritability (“nature”). The trait might have a large genetic component.

Twin Studies Criticisms: (Don’t write this, just think about it) Do fraternal twins really have equally similar environments to identical twins? Differences in looks or inborn temperament may cause people to treat you differently. 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.

Identical Twins Raised Together Twin Studies Identical Twins Raised Apart same genes, different environments Identical Twins Raised Together same genes, same environment v. Greater difference between these two groups indicates greater role of the environment (“nurture”).

Twin Studies Criticisms: (Don’t write this, just think about it) Adoption agencies try to place twins in similar families so the variation in environment may be small There is a very limited sample.

Adoption Studies Is the child more like adoptive parents… …or biological parents? Biological Parents - Nature Adoptive Parents - Nurture

Adoption Studies Note: Two related siblings that grow up in the same family are strikingly different in personality.

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) Interests (50% heritability) NATURE NURTURE Intelligence Levels – separated identical twins have similar intelligence levels, BUT adopted children score higher on intelligence tests than their biological parents

How heritable is that trait? 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% 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