DEV - Nature and Nurture

Lecture

Differences between schools account for only 10% of the variance. 

90% of the variance is within school (individual differences). 

The role of nature and nurture in individual diffs in various cognitive and learning abilities

Behavioural genetics research deigns:

• Nature and nurture --> Twin design --> MZ vs DZ twins OR twins reared apart and together
• Nature and nurture --> Adoption design --> Biological and adoptive parents and adoptees OR adoptive vs. non-adoptive families

Twin method

• Dizygotic (DZ) = share 50% of genes
• Monozygotic (MZ) = share 100% of genes
• We can compare resemblance on a specific trait eg. IQ
• Allows us to get a rough estimate of separate genetic and environmental contributions to a trait 
• Identical (MZ) twin resemblance (rMZ) = 100% G + E
• Fraternal (DZ) twin resemblance (rDZ) = 50% G + E

• Components of a Phenotypic variation (P):
• Herability: (A, h squared): influences of genetic factors
• Shared environment: (C, c squared): any environmental influences that contribute to the similarity between co-twins 
• Nonshared environment (E, e squared): any aspect of environmental influence that makes co-twins different from each other

P = h squared + c squared + e squared 

rMZ = A+C

rDZ = 1/2A + C

A = 2(rMZ-rDZ)

E = 1 - rMZ

C = rMZ - A

Polygenic trait model

• Cognitive traits are controlled by more than one genetic variant 
• Each genetic variant is additive to the others
• The hallmark of polygenic traits:
  A bell curve distribution
  A continuous distribution

Summary

• Both genetic and environmental influences are important for individual diffs in cognitive abilities
• Genetic influences are not static: eg. heritability of IQ increases during development
• Cognitive abilities are polygenic traits: multiple genetic variants with small effects contribute to these traits
• Common…