Family/Kinship Studies

• Compare individuals with different degrees of genetic relatedness on a specific characteristic or behavior

– Exs: adoption studies, twin studies

Twin Studies:

– Compare identical twins to fraternal twins on a particular characteristic/behavior

• Identical twins share 100% of their genes and fraternal twins share 50% of their genes

• If (as a group) identical twins are more similar than fraternal twins, assumed that genes influence the characteristic

Heritability Coefficient (h2) – Estimates the proportion of variability in

individual characteristics that is due to genetic differences

– Ranges from 0 to 1• Ex: A heritability coefficient of .60 indicates that

60% of the measured variation in a specific characteristic is due to genetic differences in the sample

– Can estimate environmental influences using comparable statistics

Interpretation of Heritability Coefficients

• Apply to populations (groups), not individuals

– Correct: 50% of the variation in IQ in a specific sample is due to genetic differences

– Incorrect: 50% of an individual’s IQ is due to their genes

• Heritability coefficients reflect genetic and environmental diversity

– Variability in a specific characteristic is due to genetic and environmental influences

• G + E = 100% of the variability

– The more environments vary, the lower heritability estimates will be (and vice versa)

• Heritability coefficients change with development

• Heritability coefficients are specific to a particular sample in a specific environment at a single point in time

• Characteristics that are heritable can be modified by environmental influences

– Heritability coefficients do not indicate lack of malleability

Shared and Non-Shared Environmental Effects

• Behavioral genetics research allows estimation of two types of environmental effects

– Shared: Environmental influences that make individuals similar in a common environment

– Non-shared: Environmental influences that make individuals different in a common environment

General Criticisms of Heritability Estimates

– Not useful because they cannot be generalized across samples and will change if environments change

– Not useful because they tell us nothing about specific genetic and environmental influences

General Criticisms of Behavioral Genetics Research Designs

• Attempt to partition variance attributable to genes and environment—assume independence

– Gene-environment correlations?

– Failure to take into account gene-environment interactions

• Even if g-e interactions are tested, most studies have insufficient power to detect them

Gene-Environment Correlations

Passive G-E Correlations

• Parents provide environments for children that are influenced by their own genes

• Because the child’s genes are correlated with parents’ genes, the child’s genes are correlated with the environment that parents provide

• The environment the child experiences is correlated with his/her genes

Evocative G-E Correlations • Children evoke or elicit responses from others

that are influenced by the child's genes

• Children’s environments are correlated with their genes

Active G-E Correlations

• Children (and adults) seek out environments that are compatible with their genes (niche-picking)

• Environments children choose are correlated with their genes