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CHAPTER 9:Behavioral Genetics
Introduction
People often resemble their biological relatives in physical appearance, which can be
observed in family gatherings.
The specific basis for these similarities can be complex and includes factors like eye
shape, hair curl, and facial expressions.
The underlying cause for such resemblances is shared genes.
Family Resemblance Beyond Physical Traits
Beyond physical traits, questions arise about family resemblance in personality.
Key questions in this area include:
Do we inherit our personality traits from our parents?
Are siblings psychologically similar due to biological relations?
Field of Behavioral Genetics
Objective: Investigates how inherited biological materials (genes) influence broad
patterns of behavior.
Definition: A pattern of behavior that is consistent across various situations is termed a
personality trait (Plomin, Chipuer, & Loehlin, 1990).
Terminology: While "behavioral" genetics is the traditional term, "trait" genetics might
be more accurate in this context.
Conclusion
Behavioral genetics explores the genetic basis of personality traits, aiming to
understand how our genes influence who we are behaviorally and psychologically.
Controversy
Historic Associations
Eugenics:
A belief that humanity can be improved through selective breeding.
Led to activities like:
Campaigns against “inferior” immigrants.
Nobel Prize winners' sperm banks.
Associated with historical figures like Adolf Hitler, implying dystopian futures.
Cloning:
, The belief in creating a complete duplicate of a human being, both
psychologically and physically.
Seen as technologically speculative and ethically controversial.
Concerns
Research might lead to the misconception that traits like intelligence, poverty,
criminality, mental illness, and obesity are fixed by genes.
Public might overlook the role of experience and social circumstances in shaping these
outcomes.
Modern Behavioral Geneticists' Stance
Basic Scientists:
Pursue knowledge for its sake and to develop treatments for behavioral
disorders.
Emphasize the importance of understanding genetic influences alongside
environmental factors.
Reject the feasibility and desirability of eugenics and cloning.
Complex Interaction Between Genes and Environment
Personality results from a complex interaction between genes and the environment.
Impracticality of Eugenics and Cloning:
Breeding to specification or duplicating individuals is unlikely.
A genetic clone would still di er significantly due to environmental influences.
Contribution to Understanding Personality Development
Balanced Perspective:
Recognizes both genetic and environmental bases of personality.
Challenges the idea of traits being inevitably fixed.
Calculating Heritability
Basic Idea
Traits influenced by genes result in greater similarity among genetic relatives compared
to non-relatives.
Closer genetic relationships correlate with higher similarity in traits.
Types of Twins
Identical (Monozygotic, MZ) Twins:
Arise from a single fertilized egg.
, Genetically identical.
Fraternal (Dizygotic, DZ) Twins:
Arise from two di erent eggs fertilized by di erent sperm.
Genetically similar to ordinary siblings.
Genetic Similarity
Over 99% of human genes are identical across individuals.
Behavioral genetics focuses on the less than 1% of genes that vary among individuals.
Genetic Sharing:
MZ twins: identical genes.
DZ twins: share about half of the variable genes, similar to parents and o spring.
Research Methodology
Twin Studies:
Identifies MZ and DZ twins, including those reared apart.
Measures personality using self-report instruments like the Eysenck Personality
Questionnaire (EPQ), California Psychological Inventory (CPI), and NEO-PI.
Less common: direct observation of twins in laboratory settings.
Heritability Calculation
Correlation Coe icients:
Compute correlations separately for MZ and DZ twins.
Heritability coe icient: reflects the degree to which trait variance in a population
is due to genetic variance.
Formula: Heritability quotient = (rMZ – rDZ) × 2.
Example: Average correlation for MZ twins is .60 and for DZ twins is .40, resulting
in a heritability coe icient of .40 (40% of trait variance attributed to genetic
variance).
Additional Relatives
Other Relatives:
Children share 50% of variable genes with each biological parent.
Adopted children share no more personality-relevant genes with adoptive
parents than with any random person.
Full siblings: 50% shared genes on average.
Half-siblings: 25% shared genes.
First cousins: 12.5% shared genes.
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