Personality and Heredity – Adoption & Twin Studies
- Is your personality dictated by the genes your parents gave you or a product of
your experiences and your environment?
- This “nature v. nurture” question has been very important in understanding the
origins of personality
- The question is also important to applications of personality psychology. For
example, if personality is solely hereditary could you expect therapy to be
successful? Would biological interventions, such as drugs, be the proper way to
treat personality disorders? Would we have discrimination laws for people
based on their personality type?
- Discussion of nature v. nurture boils down to a question of whether there is a
biological basis for personality.
- This section is intended to clarify the different research designs used to answer
this question and summarize the results we now know.
• Does Biology Relate to Personality?
Sheldon’s Research on Body Type & Personality Characteristics
- Sheldon collected photographs of college students and rated their body type on
three dimensions: Endomorphy, Mesomorphy, & Ectomorphy
- He also collected self-ratings of personality and correlated their body type scores
with the personality characteristics
- He found that extreme body type scores were correlated with personality
• Viscertonia -- jolly, sociable, persons who love comfort and eating
• Somatonia – bold, adventurous, assertive, dominating, noisy, callous,
youthful orientation, love physical activity and exercise
• Cerebrotonics -- are restrained, introversive, anxious, sensitive to pain, have
- Although Sheldon’s results suggest a relationship between body type (biology)
and personality, they cannot specify the causal mechanism involved. One or
more of the following are possible:
- Different body types (biology) may cause certain personality characteristics
- Certain personality characteristics and the lifestyle that they foster may
produce different body types
- Body type and personality may be causally unrelated but their correlations
with a third variable (such as cultural stereotypes) produce Sheldon’s results.
- This is just one of a number of studies in early research attempting to prove that
Personality is due to Heredity
Two Examples of Family Tracing Studies (from the turn of the century)
• Gaulton believed that intellectual ability was due to hereditary.
- He identified a number intellectually gifted historical individuals then
examined their family trees to identify descendents who had achieved a high
level of accomplishment in an intellectual field.
- He found that 30-50% of the descendents of the gifted men were themselves
judged to be eminent in some field (mathematics, medicine, etc.)
- Gaulton estimated that in the population at large for that time period only 250
out of every million people achieved this level of intellectual achievement
(that’ s about .025%).
- From these results, Gaulton concluded that intellectual ability has genetic
• Henry Goddard believed that social deviance (juvenile delinquency, aggression,
criminality, etc.) was due to heredity
- He traced the lineage of Martin Kallikak -- a revolutionary war soldier who
had children by two women: “a feeble minded tavern girl” & his wife “a
- The tavern girl had “old horror” who himself produced 10 kids – producing
“hundreds the lowest types of human beings”
- Quaker woman had 7 kids who descendents were “the highest types of
- Goddard concluded that heredity alone was responsible for the high
incidence of social deviance in old horror’s descendents
• What can one conclude of about the biological basis of either intellectual ability or social
degeneracy from these family tracing studies?
- Nothing! The effects of the environment and heredity are completely confounded!
- People in different families not only have similar genetics but also have
similar family environments, social standings, etc. as well.
- In other words, it is impossible to separate the effects of genes from the effects
of environment in these studies.
- To research the origins of personality and the role of heredity we need to
identify research methods that enable us to separate the role of heredity and
the role of environment.
• Some Basic Concepts in Behavioral Genetics
- Heredity: your parents, bloodline, etc.
- Genes: sequences of DNA that provide the blueprint for individuals’ traits and
- Genes can act singly or it may take multiple genes to produce a particular
trait or characteristics
- Genotype: all the possible ways in which multiple genes can combined and
expressed. How genes are combined will influence how much of a particular
trait is expressed and will produce variations that we can observe.
- Phenotype: actual, observable variations in some trait or characteristic.
Phenotypes are caused by genes (genotype) and environment, that is…
Phenotype = effects of genes + environment
- Heritability: how much of the variability in phenotype is due to genotype.
Heritability = Genes / (Genes + Environment)
Ranges from 0 (only caused by environment) to 1.0 (only caused by genes
• Approaches to Determining the Heritability of Personality Traits
• Selective Breeding (with animals)
• Adoption Research Studies
• Twin Study Method
• Adoption Studies with Twins Raised Together and Apart
These research designs try to determine how much of the variation in a personality
trait (phenotype) is due to genetics and how much is due to environment.
Selective Breeding Studies
- Animal breeding demonstrates that animals’ temperament (i.e. their
aggressiveness, sociability, etc.) can be influenced by breeding.
- Compares the similarity of children and their parents (adoptive or biological) on
specific particular personality traits
- High similarity between biological parents and their children could indicate an
- either genes or family environment when raised with the biological parents
- only genes when raised with adoptive parents
- High similarity between adoptive parents and their adopted children indicates
only an effect of family environment
• Authoritarianism using the adoption research approach
What is authoritarianism?
- Personality trait indicating a tendency to conform follow rules and rigidly
adhere to authority while feeling resentful at the same time
- People high in authoritarianism are more likely to discriminate against
members of their out-groups (e.g., members of the arayan nation)
- Collected parent-child correlations on authoritarianism for adopted and non
- Correlations between biological parents and their (non adopted) children
could be due to effects of either genes or family environment
- Correlations between parents and their adopted children can’t be due to
biology—only family environment
Authoritarianism Correlations Among Family Members
Relationship Non Adopted Adopted Difference
Father-Child .37 .14 .23
Mother-Child .41 .00 .41
- Genetics has a stronger effect on children’s authoritarianism than family
- Maternal genetic effect is stronger than paternal genetic effect
- To the degree that authoritarianism is taught, the effect is through the father
• Criminal Behavior using the adoption research approach (Mednick et al., 1984,
Science, 224, 891-894)
- Examined the conviction rate of sons when biological/adopted parents had
or had not been convicted of a crime
- Sample consists of 15,000 adopted boys from Denmark
Conviction Percentages of Sons From Convicted/Not Convicted Parents
Adopting Parents Convicted Not Convicted Row Total
Convicted 24.5% 14.7% 19.6%
Not Convicted 20.0% 15.5% 17.75%
Column Total 22.25% 15.1%
- Greater evidence of criminality if biological parent was convicted of a crime
(22.25% v. 15.1%) than if adopted parent was convicted (19.6% v. 17.75%).
• Weaknesses of Adoption Research Studies
- Compares the similarity of people (parents and children) at very different
times in their lives (they are at different stages of development)
- Can’t determine the impact of other environmental effects outside of the
family (such as peer group or educational experiences)
- Parents & children do not have the exactly the same genes
Twin Study Method
- There are different types of twins:
- Identical or monozygotic twins (MZTs): a single egg divides 1 to 14 days after
conception to create to individuals with identical genetics.
- Behavioral and physical differences between the individuals are
associated with environmental differences.
- Different environments may occur at prenatal (in the womb), peri natal
(during birth), or post natal (after birth) time periods.
- Fraternal or dizygotic twins (DZTs): two eggs are fertilized by separate sperm.
- Theoretically DZ siblings may share between 0-100% of their genes, but
most likely is between 25-75%.
- On average, their genetic makeup is the same as that of non twin siblings
- Measure the similarity of personality traits for identical and fraternal twin pairs.
That is, how similar is one twin of the pair to the other?
- If identical twins tend to be more similar than fraternal twins then the
characteristic likely to be caused by genetics.
- If there are no differences in the similarity of identical and fraternal twins then
the characteristic is likely to be caused by environment.
• Eysenck (1964) estimated twins’ similarity on various social behaviors.
- The concordance rate is the likelihood that both twins will share a personality
trait or behavioral pattern.
- Identical twins were more similar than fraternal twins in terms of their:
- Childhood behavioral problems
- Adult criminal behavior
- Sexual Orientation
- This does not say that identical twins are more likely to have behavioral
problems, but if one twin does (or doesn’t) then so is the other
• Weakness of the twin study method
- People tend to treat identical twins more alike than fraternal twins
- Some of what gets attributed to genes may be due to identical twins
experiencing a more similar environment than fraternal twins (violates
equivalent environment assumption)
Adoption Studies with Twins Raised Together and Apart
- Includes identical and fraternal twins, raised together and apart
- Provides a natural experiment that varies:
- Genetic similarity (identical v. fraternal) twins
- Environmental similarity (raised together or apart)
- Effects of genes and environment are estimated most accurately with this approach
• Minnesota Twin Study
- Includes the different twin pairs mentioned above:
- DZA – Fraternal twins raised apart
- DZT – Fraternal twins raised together
- MZA – Identical twins raised apart
- MZT – Identical twins raised together
- Measured personality using Eysenck’s three factor model
- Positive emotionality
- Includes well-being, social potency, achievement, & social closeness
- Correlates with Extraversion & Openness dimensions of the FFM
- Negative emotionality
- Includes stress reaction, alienation, & aggression
- Correlates with Neuroticism & Agreeableness dimensions of the FFM
- Includes control, harm avoidance, & traditionalism
- Correlates with Conscientiousness dimension of the FFM
- Also measured shared and non shared environmental influences
- Shared environments
- aspects of the environment shared by family members same family
- parenting style, religion, values, socioeconomic status, etc.
- Non shared environments
- aspects of growing up together that are not shared among siblings
- differences due to birth order, gender, or other unique life events
Similarity of Twins Reared Together & Apart
Personality Trait DZA DZT MZA MZT
Positive Emotionality -.07 .18 .34 .63
Negative Emotionality .29 .41 .61 .54
Constraint .04 .25 .57 .58
- Identical twins raised apart are as similar as when raised together
- Suggests a strong genetic component
- Estimates for genetic, shared environment, & non shared environment:
Genetic & Environmental Determinants of Personality
Personality Trait Genetic Environment Environment
Positive Emotionality .40 .22 .38
Negative Emotionality .55 .02 .43
Constraint .58 .00 .43
- Shared environment accounts for little variation in personality
- Genetic effects and unique life events appear to have the greatest influence
• Heritability of personality dimensions of the Five Factor Model:
Openness to Experience .46