Unit 2: How Twins Saved Psychology

MOOC Summaries - Introduction to Human Behavioral Genetics - twins

Unit 2: How Twins Saved Psychology

“Twins: A Natural Experiment… How to Assess Twin Similarity… Findings from Twin Studies… Are Twin Studies Valid?… Adoption Studies: Not by Twins Alone… Minnesota Study of Twins Reared Apart (MISTRA)… Supplemental – Interview with Tom Bouchard, Director of MISTRA…” 


  • Module A: Twins - A Natural Experiment
  • Module B: How to Assess Twin Similarity
  • Module C: Findings from Twin Studies
  • Module D: Are Twin Studies Valid?
  • Module E: Adoption Studies - Not by Twins Alone
  • Module F: Minnesota Study of Twins Reared Apart (MISTRA)
  • Module G: Supplemental - Interview with Tom Bouchard, Director of MISTRA

Module A: Twins – A Natural Experiment

  • Twin studies are based upon the existence of two types of twins – monozygotic (MZ) and dizygotic (DZ).
  • Monozygotic twins occur because of a single sperm fertilizing a single egg, and within the first two weeks post conception, the developing embryo divides into two identical zygotes i.e. they have 100% genetic overlap, and they grow up together.
  • Dizygotic twins occur because two sperms independently fertilize two eggs but in one ovulatory cycle i.e. on average 50% genetic overlap, and they also grow up together.
  • Twin studies take advantage of this natural occurrence to understand whether or not genetics influences individual differences in a trait i.e. they are natural experiments.
Chop Chop MOOCs’ summary of https://class.coursera.org/behavioralgenetics-002/lecture/view?lecture_id=35

Module B: How to Assess Twin Similarity

  • In twin studies, phenotypes (i.e. traits) can be:
    • categorical i.e. either or traits like whether one has schizophrenia, or whether one has a college degree, whether a marriage ended in divorce etc.
    • quantitative i.e. distributed along a continuum and are numeric, such as IQ, extraversion and amount of alcohol consumed in a week etc.
  • For categorical phenotypes, the measure of twin similarity is usually (twin) concordance i.e. the probability a twin has that phenotype given the co-twin has the phenotype.
  • For quantitative phenotypes, twin similarity is usually measured using a correlation coefficient i.e. an index of the strength of the linear relationship between two quantitative scores.
  • Second major type of phenotype is quantitative trait like IQ or extraversion, these are traits that are distributed along a continuum so they are numeric
  • Step-by-step examples of calculation concordance and correlation coefficient:
    • Concordance: drug abuse – concordance for monozygotic twins (MZ) is 62%, dizygotic twins (DZ) is 53%, and the population in general is 20%.
    • Correlation coefficient: height – MZ twins has a correlation of 0.92, a very strong correlation whereas dizygotic twin has a correlation of 0.56
  • One additional interpretation that will be useful for quantitative genetics and molecular genetics is to multiply it by itself i.e. square it to get the r-squared, which is the percentage of the variance accounted for in one variable by another.
    • For the height example above, take 0.92 multiplied by 0.92, giving us 0.81 or 81%, which means we can account for 81% of the individual differences in height of one twin once we know the other twin’s height.
Chop Chop MOOCs’ summary of https://class.coursera.org/behavioralgenetics-002/lecture/view?lecture_id=37

Module C: Findings from Twin Studies

  • Descriptions of various ways to study twins.
  • Three findings on behavioral phenotypes (i.e. traits) that are fairly consistent across studies:
    • Monozygotic twins (MZ) are more similar than dizygotic twins.
    • MZ twins are not perfectly similar.
    • Similarities in twins are not that different from the similarities in physical and medical traits.
  • Concordance for MZ higher than for DZ twins on various physical and mental disorders/behaviors such as alcoholism, schizophrenia, Alzheimer’s disease, autism, reading disability, Parkinson’s disease, hypertension, breast cancer, heart disease, peptic ulcers etc.
  • Correlation coefficient for MZ higher than for DZ twins on psychological characteristics such as memory, neuroticism, extraversion, spatial reasoning, scholastic achievement at adolescence, verbal reasoning, processing speed, general intelligence etc.
  • Greater MZ and DZ similarity too on “improbable” trains like divorce (concordance), religiousness (correlation), and political attitude (correlation); but we should not conclude there are genes for these traits.
  • Two explanations for all these results:
    • Genetic factors might have indirect and multiple effects (i.e. pleiotropic) that influence different traits (because the underlying personality characteristics e.g. the neurotransmission systems in brains might be heritable);
    • Twin studies are wrong – this will be covered in the next module.
Chop Chop MOOCs’ summary of https://class.coursera.org/behavioralgenetics-002/lecture/view?lecture_id=39

Module D: Are Twin Studies Valid?

  • Validity of twin studies is based on two key assumptions:
    • Representativeness: twins are psychosocially like the non-twin population i.e. the findings can be generalized
    • Equal Environmental Similarity (EES): environmental similarity of MZ twins is not greater than that of DZ twins.
  • On the first assumption, studies of psychiatric illnesses, personality and cognitive ability for twins showed they were were similar to those in the general population.
  • On the second assumption:
    • Pre-natal environment: the studies that have been done (even though there are not many as it is difficult given the pre-natal environment) do not seem to invalidate the assumption.
    • Post-natal environment: studies also supported the assumption.
  • What some studies e.g. on life stress do show is that:
    • For environmental factors that are externally imposed (such as family-related factors), MZ similarity is not higher than DZ.
    • For environmental factors that reflect our own behavior (e.g. how we do in school and work), there is greater MZ similarity than DZ similarity.
    • These suggest that genetic influences on behaviour may be due to the environments we create.
  • That said, behavioral genetics does not only rely on twin studies (otherwise there might be reservations on the inferences drawn); there are also other studies e.g. adoption studies.
Chop Chop MOOCs’ summary of https://class.coursera.org/behavioralgenetics-002/lecture/view?lecture_id=41

Module E: Adoption Studies – Not by Twins Alone

  • Behavioral genetics does not rely only on twin studies; there are other research design such as adoption studies which also have similar conclusions.
  • Types of adoption studies:
    • Adopted siblings: not genetically related but raised in the same home;
    • Parent offspring: genetic and environmental contributions from one generation to another where the parent and the child have been separated (the latter was adopted by someone).
  • These adoption studies support the conclusions of the twin studies; they also illustrate that environments can be important.
Chop Chop MOOCs’ summary of  https://class.coursera.org/behavioralgenetics-002/lecture/view?lecture_id=43

Module F: Minnesota Study of Twins Reared Apart (MISTRA)

  • Combine the previous two adoption studies in the form a Reared Apart Twins study – this is a very rare type of study.
  • One such study is MISTRA, and arguably the most visible behavioral genetic study.
  • Premise of the study: twins separated at or about the time of birth, and then reunited in adulthood.
  • Finding: physical and psychological correlations for MZ and DZ were similar for reared apart twins as for reared together twins.
  • The one exception: IQ – there is still a lot of similarity but not as similar as reared together twins,  suggesting the home environments are shaping IQs.
  • Three general findings from MISTRA (and why it is so influential):
    • MZ and DZ twins are similar even when reared apart – strengthens the case for genetic influences
    • Reared apart twins not much less similar than reared together – implies common rearing environment not so influential
    • Reared apart MZ twins are not psychologically identical – implies environment is important
  • The first corresponds to A – Additive Genetics, the second to C – Shared Environment, and the third to E – Non-Shared Environment.
Chop Chop MOOCs’ summary of  https://class.coursera.org/behavioralgenetics-002/lecture/view?lecture_id=45

Module G: Supplemental – Interview with Tom Bouchard, Director of MISTRA

  • Some key points from the interview:
    • Genetic explanations within psychology back in 1979 was mostly negative, particularly true for people who might work on the east coast
    • On mainstream psychology, the major impact is that students are not taught about behavior genetics
    • The atmosphere at Minnesota was entirely different as there was an atmosphere that discussed individual differences.
    • Biggest challenge for MISTRA was identifying the reared apart twins for the study
  • One phenomena of interest: one of the findings on psychopathology and intelligence is that you can be mentally ill and still be brilliant
  • Another big issue: measurement in the field of personality is a mess, and no real progress has been made in about 75 years except for finding genetic influence.
  • Some interesting linkages seem to be coming out of Genome-Wide Association Studies (GWAS) – to be covered later in the course.
Chop Chop MOOCs’ summary of  https://class.coursera.org/behavioralgenetics-002/lecture/view?lecture_id=157

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