“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 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.
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.
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.