Twin study
Twin studies are studies conducted on identical or fraternal twins. They aim to reveal the importance of environmental and genetic influences for traits, phenotypes, and disorders. Twin research is considered a key tool in behavioral genetics and in related fields, from biology to psychology. Twin studies are part of the broader methodology used in behavior genetics, which uses all data that are genetically informative – siblings studies, adoption studies, pedigree, etc. These studies have been used to track traits ranging from personal behavior to the presentation of severe mental illnesses such as schizophrenia.
Twins are a valuable source for observation because they allow the study of environmental influence and varying genetic makeup: "identical" or monozygotic twins share essentially 100% of their genes, which means that most differences between the twins are due to experiences that one twin has but not the other twin. "Fraternal" or dizygotic twins share only about 50% of their genes, the same as any other sibling. Twins also share many aspects of their environment because they are born into the same family. The presence of a given genetic or phenotypic trait in only one member of a pair of identical twins provides a powerful window into environmental effects on such a trait.
Twins are also useful in showing the importance of the unique environment when studying trait presentation. Changes in the unique environment can stem from an event or occurrence that has only affected one twin. This could range from a head injury or a birth defect that one twin has sustained while the other remains healthy.
The classical twin design compares the similarity of monozygotic and dizygotic twins. If identical twins are considerably more similar than fraternal twins, this implies that genes play an important role in these traits. By comparing many hundreds of families with twins, researchers can then understand more about the roles of genetic effects, shared environment, and unique environment in shaping behavior.
Modern twin studies have concluded that all studied traits are partly influenced by genetic differences, with some characteristics showing a stronger influence, others an intermediate level and some more complex heritabilities, with evidence for different genes affecting different aspects of the trait – as in the case of autism.
History
have been of interest to scholars since early civilization, including the early physician Hippocrates, who attributed different diseases in twins to different material circumstances, and the stoic philosopher Posidonius, who attributed such similarities to shared astrological circumstances. Saint Augustine in his Confessions on the other hand uses the diverging lives lived by twins as an argument against astrology.Gustav III, King of Sweden was the first to commission a medical study using identical twins.
Gustav's father, Adolph Frederick had been an opponent of stimulating drinks such as tea and coffee, signing the Misuse and Excesses Tea and Coffee Drinking Edict in 1757. Both Gustav III and his father had read and been strongly influenced by a 1715 treatise from a French physician on the dangers of what would later be identified as caffeine in tea and coffee. After assuming the throne in 1771 the king became strongly motivated to demonstrate to his subjects that coffee and tea had deleterious effects on human health. To this end he offered to commute the death sentences of a pair of twin murderers if they participated in a primitive clinical trial. Both condemned men agreed and subsequently spent the rest of their lives in prison fulfilling the king's demands: that one twin drink three pots of coffee every day and the other three pots of tea. The tea drinking twin died first at the age of 83, long after Gustav III, who was assassinated in 1792. The age of the coffee-drinking twin at his death is unknown, as both doctors assigned by the king to monitor this study predeceased him. The ban on coffee and tea in Sweden was lifted in 1823.
A more recent study is from Sir Francis Galton's pioneering use of twins to study the role of genes and environment on human development and behavior. Galton, however, was unaware of the difference between identical and DZ twins. This factor was still not understood when the first study using psychological tests was conducted by Edward Thorndike using fifty pairs of twins. This paper was an early statement of the hypothesis that family effects decline with age. His study compared twin pairs age 9–10 and 13–14 to normal siblings born within a few years of one another.File:Francis Galton.jpg|thumb|140px|right|Francis Galton laid the foundations of behavior genetics as a branch of science.
Thorndike incorrectly reasoned that his data supported for there being one, not two, twin types. This mistake was repeated by Ronald Fisher, who argued
An early, and perhaps first, study understanding the distinction is from the German geneticist Hermann Werner Siemens in 1924. Chief among Siemens' innovations was the polysymptomatic similarity diagnosis. This allowed him to account for the oversight that had stumped Fisher, and was a staple in twin research prior to the advent of molecular markers.
Wilhelm Weinberg and colleagues in 1910 used the identical-DZ distinction to calculate respective rates from the ratios of same- and opposite-sex twins in a maternity population. They partitioned co-variation amongst relatives into genetic and environmental elements, anticipating the later work of Fisher and Wright, including the effect of dominance on similarity of relatives, and beginning the first classic-twin studies.
A study conducted by Darrick Antell and Eva Taczanowski found that "twins showing the greatest discrepancies in visible aging signs also had the greatest degree of discordance between personal lifestyle choices and habits", and concluded that "the genetic influences on aging may be highly overrated, with lifestyle choices exerting far more important effects on physical aging."
Examples
Examples of prominent twin studies include the following:- Maudsley Bipolar Twin Study
- Minnesota Twin Family Study
- Twins Early Development Study
- NASA Twins Study
Methods
The basic logic of the twin study can be understood with very little mathematical knowledge beyond an understanding of the concepts of variance and thence derived correlation.
Classical twin method
Like all behavior genetic research, the classical twin study begins by assessing the variance of behavior in a large group, and attempts to estimate how much of this is due to:- genetic effects
- shared environment – events that happen to both twins, affecting them in the same way
- unshared, or unique, or nonshared environment – events that occur to one twin but not the other, or events that affect either twin in a different way.
The ACE model indicates what proportion of variance in a trait is heritable, versus the proportion due to a shared environment or unshared environment. Research is typically carried out using Structural equation modeling programs such as OpenMx capable in principle of handling all sorts of complex pedigrees. However the core logic underlying such programs is the same as the one underlying the twin design described here.
Monozygotic twins raised in a family share 100% of their genes, and all of their shared environment. Any differences arising between them in these circumstances are random. The correlation between identical twins provides an estimate of A + C. Dizygotic twins also share C, but share, on average only 50% of their genes: so the correlation between fraternal twins is a direct estimate of ½+C. If we denote with r the correlation, we can define rmz and rdz as the correlations of a trait among identical and fraternal twins, respectively. For any particular trait, then:
Stated again, the difference between these two sums then allows us to solve for A and C. As the difference between the identical and fraternal correlations is due entirely to a halving of the genetic similarity, the additive genetic effect A is twice the difference between the identical and fraternal correlations:
given the estimate for A, the one for C can be derived, for instance, from the first equation:
Finally, since the trait correlation among identical twins reflects the full contribution of A and C, the residual variation E can be estimated by subtracting this correlation from 1
To summarize therefore, the additive genetic factor A is twice the difference between MZ and DZ twin correlations, C is the MZ twin correlation minus this estimate of A, and the random factor E is, i.e. MZ twins differ due to unique environments only.
Modern modeling
Beginning in the 1970s, research transitioned to modeling genetic, environmental effects using maximum likelihood methods. While computationally much more complex, this approach has numerous benefits rendering it almost universal in current research.An example structural model is shown:
Model A on the left shows the raw variance in height. This is useful as it preserves the absolute effects of genes and environments, and expresses these in natural units, such as mm of height change. Sometimes it is helpful to standardize the parameters, so each is expressed as percentage of total variance. Because we have decomposed variance into A, C, and E, the total variance is simply A + C + E. We can then scale each of the single parameters as a proportion of this total, i.e., Standardised–A = A/. Heritability is the standardised genetic effect.