Monozygotic (identical) twins share all of their genes, while dizygotic (fraternal) twins share only about 50 percent of them, the same as non-twin siblings. If a researcher compares the similarity between sets of identical twins to that of fraternal twins for a particular trait, then any excess likeness between the identical twins should be due to genes rather than environment.
Twins. Depending on the question you may need monozygotic or dizygotic twins, or a combination of the two.
Use of Twin Studies
Researchers use this method, and variations on it to:
- Estimate the heritability of traits
- Quantify the effect of a person's shared environment (family) and unique environment (the individual events that shape a life) on a trait
Variations of Twin Studies:
- Twins-reared-apart design focuses on twins who were adopted into separate homes when they were infants. These twins do not share their environments, so genetic influences are thought to account for their similarities.
- Longitudinal studies of twins follow twins over time to assess the development of adult-onset conditions and traits. This approach allows for a more complete and accurate assessment of environmental factors over time.
- Combining classic twin studies with molecular genetics research incorporates data about the presence or absence of specific genetic variants to determine the impact on the trait of interest.
Assumptions in the design
Random mating. Research on twins assumes that people are as likely to choose partners who are different from themselves as they are to choose partners who are similar for a particular trait. If, instead, people tend to choose mates like themselves, then fraternal twins could share a greater percentage of their genes than expected. In the case of non-random mating, fraternal twins would have more genetically influenced traits in common than expected, because the genes they receive from their mothers and fathers would be similar to each other.
Equal environments. Research on twins also assumes that fraternal and identical twins raised in the same homes experience equally similar environments. However, some research suggests that parents, teachers, peers, and others may treat identical twins more similarly than fraternal twins.
Gene-environment interaction. Twin studies typically assume that genes and environment have only separate and distinct contributions to a trait. However, for some research outcomes, interactions between genes and environment appear to influence the development and intensity of traits. This suggests that both the gene variant and the specific environmental factor must be present for the trait to occur. Many twin-study designs fail to take this possibility into account.
Genetic mechanisms. Twin studies, in general, assume that only one type of genetic mechanism--usually additive--is operating for a particular trait. However, traits can be inherited through different genetic mechanisms:
- Additive genetic mechanisms mix together the effects of each allele. For example, if genes for height were additive, a very tall father and a very short mother might have a child who is of average height.
- For traits governed by dominant genetic mechanisms, a dominant gene inherited from one parent trumps a recessive gene inherited from the other parent. For example, if a person inherits a recessive gene for blue eyes from one parent and a dominant gene for brown eyes from the other parent, then the dominant brown gene "wins", and the person's eyes are brown.
- Epistatic mechanisms are complex cases where interactions among multiple genes may determine the outcome of one trait.
Note: Traditional concepts of dominant and recessive traits and diseases are being challenged as new evidence is gathered about subtle variations in phenotype and the complex etiology of common conditions. Some carriers of some recessive conditions have been found to show mild symptoms of the condition.
Advantages of Twin Studies
- Allows disentanglement of the shared genetic and environmental factors for the trait of interest. Researchers can estimate the proportion of variance in a trait attributable to genetic variation, versus the proportion that is due to shared environment or unshared environment.
Limitations of Twin Studies
- The use of twins does not allow the researcher to consider the effects of both shared environment and gene-environment interaction simultaneously. This can be addressed by including additional siblings in the design.
- Results from twin studies cannot be directly generalized to the general population. Twins are not a random sample of the general population as they differ in their developmental environment because two fetuses are growing simultaneously. Genetic factors may lead to a higher incidence of twinning (e.g., women more likely to release two eggs during ovulation).
Hints for use
- Research on mate selection patterns may help guide whether non-random mating can be assumed for a particular trait. For example, some studies suggest that people are more likely to select mates with similar levels of intelligence than they are to select mates with similar personality traits (i.e., introversion). Therefore, non-random mating may be more of a concern for researchers studying intelligence than those studying introversion.
- Many of the assumptions of twin studies can be tested if appropriate data is available. For example, the effects of non-random mating may be investigated by including spouses of twins in studies.
Examples of Twin Studies
- Boomsma, D et al. 2002 Classical twin studies and beyond. Nature Reviews Genetics, 3(11): 872-882.
- Hasselbalch AL. 2010. Genetics of dietary habits and obesity - a twin study. Dan Med Bull, 57(9): B4182.
- Kendler, KS et al. 1993 A test of equal-environment assumption in twin studies of psychiatric illness. Behavior Genetics, 23: 21-28.
- Slutske WS et al. 2010. Genetic and environmental influences on disordered gambling in men and women. Arch Gen Psychiatry, 67(6): 624-30.
- Van der Aa N et al. 2010.Moderation of genetic factors by parental divorce in adolescents' evaluations of family functioning and subjective wellbeing. Twin Res Hum Genet, 13(2): 143-62.
In this Program
- A discussion of hereitability in our Gene Environment Interaction module
- Heritability in the context of nicotine addiction in our Research in Action example, Exploring Genetic Methods
Outside This Program
- Neale, MC & Cardon, LR. 1992. Methodology for genetic studies of twins and families. Dordrecht, The Netherlands: Kluwer Academic Press.
- Read a review and critique of the use of twin studies in the field of behavioral genetics.
Specific twin studies