Why psychologists study twins

In the second of our on-going series of guest features for psychology students, Dr. Angelica Ronald of London's Institute of Psychiatry describes the use of twin studies in psychology.

Psychologists are often trying to control one thing to look at its effect on something else. This results in the plethora of artificial experiments and carefully-matched control groups in psychological studies. The beauty of twin studies is that they provide psychologists with a natural experimental design – there’s no need for any additional control groups.

This natural design comes about because there are two types of twins: those who share all their genes (because they were formed from the same egg which split early on in development), called identical or monozygotic twins, and those who, just like non-twin siblings, share on average half their genes (they are formed from two separate eggs), called fraternal or dizygotic twins.

Twin designs address the nature-nurture question. Behaviour geneticists compare how alike one twin is with the other twin on whatever variable they are interested in; in my case this is autistic behaviours. If genes influence variation in autistic behaviours, identical twin pairs who share all their genes will be highly similar in their degree of autistic behaviours whereas fraternal twins will be much less similar. This is what we have found.

It’s the same for a diagnosis of autism: when one identical twin has autism, in 60 per cent of cases their co-twin also has autism. With fraternal twins there is a different pattern: most of the time when one twin has autism, the other does not have a diagnosis.

Just as much as twin studies have told us about genetics, they have been paramount in revealing the importance of the environment. For example, it is true that about 60 per cent of identical twins have the same autism diagnosis i.e. if one is autistic, the other is too. But in the other 40 per cent or so of identical twins, if one has autism, the other does not. This is sound proof that autism is not completely genetically determined – because if it were, both identical twins in a pair would always show the same degree of autistic problems. Genes play a role in risk but there must be some influence of the environment on the child’s outcome as well.

This example represents just the tip of the iceberg of how twin studies can contribute to psychology and our understanding of the causes of human behaviour. Of course no study design is perfect: like most research designs, the twin design has a number of assumptions, and even though it’s a natural experiment and we don’t have to control any variables, behavioural geneticists have to collect huge samples of twins (usually in the 1000’s) to be able to be certain about their findings.
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Further reading:

Plomin, R., DeFries, J.C., McClearn, G.E. & McGuffin P. (2001). Behavioral Genetics (4th edn). New York: Worth Publishers.

Ronald, A., Happé, F., & Plomin, R. (2005). The genetic relationship between individual differences in social and nonsocial behaviours characteristic of autism. Developmental Science, 8, 444-458.

Ronald, A., Happé, F., Bolton, P., Butcher, L. M., Price, T. S., Wheelwright, S., Baron-Cohen, S., & Plomin, R. (2006). Genetic Heterogeneity Between the Three Components of the Autism Spectrum: A Twin Study. Journal of the American Academy of Child & Adolescent Psychiatry, 45, 691-99.

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