Friday, 9 February 2007

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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11 comments:

  1. A confounding factor in your interpretation is that mz twins may share more environmental factorsh than dz twins. For example, people may treat mz twins more equally than dz twins.

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  2. Anonymous12:06 am

    thats why we study mz that have been brought up apart you doofus

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  3. Richard1:18 pm

    You can't call him a doofus on that basis - analysis of most studies of MZ reared apart are in situations where environments are similar (very often two different branches of the same family) and twins are often in contact with each other. These are issues that can't simply be ignored like this. Good coverage of the issues can be found here:

    http://www.jayjoseph.net/files/Twin_2002_pdf.pdf

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  4. Thanks for the nice post!

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  5. Anonymous3:13 pm

    Its the links the parents of twins share that provides more interesting data.

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  6. I wonder if the point at which the MZ cells split apart is considered when assuming that the twins are truly identical. They have the same genes, but perhaps the longer/shorter amount of time spent as one could be considered a rogue environmental factor, as well. Due to the comparatively violent nature of the split, could one (theoretically) be more affected by the process than the other?

    A related issue to consider: Do individual experiences start at birth, or before? ("Ouch - move over - stop kicking me in the head like that..." ;)

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  7. Rachel11:59 pm

    Those are some good points there. I suppose the length of time before the cells split apart may have some effect, but probably very little. It would probably be too early on for it to make a lot of difference.
    I do believe that individual experiences start before birth. My parents always tell me that before we were born, my twin sister and I were always kicking each other. This behaviour and possibly conditions before birth could be considered as an environmental factor.
    One twin could potentially be more affected by the split than the other; that's a brilliant point.

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  8. Anonymous2:44 am

    well, as a fraternal twin myself, my brother was more affected by the split in the womb, as he is bigger than I. we always joke that he took all of the nutrients,which is probably true. I think that twin studies are actually pretty cool :)

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  9. Anonymous4:41 am

    Now who is the doofus ?? How could you and your brother be monozygotic twins when you are male and female ? There was no split as mz twins are ALWAYS the same sex !! I suggest you do a bit more study on twins before you show your ignorance.

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    Replies
    1. Anonymous1:31 am

      not always doofus

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    2. Anonymous1:33 am

      You should study further it's very rare - but possible! I know

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