Twin studies have had a long but not always honorable place in the history of medical science since their introduction by Sir Francis Galton in the last century. This is particularly true of studies which have been applied to such complex traits as intelligence where the image of this approach has been tarnished in part by the failure to recognize the limitations inherent in the twin method and by the attempt to extrapolate from highly selected twin samples to the population in general. On the other hand twin studies, particularly when multidisciplined and placed in the perspective of population and family studies, can be of great value. Multiple sclerosis (MS) is, most certainly, a multifactorial disorder; both genetic and environmental factors are thought to contribute to the disease. Twins provide a unique opportunity for performing studies in an effort to identify the relative influences of these factors. Further, this approach could identify actual causative factors or specific genetic determinants relating to disease susceptibility. However, it is important to remember the inherent weaknesses of twin studies such as sample bias. Also, possible pitfalls such as subclinical disease, although representing a difficulty in evaluating results, actually emphasizes the usefulness of twins in magnifying this difficulty. It is hoped that more detailed investigations of these and additional twins along with longitudinal studies will help to define further the relative influences of genetic and environmental factors. More importantly it is hoped that these approaches will permit a more precise identification of environmental elements which either cause or increase the risk of MS.
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