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Studies of factors that influence an individual's vulnerability to trauma exposure or reaction to trauma have focused on intrapersonal psychological traits, interpersonal psychosocial variables or aspects of the trauma itself. Recently, attention has been turned toward genetic factors that might influence either the amount of trauma to which an individual is exposed or the responses to exposure. The establishment of twin registries, such as the Vietnam Era Twin Registry, has facilitated such research. 

There are two primary research designs that use twin pairs: the classical twin study and the co-twin control study. The classical twin study compares the degree of similarity of a specific trait or condition within monozygotic (MZ) and dizygotic (DZ) pairs. MZ twins share 100 percent of their genes, and DZ twins, as in ordinary siblings, share about 50 percent of their genetic material. If MZ twins have more similarities in a particular trait or condition than DZ twins, this is interpreted as evidence for genetic influence. The method assumes that the effect of family environment for MZ and DZ twins is the same, so that a higher degree of similarity between MZ twins, as opposed to DZ twins, is due to genetic factors. 

The goal of the classical twin study is to estimate the heritability of a trait or condition. Heritability is defined as the proportion of the variance in a trait or condition due to genetic factors. The estimation of heritability is based on statistical calculation comparing the trait or condition similarity in MZ and DZ pairs. True and his colleagues, in a 1993 Archives of General Psychiatry article, conducted a classical twin study to estimate the heritability of PTSD. They demonstrated that there was genetic influence for nearly all PTSD symptoms. 

The co-twin control study is intended to assess the association between an environmental experience and a condition. The design identifies twins who have been exposed to different environmental experiences - for example, one serving in Vietnam while the co-twin did not. The prevalence of mental health outcomes, such as PTSD, depression and alcohol use, are then compared within these environmentally discordant twins. The design is especially valuable when appropriate "controls" are hard to define. When the design is restricted to MZ twins with discordant exposure, a comparison is done on the twins' genetics and shared family environmental factors. Goldberg and colleagues used MZ Vietnam discordant twin pairs in a 1990 Journal of the American Medical Association paper to examine effects of Vietnam service on the prevalence of PTSD symptoms. 
For an excellent introduction to the twin method in psychiatry, see Kendler's (1993) article in the American Journal of Psychiatry, "Twin Studies of Psychiatric Illness." As in all research designs, the twin method has strengths and limitations. Criticism of the method has been raised by Lewontin, Rose, and Kamin (1984) in their book, Not in our Genes: Biology, Ideology, and Human Nature. 

ISTSS Research Methodology Special Interest Group sponsored this brief report. If you are interested in becoming a member of the group, please contact chairs Daniel and Lynda King, National Center for PTSD (116B-2), VA Boston Healthcare System, 150 S. Huntington Ave., Boston, Mass. 02130; e-mail: king.daniel@boston.va.gov or lking@world.std.com.