location (Harrison and Geddes, 1996).
On the other hand, another type of study, the association study, also has limitations. The association study is a study that examines large numbers of subjects looking for associations between various factors and the disease. Among the problems in association studies are selection bias, publication bias, and confounding bias (Harrison and Geddes, 1996).
Twin studies, too, have their limitations. According to McGuffin et al., 1995), major problems with twin studies include the fact that concordance rates are based on clinical diagnoses rather than explicit research criteria and that they tend to have biases in the selection process.
One of the methods currently used to determine the corresponding contribution of environment and genetics to a particular condition or characteristic is the twin study. There are two different types of twin study. In one type of twin study, rates of a particular characteristic are compared for monozygotic and dizygotic twins. Monozygotic twins share more genetic material than their dizygotic counterparts. If, therefore, there is a higher rate of correspondence between the monozygotic twins than the dizygotic twins, the likelihood of there being a genetic component involved increases.
McGuffin et al. (1995) summarized a number of separate twin studies, considering that relationship. In their chart, they indicated that monozygotic twins had nearly a 50 percent risk of sharing the characteristic of schizophrenia, while dizygotic twins had only approximately a 14 percent chance of such a result. This was based on a summary of European-based hospital register studies.
Another type of twin study, which was also reported by McGuffin et al. (1995) is a study in which twins raised apart are compared for a certain trait, disease, or characteristic. Again, if twins raised apart, in totally different environments, tend to exhibit the same characteristics ...