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Epidemiological studies using twins can elucidate the degree of genetic relatedness among, and environmental contributions to, diverse disorders (e.g., for internalizing and externalizing disorders). These researchers analyzed interview data from a Norwegian twin study (669 monozygotic pairs, 377 dizygotic pairs, and 19 individual twins) to search for overarching factors signifying correlations of genetic or environmental risks among 12 axis I disorders and all 10 axis II disorders. Both syndromal and subsyndromal illnesses were investigated.
For axis II assessments, investigators used participants' endorsed illness criteria and determined whether behaviors, cognitions, and feelings predominated in the previous 5 years. Analyses included com…