MEASURING THE SIMILARITIES BETWEEN THE LIFETIMES OF ADULT DANISH TWINS BORN BETWEEN 1881-1930

The survival of like-sex twins born between 1881 and 1930 in Denmark, of which both were alive by the age of 15, is studied by means of models for bivariate survival data, with special reference to the degree of dependence. The 8,985 pairs were followed until 1980. The dependence is assumed to be generated by a common unobserved risk level. Several families of distributions for this level, including positive stable and gamma, are compared. General aspects of such data are discussed, including how to measure the degree of dependence. A simple choice is Kendall's coefficient of concordance, which for monozygotic males and females is about .17 and .15 and for dizygotic .09 and .10. Adjusting for cohort effects by a covariate describing year of birth reduces the dependence measure only slightly. The dependence is so small that a prediction for one twin is only slightly improved if the survival status of the other is known.