Iatrogenic influences on the heritability of childhood tonsillectomy: cohort differences in twin concordance.

In 1980-82, a mailed questionnaire was completed by 3,810 pairs of adult twins enrolled on the Australian NH&MRC Twin Register. Twins were asked whether they had had their tonsils out and, if so, at what age. The sample was divided into four birth cohorts of approximately equal size, and only childhood tonsillectomy (to the age of 18) was considered. The prevalence of tonsillectomy differed markedly between cohorts, being highest in those born in the 1940s and early 1950s. Within each cohort, the prevalence was very similar in MZ and DZ twins, yet concordance was much higher in MZ twins, indicating the importance of genetic factors in predisposition to tonsillectomy. However, the proportions of variance in liability due to genetic and shared environmental factors differed markedly between cohorts. In the 1950s, when tonsillectomy was fashionable, shared environment accounted for 60% of variance and genetic factors for only 29%. However, by the early 1960s, when tonsillectomy was going out of fashion, heritability was up to 0.82 and shared environment accounted for only 10% of variance. Our results illustrate, once again, that heritability is not a constant, but depends on the precise characteristics of the population and the time at which it is studied.