GENETIC CLOCKS AND SOFT EVENTS - A TWIN MODEL FOR PUBERTAL DEVELOPMENT AND OTHER RECALLED SEQUENCES OF DEVELOPMENTAL MILESTONES, TRANSITIONS, OR AGES AT ONSET

We propose and explore a twin model to examine the basis for synchrony that often characterizes different facets of normal development. In s o doing we also present an approach to the analysis of ''soft'' events ; events for which available reports of dates or ages of occurrence ar e unreliable or inconsistent. Discrepancies among reports are accounte d for by a statistical measurement model. This combines current status error reflecting uncertain definition of onset and two mechanisms for the phenomenon of ''telescoping,'' namely, systematic compression of the time scale and heteroscedastic random measurement error. Statistic ally, the model can be viewed as a mixed generalized linear model with random effects within both mean and variance functions or, alternativ ely, as involving multiplicative random effects. We apply the model to multiple maternal reports on menarche and onset of breast development in twin daughters. Fitted to data from the Virginia Twin Study Of Ado lescent and Behavioral Development by the use of penalized/predictive quasi-likelihood, the model provided much improved estimates of the tr ue age-at-onset distribution as compared to those from a naive analysi s. Results suggested that the observed variance was made up almost ent irely of genetic variance and measurement error variance due to telesc oping and current status errors and that the timing of breast developm ent and menarche are largely under the control of a common set of gene s. Results also indicated that maternal recollections of the onset of breast development were both more poorly defined and subject to greate r recall errors than maternal recollections of menarche.