The genetic component of discrete disability traits: an analysis using liability models with age-dependent thresholds

The presence of familial and genetic effects in the Activities-of-Daily-Life (ADL) data collected in the first wave of the 1995 Longitudinal Study of Aging of Danish Twins (LSADT) older than 75 is tested using multithreshold liability models of disability with age-dependent thresholds. These models are developed for discrete scores represented by five disability scales of male and female Danish twins. The presence of familial effects is revealed in all five scales of disability data for females and in three scales of data for males. Genetic effects are found to be significant in all four levels of aggregation of the Upper Limb-T (T = tiredness) disability scale for females and in the PADL-H (H = need for help) scale for males. Genetic effects are also pronounced in the Mobility-T scale for females and in the Lower Limb-T scale for males and females. For females, the genetic effects in the T-scale seem to be more pronounced than in the H-scale. For males, genetic effects are more pronounced in the H-scale. The estimates for MZ correlations in liability tend to be higher than the estimates for DZ correlations in almost all cases, which suggests that additional genetic effects may be revealed should the sample size of the ADL data be increased.