HOW SQUARE IS THE SURVIVAL-CURVE OF A GIVEN SPECIES

An investigator-independent parameter, the prolate rectangularity inde x kappa, for describing the so-called rectangularity of biological pop ulation survival curves, is introduced, developed, and applied to real world survival datasets. This new rectangularity parameter is construc ted using an intrinsic time scaling that places the intrinsic inflecti on point time at a value of unity so that species populations may be c ompared independently of their extrinsic life span distributions. The analytical expressions for the prolate rectangularity index of the the oretical Gompertz and Weibull continuous models are obtained, as are n umerical values of this index for discrete experimental population sur vival data sets from two dissimilar species with orders of magnitude d ifference in extrinsic life span range. The values of the parameter ar e also compared for populations of a single species having differing d ietary regimens, and for human demographic populations at decade inter vals in extrinsic chronological time during the current century. It is found that scaling time, using the survival inflection point, appreci ably collapses extrinsic survival profile dispersion among similar pop ulations and allows a more meaningful comparison of profiles among dis similar populations. Using this method of scaling, demographic populat ions within the United States are seen to have rectangularity paramete r values that have been slowly drifting during this century toward val ues indicating a higher degree of rectangularity. In recent decades, h owever, the trend appears to be stabilizing with kappa values indicati ng no approach towards the theoretical maximum rectangularity. This ap parent submaximal stabilization of kappa supports a hypothesis of no g enetically pre-determined maximum life span in human populations. Or, if such a maximum exists, we are not currently near it.