A joint explanation of infant and old age mortality

TL;DR

This paper proposes a unified biological mechanism explaining both infant and old age mortality, linking their distinct age-specific death rates to the severity and spread of anomalies affecting organisms.

Contribution

It introduces a theoretical framework that accounts for the contrasting mortality patterns in infancy and old age within a single explanatory model.

Findings

The same biological effects can explain both declining and increasing death rates.

Severity of death effects correlates with organism complexity and number of organs.

The model suggests larger organisms may experience higher severity in mortality effects.

Abstract

Infant deaths and old age deaths are very different. The former are mostly due to severe congenital malformations of one or a small number of specific organs. On the contrary, old age deaths are largely the outcome of a long process of deterioration which starts in the 20s and affects almost all organs. In terms of age-specific death rates, there is also a clear distinction: the infant death rate falls off with age, whereas the adult and old age death rate increases exponentially with age in conformity with Gompertz's law. Clearly, it would be satisfactory to explain the two phenomena as being two variants within the same explanatory framework. In other words, a mechanism providing a combined explanation for the two forms of mortality would be welcome. This is the purpose of the present paper. We show here that the same biological effects can account for the two cases provided there is a difference in their severity: death triggered by isolated lethal anomalies in one case and widespread wear-out anomalies in the second. We show that quite generally this mechanism leads indeed, respectively, to a declining and an upgoing death rate. Moreover, this theoretical framework leads to the conjecture that the severity of the death effects, whether in infancy or old age, are higher for organisms which are comprised of a larger number of organs. Finally, let us observe that the main focus of the paper is the drastic difference of the age-specific death rates (i.e. decreasing versus increasing) because this difference is found in many species, whereas the question of the best fit (e.g. Gompertz versus Weibull) is rather specific to human mortality.