The reverse hygiene hypothesis: the effect of children on the probability of dying from infectious disease

Paul Schweinzer, University of York and Miguel Portela, Universidade de Minho, Braga, Portugal

[Project number 30130]

Children are born without defences against a significant number of infections or diseases. They acquire this immunity through exposure and consequently are often and repeatedly ill in their first years of life. Nursery, Kindergarten or school attendance ensures that no major virus is missed. This protects them in later life. The involved viruses and bacteria adapt and mutate quickly, though, and therefore this early-life immunisation does not last forever.

This project aims to test the hypothesis that a fresh immunisation at the adult stage is obtained through having or being around children. More precisely, exposure to the pathogens which cause the child to acquire its own immunisation have a secondary effect on the parents' which boosts their own immune system. (A similar effect should be observable for individuals in child caring professions.) Thus, given that parents survive the initial exposure, they are better equipped for older age.

Evolutionary speaking, it is beneficial for the gene-pool to duplicate more than once with, perhaps, more than one partner, ie. for any person to produce more than one child. Since this is facilitated through a longer life span, re-immunisation is also beneficial genetically. This effect is probably pronounced through the prolonged life expectancy of recent generations.

We attempt to answer a simple empirical question: Does having children - given that one survives the first three or so years of their lives - make one live longer? The hypothesis we offer is that a parent's immune system is refreshed at a time when their own protection starts wearing thin. With the boosted immune system, the parent has a better chance to fend off whatever infections might strike when old and weak.