# Evolutionary stability and the rarity of grandmothering

## TL;DR

This paper uses life-history and game theory to explore why grandmothering, despite its benefits, is evolutionarily rare due to conflicting thresholds for its emergence and stability.

## Contribution

It introduces simple thresholds for the evolution and stability of grandmothering, revealing a fundamental conflict explaining its rarity in evolution.

## Key findings

- Thresholds for evolution and stability are conflicting.
- Grandmothering evolution is inherently difficult to achieve and maintain.
- The rarity of grandmothering can be explained by these evolutionary constraints.

## Abstract

The provision of intergenerational care, via the Grandmother Hypothesis, has been implicated in the evolution of post-fertile longevity, particularly in humans. However, if grandmothering does provide fitness benefits, a key question is why has it evolved so infrequently? We investigate this question with a combination of life-history and evolutionary game theory. We derive simple eligibility and stability thresholds, both of which must be satisfied if intergenerational care is first to evolve and then to persist in a population. As one threshold becomes easier to fulfill, the other becomes more difficult, revealing a conflict between the two. As such, we suggest that, in fact, we should expect the evolution of grandmothering to be rare.