  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.
