Math says that being a jerk to your own family can actually be a smart move for survival if you're in a group with few kids.
March 20, 2026
Original Paper
Permissible Spite: Kin Selection, Demography, and the Inverse Hamiltonian Equation
EcoEvoRxiv · 10.32942/X2D95B
The Takeaway
Biologists have long used Hamilton's Rule to explain altruism—why animals help relatives who share their genes. This paper proposes an 'inverse' equation showing that when birth rates are low, the same evolutionary logic that usually encourages kindness actually makes aggressive, spiteful behavior toward close kin a stable and permissible trait for survival.
From the abstract
This article revisits Hamilton’s rule by proposing an inverted formulation to evaluate the evolutionary permissibility of spiteful behavior within kin-based populations. We formalize a reverse Hamiltonian equation and apply replicator dynamics to investigate the demographic and genetic conditions under which within group aggression may become evolutionarily stable. The model shows that in low-fertility populations, even close kin may be treated as expendable, while high fertility expands the ran