In-group bias (in-group favoritism)

Biological Parallel

Naked mole rats in a colony share 75-80% of their DNA, creating genetic incentives to favor colony-mates over strangers. Worker mole rats will die defending their queen and siblings because they're defending copies of their own genes—inclusive fitness makes nepotism mathematically rational. In-group bias evolved through kin selection: organisms that preferentially helped genetic relatives propagated more copies of the helping-kin genes, making in-group favoritism an evolutionary inevitability whenever groups correlate with kinship.