Collections and analyses of linguistic data by philologists, among others, demonstrated that while some cultures differentiated "lineal kin" (those in a direct parent-child relationship) from "collateral kin" (such as cousins, aunts, and uncles), others did not.
These findings have been attributed to direct family provisioning (i.e. kin selection, [3]), and to hunters being preferred mating partners due to their high social status, through which they might be signaling their good genes (i.e. costly signaling theory, [4], [5]).
The helping traits are preserved and proliferate through relatives, not direct offspring: kin selection.
We possess greed, and also "limited generosity" — dispositions to kindness and liberality which are more powerfully directed toward kin and friends and less aroused by strangers.
A classic solution to this problem is kin-selection [6], [7], [8]; here cooperation can be favoured if the benefits of cooperative acts are directed towards kin with whom the cooperator shares genes.
In many taxa, cooperation is preferentially directed towards kin or members of the same social group because these individuals have a higher probability of sharing cooperative genes and/or reciprocating the behaviour in the future.
