Why costly help tends to go to close kin
When helping is expensive, the helper needs a strong fitness return to make it worthwhile. Close kin offer that return because aid to a relative can raise the helper's inclusive fitness—the indirect genetic benefit gained by helping someone who shares genes. In a study of Mosuo villages, people helped kin in need but not needy non-kin, and they traveled farther to help kin [3]. That pattern is consistent with costly cooperation being underwritten by kinship.
The same logic appears in decision rules for altruism: people are more willing to do costly things for close kin than for distant kin or non-kin, and the biological importance of the decision shifts how strongly kinship matters [4]. In cooperatively breeding cichlids, helpers that were not kin compensated for the lack of indirect fitness benefits in other ways, and the care they provided involved temporary fitness costs that resemble an exchange [5]. Together these findings suggest that when help is costly, kinship is the more reliable route to a fitness payoff.
Non-kin help depends on getting something back
Between non-kin, the fitness return from helping is less automatic. Reciprocal exchange—helping now in expectation of help later—is one way non-kin helping can be viable. A model of reciprocal exchange found that exchange does occur in small networks based on kinship, but it is also viable in fitness terms for one individual to help another between non-kin in a notional population [1]. That suggests reciprocity can make non-kin helping pay, at least under some conditions.
Vampire bats offer a concrete example. Food sharing among close kin is expected because it can raise inclusive fitness, but non-kin dyads also appeared to exchange food reciprocally, and sharing with non-kin can expand a donor's network [2]. The fitness benefit of helping non-kin therefore hinges on the relationship being reciprocal rather than on shared genes.
How cost shifts the balance between kin and non-kin help
The cost of help changes which strategy is favored. When help is cheap, helping non-kin can be a low-risk way to build reciprocal relationships and widen a social network, as the vampire bat findings suggest [2]. When help is expensive, the helper needs a more dependable return, and kinship provides one through inclusive fitness [3][4].
In the cichlid study, non-kin helpers faced temporary fitness costs and their care resembled an exchange, while kin helpers did not need to compensate for missing indirect benefits [5]. This fits a broader pattern: costly help is more tightly linked to kinship, while non-kin help is more conditional on reciprocity. The fitness consequences of helping therefore are not fixed by kinship alone—they depend on how much the help costs and whether it is returned.
Sources used in this answer
Social markers and the evolution of reciprocal exchange
Reciprocal exchange occurs in small kin-based networks, but it can also be fitness-viable for one individual to help another between non-kin in a notional population.
Social benefits of non-kin food sharing by female vampire bats
Vampire bats share food reciprocally among non-kin dyads as well as among close kin, and sharing with non-kin can expand a donor's network.
Kinship underlies costly cooperation in Mosuo villages
In Mosuo villages, people helped kin in need but not needy non-kin, and traveled farther to help kin, consistent with inclusive fitness explanations for costly cooperation.
Some neo-Darwinian decision rules for altruism: Weighing cues for inclusive fitness as a function of the biological importance of the decision.
People are more willing to do costly things for close kin than for distant kin or non-kin, and the biological importance of the decision affects how strongly kinship guides altruism.
Kinship reduces alloparental care in cooperative cichlids where helpers pay-to-stay
In cooperatively breeding cichlids, non-kin helpers compensated for the lack of indirect fitness benefits, and their care involved temporary fitness costs resembling an exchange.