American Society of Naturalists

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“Kin blackmail as a coercive route to altruism”

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Patrick Kennedy and Andrew N. Radford (Feb 2021)

Blackmail can lead to altruism evolving at payoff values that would otherwise be against Hamilton’s rule

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Two <i>Polistes</i> wasps cooperating to build a nest on a banana frond in Ghana.<br />(Photo: Patrick Kennedy)
Two Polistes wasps cooperating to build a nest on a banana frond in Ghana.
(Photo: Patrick Kennedy)

Raising kids can be tough. Sometimes, you need all the help you can get. But what can you do if your family aren’t interested? In this study, researchers at the University of Bristol suggest that animals might be able to blackmail relatives into becoming helpers.

Recently, there has been renewed interest in how manipulation could drive the evolution of ‘altruism’ (personal sacrifices to help others) among animals. However, the focus has been on recipients limiting options for potential helpers, such as by impairing their abilities to reproduce. A more Machiavellian manipulation may also be possible: a recipient may deliberately limit its own options, holding its own fitness hostage to helpers’ behaviors.

In a Darwinian sense, relatives are valuable to individuals to the extent that they are vehicles for shared genes. Whether helping evolves depends on a simple cost–benefit calculation known as ‘Hamilton’s rule’: help if it will lead to a net increase in copies of your genes in the population. However, one aspect of Hamilton’s rule has been overlooked: individuals might increase their value to kin by ensuring that shared genes will be in jeopardy if reluctant relatives fail to help. The authors use kin selection models to extend the theory of ‘blackmail’ (first suggested four decades ago by Israeli biologist Amotz Zahavi) to the evolution of altruism beyond parental care.

Shrewd use of energy reserves may be a promising focus to uncover blackmail empirically. For instance, if two tasks are required for the kids to survive, a mother could lay a large clutch but invest most of her energy in only one task. She has tied her hands: unless relatives step in to complete the other task, the babies will not leave the nest alive.

Blackmailed altruism might often give the illusion of harmonious cooperation, since helpers would appear to be making completely voluntary choices to help. In reality, a sly form of coercion would underlie the apparent harmony: make your family an offer they simply can’t refuse.


The evolution of altruism (helping a recipient at personal cost) often involves conflicts of interest. Recipients frequently prefer greater altruism than actors are prepared to provide. Coercion by recipients normally involves limiting an actor’s options. Here, we consider the possibility of a coercive recipient limiting its own options. Forty years ago, Amotz Zahavi suggested that nesting birds may be ‘blackmailed’ into increased parental care if offspring threaten to harm themselves (and so jeopardize the direct fitness of their parents). In a simple kin selection model, we expand blackmail to indirect fitness, and highlight that blackmail can occur between any kin to drive reproductive division of labor. In principle, a recipient may place its own fitness at risk (brinkmanship), imposing sanctions on a relative’s indirect fitness if the relative fails to cooperate. To use its own survival or reproduction as leverage in a sequential game, a recipient must increase the extent to which its existing fitness depends on the actor’s behavior and so credibly commit to a cost if the actor does not comply. As it requires opportunities for commitment, kin blackmail can arise only under stringent conditions, but existing kin blackmailers may pass unnoticed due to their strategic success.