Hamiltonian spite

Females of Wattled Jacana often carry out infanticide.

Within the field of social evolution, Hamiltonian spite is a term for those behaviours that occur between (among) conspecifics, and have a cost for the actor and a negative impact (harm) for the recipient(s). Why do animals and humans exhibit spiteful behaviours?

Theories on altruism and spitefulness

W. D. Hamilton published an influential paper on altruism in 1964 to explain why genetic kin tend to help each other. He argued that genetically related individuals are likely to carry the copies of the same alleles, thus helping kins may ensure that the copies of the actors' alleles pass into next generations.

While this became a widely accepted and highly popular idea ever since then, however, few scientists noticed that he also published a second paper that modifies this view, or at least adds further important points to it. This paper argues that by measuring the genetic relatedness between any two (randomly chosen) individuals of a population several times, we can identify an average level of relatedness. Theoretical models predict that (1) it is adaptive for an individual to be altruistic to any other individuals that are more closely related to it than this average level, and also that (2) it is adaptive for an individual to be spiteful against any other individuals that are less closely related to it than this average level. The indirect adaptive benefits of such acts can surpass certain costs of the act (either helpful or harmful) itself. Hamilton mentioned birds and fishes exhibiting infanticide (more specifically: ovicide) as examples for such behaviours.

Briefly, an individual can increase the chance of its genetic alleles to be passed to the next generations either by helping those that are more closely related, or by harming those that are less closely related than relationship by chance.

Doubts about the adaptive nature of spiteful behaviour

Though altruism and spitefulness appear to be the sides of the same coin, however, there are good reasons why the latter one appears to be a less accepted idea among evolutionary biologists.

First, unlike in the case of an altruist act, target individuals of aggression are likely to act in revenge: bites will provoke bites. Thus harming non-kins may be more costly than helping kins.

Second, presuming a panmictic population, it is easy to see that the vast majority of pairs of individuals exhibit a roughly average level relatedness. For a given individual, the majority of others do not worth to help or harm. While it is easy to identify the few most closely related ones (see: kin recognition), it is also hard to identify the few genetically most distant ones.

Most terrestrial vertebrates exhibit a certain degree of site fidelity, thus levels of kinship tend to correlate negatively with spatial distance. On one hand, this may provide some cues for them to identify the least related non-kin individuals. On the other hand, this may ensure that non-kins rarely if ever meet each other.

Spiteful behaviour in animals

Infanticide

Many animal species exhibit infanticide, i.e. adults tend to kill the eggs or the offspring of conspecifics, even if they do not feed on them (in the absence of cannibalism). This form of spitefulness is relatively free from the threat of revenge – provided that the parents and relatives of the target are either weak or far away.

When infected by tuberculosis, badgers emigrate from their natal group.

Aggression by means of pathogens

An individual carrying a long-lasting infection of virulent pathogens may benefit from (1) channelling the flow of pathogens from its own body away from its kins and (2) directing them toward non-kin conspecifics. The adaptive nature of this behaviour has been supported by the analysis of theoretical models and also by the analyses of the behavioural repertoire of different animal species. Thus, tuberculosis-infected European badgers and rabies-infected dogs equally tend to emigrate from their natal ranges before starting to distribute the pathogens. Similarly, wild herds of Asian elephants tend to defecate into drinkwater holes apparently to keep rival herds away..

Wartime infanticide is a characteristic aspect of human aggression.

Spiteful behaviour in humans

Wartime infanticide

Throughout human history, war often emerges as a costly form of aggression typically targeting the non-kin enemy. Naturally, most wars appear to be motivated by potential benefits other than genetical. Nevertheless, the widespread occurrence of rape and infanticide during periods of war indicates Hamiltonian elements as well.

Economy

It was recently suggested that spiteful motivations may also play a role in economic decisions.

Aggression by means of pathogens

Moreover, another paper argues that the motivation for biological aggression – i.e. using pathogens as weapons – is an inherent and common aspect of the human behavioural repertoire. This argument contradicts the widespread belief that biological weapons are rare and aberrant tools of warfare and it is supported by the following observations:

• the use of pathogen weapons predates the scientific understanding of infections and epidemics;
• attempts of biological aggression are easy to hide and to overlook, thus it appears to be underrepresented in written history;
• it is a frequent component of our expectations concerning the likely behaviour of an enemy;
• it is also a frequent component of threats (see e.g. white powder letters);
• several languages appear to bear linguistic references to our motivation for biological aggression in profanity.
• given that wartime epidemics tend to kill people at a rate exceeding (or at least comparable to) that of traditional weapons, all wars fought in recorded history incorporated an implicit element of aggression by biological means.