Why spite?

A yellow-rumped caciques. Photo by Thinkstock.

A yellow-​​rumped caciques. Photo by Thinkstock.

Spite, in the words of Mer­riam and Web­ster, is “the desire to hurt, annoy, or offend someone.” But I like this def­i­n­i­tion better: “Spite [is] the shady rel­a­tive of altruism.” If altruism is costly behavior that helps another, spiteful char­ac­ters pay to harm another.

This is how North­eastern assis­tant pro­fessor of phi­los­ophy Rory Smead describes the vengeful behavior in a paper released ear­lier this year in the journal Evo­lu­tion.

Turns out big sis­ters and Draco Malfoy aren’t the only crea­tures that par­tic­i­pate in the activity. Birds do it. Bac­teria do it. Even ado­les­cent yellow-​​rumped caciques do it (young males of the species harass the females and fledg­lings for no apparent reason, costing them­selves time and energy and some­times get­ting them­selves hurt when the girls fight back).

But it’s curious that we observe this behavior in nature: it’s expen­sive (remember the time Malfoy got turned into a ferret for being a jerk?) and it doesn’t seem to have much ben­efit to the perpetrator. “Spite hurts both the actor and the recip­ient, so it would gen­er­ally be better not to be spiteful,” said Smead. So what do behav­iors that cost resources and are appar­ently use­less still doing in ecology after bil­lions of years of selec­tion? Shouldn’t this kind of thing have run its course by now?

The costs here, in evo­lu­tionary biology, are under­stood in terms of fit­ness, or repro­duc­tive poten­tial,” explained Smead. “Having off­spring is impor­tant for evo­lu­tion, but so is having more off­spring than others. The key for the evo­lu­tion of spite is that an organism might be able to sac­ri­fice some of its own fit­ness in order to gain a rel­a­tive advan­tage over others. In other words, spite involves a trade-​​off: lower your own fit­ness, but lower the fit­ness of everyone else more.”

Back in the 1960s a guy named Bill Hamilton fig­ured out that this mech­a­nism would allow spite to evolve in small pop­u­la­tions, where a little bit of spite can go a long way. The idea was dropped because everyone assumed that these small pop­u­la­tions were on their way to extinc­tion anyway, so it didn’t matter much. Those obser­va­tions of spite found in nature were chalked up to misiden­ti­fied cases of selfishness–that’s when harming another indi­vidual costs nothing or even ben­e­fits the actor.

Assistant professor of philosophy and religion Rory Smead studies the evolution of social behavior. Photo by Mary Knox Merrill.

Assis­tant pro­fessor of phi­los­ophy and reli­gion Rory Smead studies the evo­lu­tion of social behavior. Photo by Mary Knox Merrill.

But in their new research, Smead and his col­lab­o­rator Patrick Forber of Tufts Uni­ver­sity, revis­ited Hamilton’s old models and have come to some new con­clu­sions. Instead of looking at static pop­u­la­tions like Hamilton did, the duo looked at dynamic ones, where pop­u­la­tion size and growth rate were con­stantly changing, which is much closer to the real world sce­nario anyway. Now they found that spite could evolve and per­sist even in larger populations.

In the models we’re looking at all that you need is the right set­ting (small enough pop­u­la­tion com­bined with a low cost-​​to-​​harm ratio for spiteful behavior) and a single muta­tion that intro­duces spite will start spreading in the population.”

At a cer­tain point it even starts to be good to be bad, because the spite ends up acting to reg­u­late pop­u­la­tion size, some­thing that can be very impor­tant when resources are lim­ited. So, in fact, spite can actu­ally create the very cir­cum­stances nec­es­sary for it to per­sist, write the authors.