Daniel Brian Krupp answers
There is an absurd variety of meanings given to the words “altruism” and “spite.” A few years ago, I tried to tidy things up by providing precise definitions for these and other misused terms. Having done that, I and Peter Taylor have since discovered several patterns in the evolution of these behaviors that are equal parts breathtaking and bizarre.
The evolution of altruism and spite depends in part on how likely social partners are to share identical copies of the genes causing the actor’s behavior. The greater the chance that actors and their recipients share copies of the causal gene, the more easily altruism evolves; the lesser the chance that they share copies—and, hence, the more likely they are to bear copies of rival genes—the more easily spite evolves. In many cases, actors determine the likelihood that recipients share their genes through phenotypic resemblance—the similarity in appearance, smell, or sound of the recipient to themselves. For instance, identical twins are on average both more genetically and phenotypically similar than fraternal twins or other full siblings, and these siblings are in turn more similar to one another than are half-siblings.
Taylor and I took this logic a step further by studying the effects of phenotypic similarity not just between actors and recipients, but also between actors and their neighbors. We found:
1. Actors with rare phenotypes are altruistic to recipients with a wide range of phenotypes
2. Conversely, actors with common phenotypes are spiteful to recipients with a wide range of phenotypes
3. At the extreme, actors with common phenotypes can pay costs that are vastly larger than the ones they spitefully impose on their partners.
Together, the first two findings imply that the behavior of individuals can be “asymmetrical,” with one party inflicting more harm and offering less help than the other. The third finding is astonishing, because it suggests that some individuals will go to extraordinary lengths to hurt others. This video illustrates our model and results: