It is often claimed that the exceptional severity of the Spanish flu, one of the most deadly events in recorded human history, is an unsolved mystery. However, even detailed aspects such as its W-shaped mortality curve are well explained by Paul Ewald’s theory of the evolution of virulence. Understanding the causes of the Spanish flu will help to prevent future epidemics.

This article is divided into two main sections. The first discusses “Female Inheritance and the Male Retention Hypothesis.” Permanent groups exist in several species because over generations members share important interests. Considering the association between cooperation and degree of relatedness, it seems to follow that a collective interest is more likely to be achieved when members show a higher degree of relatedness. I argue that if membership is inherited by only one sex, and this is the female sex, this results (...) in a higher degree of relatedness between group members than when membership is inherited by both sexes, or by males only. Indeed, this is found in the overwhelming majority of species of insects, fish, birds, and mammals living in permanent groups. The exceptions to the rule are briefly discussed. Humans are of special interest because human preindustrial societies tend to show either male or female inheritance. The second section asks, “Do Moralizing Gods Raise Paternity Confidence?” Since males inherit valuable membership in patrilocal/lineal societies, they are expected to be more concerned about the probability of paternity than males in matrilocal/lineal societies. Moral rules, and specifically belief in moralizing gods, are expected to reflect this difference. An analysis of cross-cultural data of preindustrial societies does not refute the hypothesis that moralizing gods are more often found in patrilocal/lineal societies, nor is this hypothesis unambiguously supported. (shrink)

This commentator has no argument with the explanations given by Soltis. Yet a different approach to the phenomenon of crying might be fruitful. Neonates elicit care. It is hypothesized that non-neonates evolved to mimic, when in need, the appearance of the neonate by crying and by shedding tears, thus inducing helping behavior by the spectator.

Ideas formulated by Paul Ewald about the “evolution of virulence” are used to explain why bats, more often than other mammals, are a reservoir of virulent viruses, and why many of these viruses severely affect other mammals, including humans, but are apparently less pathogenic for bats. Potential factors contributing to bat viruses often being zoonotic are briefly discussed.