Massimo Pigliucci writes in the November - December, 2004 issue of Skeptical Inquirer:
Monogamy isn’t very common in the animal world, but male birds of species that exhibit this behavior, and that stick with the progeny to do their part in parenting, tend to have low levels of the hormone testosterone. This is an interesting correlation that suggests an obvious causal link between hormone levels and behavior. When scientists tested such hypothesis by injecting testosterone in these male birds, their behavior changed dramatically: they abandoned their offspring and went off to court other females.
So what, you may say, we (Homo sapiens) are a heck of a lot more complicated, both physiologically and behaviorally, than birds, and one cannot willy-nilly translate such findings to the cultural complexities of twenty-first century humanity. Except that one can: It turns out that among humans, males with high levels of testosterone are less likely to marry, more likely to engage in extramarital affairs, and divorce more often (they are also more violent, being more likely to assault their spouse). Makes you pause, doesn’t it?
Obviously hormones aren’t the final and ultimate “cause” of behavior like polygamy or monogamy, but it’s an important component. If a “soul” were ultimately responsible, there doesn’t seem to be any way to explain or justify the idea that basic chemical interactions, like those involving hormones, would have any real impact on behavior.
Moreover, there shouldn’t be any evolutionary connection between our hormone-influenced behavior and that of other animals. Our physical bodies may have evolved and thus be related to the physical bodies of other animals, but surely our souls didn’t evolve according to the same natural processes. If our souls are unique, supernatural creations of a supernatural being, then there shouldn’t be any connection between what our souls cause us to do and the behavior of other creatures.
Of course, all of these problems are questions are eliminated if we accept the simpler explanation that there simply aren’t any souls — that our behavior is determined by the same material, biological, and chemical processes which we find in other animals. If souls are nothing more than fantastic stories our ancestors told to each other, with no more reality than fairies and elves, then all of these problems just evaporate into so much mythological nonsense.