As an example, consider how oxytocin contributes to the explanation of parental care as one of the 20 natural desires that I first listed in Darwinian Natural Right, published in 1998. In my chapter on "Parent and Child," I agreed with biological anthropologists like Aristotle, Charles Darwin, and Sarah Hrdy who argue that the social nature of human beings is a natural extension of the maternal care of children. The maternal love for children is the model for, and natural origin of, all forms of love, friendship, and affiliative behavior.
The Darwinian biology of animal behavior can support this conclusion by explaining parental care at four levels of analysis: (1) functional causes, (2) phylogenetic causes, (3) developmental causes, and (4) immediate causes. Human parental care has a functional cause in that it has been favored by natural selection as serving the survival and reproductive success of human beings. It has a phylogenetic cause in that it has emerged from an evolutionary history that human beings share with other primates and other mammals. It has a developmental cause in that the desire to care for children emerges as an outcome of the normal development of children into adults. And, finally, it has immediate causes in that it arises as a response to physical and social stimuli in the environment that elicit processing in the neural circuitry in the brain that produces parental caregiving behavior.
A full explanation of parental care--and of all the other natural desires--would have to move through these four levels, with each level requiring a complex explanation. Although the complexity here is so great that we will probably never achieve full scientific explanations, we can still make progress.
So in my chapter on parental care, I spoke of oxytocin and vasopressin as part of the immediate causes of parental care. At least in prairie voles, oxytocin promotes the female's care of her offspring and bonding to her mate, while vasopressin promotes the male's paternal care and pair-bonding.
But now, 20 years after the publication of Darwinian Natural Right, we know much more about the biological causality of parental care, in which oxytocin and vasopressin act only through complex interactions with many other factors. Last year, a team led by Catherine Dulac at Harvard University published the first description of the neural circuitry in the brains of mice supporting parental care (Kohl et al. 2018; Kohl 2018). A summary of their study can be found here.