The psychology of morality can be viewed in terms of an evolutionary perspective. Evolution has conditioned us to behave morally, because it has a few beneficial products worth putting on the table. First, it aids in our survival. Over 98% of all species that have ever been on this planet have gone extinct, and I attribute some of our success as a species to our ability to form tribes, work in groups, to love one another, and to act morally. Another positive aspect is that being moral increases our own happiness. The website Digital Journal ponders over a study that claims having no friends has similar effects to smoking 15 cigarettes daily. I would consider moral behavior towards people as an essential component of friendship. One can also look at morality from a utilitarian perspective, which is to say “you scratch my back, I scratch yours.” This is observable in other species besides homo sapiens. Egyptian plovers have been known to fly into the mouths of crocodiles in order to eat the food stuck in between the reptile’s teeth. Instead of eating the bird, the crocodile allows the bird to eat, as he gets his teeth cleaned in the process. So morality can be a means to an end, but heartwarming no less.
The best philosopher I know on the subject of morality is the author Sam Harris. In Sam’s book, The Moral Landscape, he attempts to describe how morality relates to changes in conscious creatures. He uses the example of a universe only constituting rocks. Since rocks are not conscious, there cannot be anything known as a moral decision. If rocks cannot feel joy or pain, there is no basis for morality. So conscious life is needed for morality. But who’s to say which conscious creatures deserves the more attention than others? Harris argues that the complexity of the creature is the answer. The more intricate a creature is in terms of a brain and nervous system, the more pain or happiness it can feel. The larger the delta between the best possible scenario and the worst possible misery, the more careful it should be treated. This would explain why we kill flies without hesitation but are more distressed about killing mammals.