What is the source of morality? There are many proposed sources of morality. Here are eight, with links to peer-reviewed resources where you can find out more about each proposed source of morality.
What? Science as a source of morality? But, but—calm yourself.
Morality may have begun as something unscientific—e.g., a way to deal with failures to cooperate as the human population grew and expanded into just about every habitable portion of the globe. However, now we can use the tools of science to determine the most reliable paths to the moral outcomes that we prefer.
What decisions, policies, etc. tend to promote cooperation and minimize violence? That’s an empirical question. And science offers the most reliable methods for answering empirical questions. At least, that’s the kind of claim you will expect from those who adopt a naturalist view of ethics. To learn more about naturalist ethics, see The Ethical Project and How Science Can Determine Human Values.
Evolution is invoked to explain many things. One of them is human morality.
An evolutionary psychological view of morality holds that ethical systems, judgments, and norms are products (or byproducts) of various forms of evolutionary selection. After all, disapproving of certain practices (e.g., killing children) and approving of other practices (e.g., cleanliness) may become more or less common depending on how they impact reproduction rates. See Morality and Evolutionary Biology for more about this.
You may be aware that much of what we consider normal has not been considered normal at other points in history and/or in other parts of the world. One source of these differences in norms is culture. Cultures change over time and differ between regions.
So cultural differences may result in differences in moral systems, judgments, and norms. For example, some cultures consider it wrong to eat certain animals that other cultures not only eat, but often take pride in eating. There are even emerging differences in whether it is ever right to eat animals. See The Culture of Morality: Social Development, Context, and Conflict for more about the relationship between culture and morality.
Much of human history involves fights to obtain and protect power. The powerful often want to maintain their power and the powerless often want to overthrow abusers of power. These power dynamics may influence the development of morality. For example, norms about property, currency, labor, and autonomy can serve as limits on how power can be wielded over others. And many of these norms seem to feature not just in politics, but in moral systems as well. For more about this, see Conflict and the evolution of social control.
Our emotional responses can condition us into liking some things and disliking others. For example, when parents reward certain behavior and punish others, their children may become conditioned to have positive feelings about the rewarded behavior and negative feelings about the punished behavior. These positive and negative feelings may influence our evaluations of what we (and others) ought and ought not to do.
Learning moral norms may also work via some sort of emotional conditioning. If parents, authorities, or society consistently shame people for certain behaviors, then we may feel disinclined to behave that way. And we may be conditioned to prioritize behaviors that parents, authorities, and society constantly praise. We may also join in these praise and blame habits. See Morality is a Conditioned Response or the book-length treatment of this topic, The Emotional Construction of Morality, for more about how emotion may be a source for morality.
Religions often involve norms that are supposed to govern behavior. Some of these norms may be construed as moral norms. For example, some of the 10 commandments seem to be not just religious norms (e.g., about God, religious days, etc.) but moral norms (e.g., about not killing or stealing). For more about the alleged relationships between religion and morality see “Morality and religion” and Chapter 4 of What Ever Happened To Good And Evil?.
7. Non-natural or Supernatural
Of course, not all religions involve gods. And not all supernatural beliefs are religious. For example, one can believe in spirits or magic without believing in a god or being committed to a religion.
Some supernaturalist beliefs can inform morality. For example, you might think that what is true and good is transcendent in way that makes it inaccessible by ordinary experience. On this view of truth and goodness, what makes 2 + 2 equal to 4 is not something we discover by implying observation and the tools of science, but by accessing something beyond the realm of experience. Similarly, one may think that what makes stealing wrong is not something that can be discovered via science. Rather, stealing is determined to be wrong by something transcendent or supernatural that science is unable to study. See this encyclopedia entry on Moral Non-Naturalism for more about this.
Pluralism is an appeal to more than one thing—e.g., authorities, principles, etc. So pluralism about the sources of morality would hold that there are multiple sources of morality. Indeed, some have proposed that more than one of sources above accounts for the origins of morality. For an example of this, see Humean Moral Pluralism.