Short Answer
Are people naturally cooperative or selfish?
Or both? Or both! But cooperation is not a question. No one disputes humans are a social animal. What you be better off asking after is 'are people naturally altruistic or selfish or both? Both! Or maybe you might be asking 'are people naturally cooperative or antagonistic or both? Both! Let's explore.
Long Answer
The Philosophy of Human Nature
In search for some optimism, who would be Machiavelli's opposite? How would one convince some one like Machiavelli to believe in the good in people?
First, Machiavelli wasn't engaged in moral philosophy, but political philosophy. Today, such politics has become known as realpolitik. In a way, Machiavelli was a pragmatist, because republics (in his case the one of Florentine) have always struggled to find a balance between cooperation and disagreement. The US republic innovated by birthing itself as a constitutional republic free of monarchs who used to intervene in affairs. But he was interested in human nature, so on to the broader question.
Human nature is quite a broad philosophical question, and what it means to be altruistic and selfish are of course likely to draw out strong beliefs of distinction. And, there's no shortage of philosophical thinking on this topic. Let's take a quick survey of some philosophical Q&A from Plato, Machiavelli, Marx, and modern cognitive science to preface a brief introduction.
What's important to understand is that understanding human nature is a complex project. Even in old religious texts from thousands of years ago, arguably very fundamentalist, morally absolutist guides to human nature, people are observed as both. A quick tour of the swath of thinking philosophy has to offer:
Plato
Read Are people inherently good according to Plato?
Machiavelli
Read How much ethics did Machiavelli convey in The Prince?
Marx
Read The Effects of Marx's Moral Character on Communism
Cognitive Science
Read Are there any prominent modern philosophers with a background in neuroscience?
A Naturalized Epistemology as a Guide to Human Nature
Philosophy draws inspiration from many places. Certainly the great philosophical thinkers through history have outlined the complexity of human behavior and relations. What might be most relevant as we move into the twenty-first century is the contributions of a naturalized epistemology. Sociobiology, evolutionary psychology, and cognitive science have a lot to say about action (SEP) and human morality (SEP).
Whereas there was the brain in Ancient Greece, now neurologists have outlined distinct pathways leading to philosophical ideas like NCCs. This means that while humans are experiencing actions and thoughts, scientists in a rough way can see what parts of the brain involved. This has been very impactful in the philosophy of language where Wernicke's and Broca's Areas are implicated in semantics and syntax two notions highly important in philosophy, particularly since the linguistic turn.
Two important works on human nature for the philosophically inclined that come out of contemporary science are Sociobiology: The New Synthesis and The Blank Slate: The Modern Denial of Human Nature. The first goes back a ways, and tries to understand the biological origins of eusociality. Most recently, Tomasello has explored such themes in his works such as Why We Cooperate. Why is it that if genes and individuals are supposed to be biologically selfish for survival, we see such cooperation in the animal kingdom? Then answer is complicated in technical terms, but amounts to the notion that a rising tide raises all ships. Related organisms share genes, so if one organism helps kin, in a way it helps itself. Of course, explanations like these have controversial teleological overtones (SEP).
In the other direction and in line with Pinker, modern social justice advocates may overestimate human cooperation with folk psychology. Communism and libertarianism are both largely failures as a product of political philosophy when it comes to actual political systems in practice. Neither has been successfully implemented. Of course, both have many advocates, but this may be more as a new religious movement, than anything else. Communism overestimates human beings willingness to cooperate, a fact notoriously lampooned by George Orwell in Animal Farm, and libertarian economic policies in the US have repeatedly met poor outcomes; its influence on other sectors of the economy are noticeable, where US health care consistently ranks as the worst health care in the developed world based on cost and clinical outcome. It seems humans need a certain level of government to get along in groups.
