What is Sharon Street's "evolutionary debunking argument," and how does it challenge moral realism?

Sharon Street, a moral philosopher at New York University, formulated in her most famous paper "A Darwinian Dilemma for Realist Theories of Value" (2006) one of the strongest contemporary challenges to moral realism. Her argument—known as the "evolutionary debunking argument"—uses evolutionary theory to undermine the idea that there are objective moral facts independent of humans. The argument is clever and influential, and the responses from realists reveal the depth of contemporary debate about the foundations of ethics.

Inadequate Responses to Avoid

From some defenders of moral realism:

"Street is just a relativist trying to destroy ethics." This mischaracterizes her position. Street is not a naive moral relativist, but a serious academic philosopher raising a precise epistemological challenge. Her argument does not deny the existence of moral standards, but questions their nature and epistemological foundation.

"Evolution has nothing to do with philosophical ethics." This is a methodological error. If our cognitive capacities—including moral intuitions—are products of evolution, then evolution has a direct impact on moral epistemology. Ignoring this influence is evasion of the question.

From some opponents of realism:

"Street has proven that morality is an evolutionary illusion." This is an exaggeration. Street's argument challenges moral realism, but it does not prove that morality is an "illusion." She argues that moral realism faces a serious epistemological dilemma, but there are other philosophical alternatives (constructivism, expressivism, etc.).

"Evolution explains everything moral." This is excessive reductionism. Even Street herself does not claim that evolution explains the detailed content of our moral judgments, but rather explains our general moral tendencies. The leap from "evolution shaped our intuitions" to "evolution determines every moral judgment" is unjustified.

Why These Responses Are Inadequate

They fail to understand the precise structure of Street's argument. The argument is neither a nihilistic attack on morality nor a naive defense of relativism, but a specific epistemological challenge to moral realism that requires careful analysis.

Structure of Street's Argument

The argument begins with a scientific observation: evolution by natural selection shaped our evaluative attitudes. We tend to:
- Care for our children
- Help our relatives
- Cooperate with those who cooperate with us
- Punish cheaters
- Prefer fairness in distribution

These tendencies have a clear evolutionary explanation: organisms that developed these tendencies were more successful in survival and reproduction.

Now Street poses the dilemma for the moral realist:

Option One: Denying the Influence
The realist could claim that evolution did not affect the content of our moral intuitions. But this is scientifically implausible. The evidence from evolutionary psychology and anthropology is strong: our moral tendencies bear clear marks of evolutionary pressures.

Option Two: Accepting the Influence
If the realist accepts that evolution influenced our intuitions, they face a deeper problem. Evolution "cares" about survival and reproduction, not moral truth. There is no reason to believe that evolutionary pressures directed us toward independent moral facts.

Street's analogy: imagine you are sailing in the ocean with a compass. You discover that an external force (evolution) has pushed you in directions random with respect to your intended destination (moral truth). What is the probability that you will reach the correct island? Very slim.

The Devastating Conclusion
If our moral intuitions are the product of evolutionary forces unrelated to moral truth, we are in a serious skeptical position. Even if there were independent moral facts, we have no reliable epistemological path to them. Moral realism becomes an epistemologically unjustifiable position.

Responses from Realists

First Response: "Lucky Coincidence" (David Enoch)
David Enoch in "Taking Morality Seriously" (2011) accepts the challenge but argues that perhaps—by mere coincidence—evolution directed us toward some moral truths. Survival sometimes requires knowing moral facts (e.g., genuine cooperation is better than pretending to cooperate).

Problem with this response: it relies on an implausible cosmic coincidence. Why should the requirements of survival match independent moral facts?

Second Response: "Third Factor Explanation" (Derek Parfit)
Parfit suggests that both evolution and moral facts might be connected by a third factor. For example, both might reflect a deeper rational structure in the universe. This explains the match without assuming coincidence.

Problem with this response: it introduces heavy metaphysical assumptions (cosmic rational structure) to solve an epistemological problem.

Third Response: "Modest Realism" (Ralph Wedgwood)
Wedgwood argues that evolution directed us toward general rational capacities, and these capacities enable us to discover moral facts. Evolution did not directly determine the content of our judgments, but gave us tools for moral thinking.

Street responds: even our rational capacities themselves were shaped by evolution for survival purposes, not for discovering abstract moral facts.

Non-Realist Alternatives

Street herself adopts "Humean Constructivism": moral facts are not independent of us, but constructed from our evaluative attitudes. This solves the epistemological problem because it does not assume moral facts "out there" to be discovered.

Others adopt Expressivism: moral judgments do not describe facts but express attitudes. This also avoids the epistemological problem.

Application to Debates about Religion

Some philosophers of religion (like Alvin Plantinga) use a modified version of Street's argument against naturalism itself: if all our cognitive capacities are products of blind evolution, why trust them in any domain, including science? Theism—by assuming a rational creator who directed evolution—solves this problem.

Opponents respond: this assumes God directed evolution toward truth, which is an additional assumption requiring justification.

Where We Stand in the Debate Today

Street's argument remains one of the strongest challenges to moral realism. Realist responses are varied but there is no consensus on the success of any of them. Many philosophers see the argument as pushing toward rethinking the nature of morality itself.

In the context of our project (rajḥān ʿaqlī), the argument shows that all parties—naturalistic or theistic—face epistemological challenges. The choice is not between a "proven" and a "doubtful" position, but between different positions that all pay philosophical costs. Wisdom lies in fairly evaluating these costs.

For Advanced Reading

- Advanced level: Constructive responses to the evolutionary debunking objection
- Advanced level: Plantinga's reverse use of the argument against naturalism
- Sharon Street, "A Darwinian Dilemma for Realist Theories of Value" (Philosophical Studies, 2006)
- David Enoch, Taking Morality Seriously (Oxford UP, 2011)
- Richard Joyce, The Evolution of Morality (MIT Press, 2006)
- Michael Ruse & Robert J. Richards, eds., The Cambridge Handbook of Evolutionary Ethics (2017)
- "Formulation: Evolutionary Debunking Arguments" page on the website