The belief among white nationalist, alt-right supporters that white people are being systematically replaced by non-white groups in society (i.e., the conspiracy theory of “great replacement”) is grounded in their broader beliefs in “race realism”—that is, the idea that racial categories are biologically determined. Here, we probe the common psychological biases that may contribute to the alt-right’s racist ideology. Surveying a national sample of Americans ( N = 1500), we find that alt-right supporters are significantly more likely to support essentialist beliefs—the notion that biology forms the core of who people are—than other Americans. Moreover, results from a two-wave experiment suggest that the alt-right interprets media narratives around behavioral science research in a way that reinforces belief in fundamental genetic differences between social groups. Consistent with expectations, narratives that emphasize deterministic genetic explanations for behavior lead all participants (regardless of alt-right support) to express increased essentialist beliefs. However, for alt-right supporters, socioenvironmental explanations for human behavior also lead to an increase in essentialist beliefs. Moreover, alt-right supporters are more likely to mistakenly interpret socioenvironmental narratives as direct evidence for the genetic basis of behavior. We suggest that essentialist thinking may provide a cognitive gateway to alt-right ideology.