In the 1800s, the Philadelphia physician Samuel Morton measured skulls and ranked races by brain size in Crania Americana, published in 1839. Others, such as Josiah Nott, argued that races had separate origins. Some physicians claimed Black people felt less pain, and the surgeon J. Marion Sims experimented on enslaved women without anesthesia. In the early 1900s, intelligence testing was used to argue for immigration limits and sterilization.
The ideas persisted after World War II. The 1994 book The Bell Curve argued that racial gaps in IQ scores were partly genetic, a claim many scientists rejected. Medicine also built race into tools, such as a kidney function test that adjusted results for Black patients. Major kidney groups recommended removing race from that test in 2021.
The Human Genome Project found that humans share about 99.9 percent of their DNA. Scientific racism still appears in online white supremacist movements and in some medical practices that treat race as biology.