In a Sunday Review essay last weekend, David Reich, a geneticist at Harvard, argued that science is changing how we think about “race” and urged a candid discussion of the findings, whatever they may be. Hundreds of readers left comments, many expressing worry about the possibility that the results could be misinterpreted or nefariously applied. Here are Dr. Reich’s responses to some of the comments. — The Editors

Unfortunately, science has always been misused. From slavery, to eugenics, to economic Darwinism, to anti-abortion politics, the latest scientific knowledge has been employed in the service of evil. The idea of forestalling such efforts is valiant but fruitless. Science must continue its pursuit of truth, and scientists, as always, must speak out when their discoveries are exploited for harmful purposes. — Syfredrick, Providence, R.I.

We scientists always need to keep in mind our social responsibilities and to think about whether what we are doing has positive or negative effects on our world.

With respect to today’s study of human variation, I am optimistic — I believe that it has been socially positive. It is making every racist view of the world untenable. In my just-published book, I explain how the ancient DNA revolution — which has provided far greater power to reveal what happened in our deep past than what was available before — has done far more to undermine racist beliefs than to support them. As I wrote in the final paragraph of my book:

The study of human variation has not always been a force for good. In Nazi Germany, someone with my expertise at interpreting genetic data would have been tasked with categorizing people by ancestry had that been possible with the science of the 1930s. But in our time, the findings from ancient DNA leave little solace for racist or nationalistic misinterpretation. In this field, the pursuit of truth for its own sake has overwhelmingly had the effect of exploding stereotypes, undercutting prejudice, and highlighting the connections among peoples not previously known to be related. I am optimistic that the direction of my work and that of my colleagues is to promote understanding, and I welcome our opportunity to do our best by the people, ancient and modern, whom we have been given the privilege to study.

My book also gives concrete examples of how it is no longer so easy to twist science to support long-cherished stereotypes because ancient DNA is now debunking the stories that used to be made up about the nature of human variation. As I wrote there:

The Nazi ideology of a “pure” Indo-European-speaking Aryan race with deep roots in Germany, traceable through artifacts of the Corded Ware culture, has been shattered by the finding that the people who used these artifacts came from a mass migration from the Russian steppe, a place that German nationalists would have despised as a source. The Hindutva ideology that there was no major contribution to Indian culture from migrants from outside South Asia is undermined by the fact that approximately half of the ancestry of Indians today is derived from multiple waves of mass migration from Iran and the Eurasian steppe within the last five thousand years. Similarly, the idea that the Tutsis in Rwanda and Burundi have ancestry from West Eurasian farmers that Hutus do not — an idea that has been incorporated into arguments for genocide — is nonsense. We now know that nearly every group living today is the product of repeated population mixtures that have occurred over thousands and tens of thousands of years. Mixing is in human nature, and no one population is — or could be — pure.

Having been immersed in the ancient DNA revolution for the past 10 years, I am confident that anyone who pays attention to what it is finding cannot come away feeling affirmed in racist beliefs. My childhood guesses about who we are and how we’re related to one another — and about the nature of differences among people — have been shown to be wrong again and again.