Here’s another way of looking at it: Instead of asking why so many mass shooters are white, we could ask what it is about mass shootings that differentiates them, even to a small extent, from the broader trend of racial inequalities in murder rates. Why might the huge disparities that we see in homicides, born of systematic disadvantage, be diminished (though not reversed) in our most extreme episodes of violence?

The USA Today data set Lankford relied on differed from the Mother Jones collection in key ways. Most significantly, Mother Jones defined “mass shootings” as only those that took place in public settings. USA Today went with a broader definition of “mass killings,” including any with four or more victims. Public massacres like the one in Las Vegas represent just one-sixth of this larger data set. (The Mother Jones data doesn’t include, for instance, killings that unfold in a private home with members of the killer’s family as the victims.) With that said, Lankford found that the USA Today data showed there was no difference between the whiteness of mass murderers and of murderers overall. (Whites made up 37.9 percent and 36.5 percent of those groups, respectively.) Black Americans committed a slightly lower percentage of mass murders than other murders, and those described as Asian committed a somewhat higher percentage of mass murders. Taken as a whole, he concluded that there is “a clear discrepancy between the popular assumptions about mass murderers in the United States and the evidence-based reality.”

http://www.slate.com/articles/news_and_politics/politics/2017/10/what_the_white_mass_shooter_myth_gets_right_and_wrong_about_killers_demographics.html?wpsrc=sh_all_dt_tw_top