Deadly attacks have been a constant in Iraq after years of American occupation, followed by a sectarian war in which Sunni and Shiite militias slaughtered civilians of the opposite sect. Still, while terrorist attacks in Europe may feel more surprising to the West — though they have become all too common there, too — that does not explain the relative indifference to attacks in Istanbul, Saudi Arabia or Bangladesh.

“That’s what happens in Iraq,” Sajad Jiyad, a researcher in Iraq who rushed to the scene of the Baghdad bombing and found that one of his friends had died there, wrote on his own blog. “Deaths become just statistics, and the frequency of attacks means the shock doesn’t register as it would elsewhere, or that you have enough time to feel sad or grieve.”

In the Muslim world, the partly sectarian nature of some conflicts shades people’s reactions, producing a kind of internal sympathy gap. People from one sect or political group often discount or excuse casualties from another.

In Iraq, the Islamic State took root within an insurgency against the country’s Shiite-led government, and Shiite militias fighting it have been accused of brutality as well. In Syria, it is just one menace; many more Syrians have been killed by the government’s attacks on areas held by Sunni insurgents, including rebel groups opposed to the Islamic State.

Mr. Jiyad added that the Islamic State was “hoping to incite a reaction and a spiral into endless violence,” and that Iraqis played into that when they mourned more for their own sect than for others.

In the West, though, there is a tendency in certain quarters, legitimized by some politicians, to conflate extremist Islamist militants with the Muslim societies that are often their primary victims, or to dismiss Muslim countries as inherently violent.

“Either Iraqi blood is too cheap or murder is normalized,” Sayed Saleh Qazwini, an Islamic educator in Michigan, wrote on Twitter.