John Bohannon points to a study on the subject:

Models of rational behavior predict many of society's patterns, such as the prevalence of tax evasion and union strikes. But seemingly irrational behaviors like warin which the measurable costs often far outweigh the measurable benefitshave stumped researchers going back to Charles Darwin. The prospect of crippling economic burdens and huge numbers of deaths doesn't necessarily sway people from their positions on whether going to war is the right or wrong choice. One possible explanation is that people are not weighing the pros and cons at all, but rather using a moral logic of "sacred values"convictions that trump all other considerationsthat cannot be quantified.

More concretely: 

[I]f people really were rational about their decision to support or oppose the Iraq War, then they should be weighing the death toll carefully. "If I think that deposing Saddam Hussein is worth 50,000 lives but no more," [Michael Spagat, an economist at Royal Holloway, University of London] says, "then I should flip when I discover that it has cost hundreds of thousands of lives. But my sense is that hardly anyone actually behaves this way." However, Spagat says that the study is limited by its reliance on hypothetical situations.

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