Shay says other factors could be making up the difference.

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Still, Shay says what strikes him is how familiar their struggles seem: When these Iraq vets speak, he hears echoes of the same loss and anger that have wounded generations of soldiers, all the way back to ancient Greece.

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"There is a second territory ... and that is secondary traumatic stress injury, " says Shay, the author of Odysseus in America and Achilles in Vietnam.

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Matthew Shay, CEO of the NRF, says the data should encourage retailers to be more aggressive in bringing in consumers.

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