We often talk about veterans of the wars in Iraq and Afghanistan in the same breath, as if American soldiers fighting in the 21st century were engaged in largely interchangeable experiences wherever they've been "over there." Researchers often combine veterans of the two wars into the same suicide and depression statistics. They're compared in a single class to veterans of earlier wars in the Persian Gulf, Vietnam, Korea or World War II.

And it's true that they deploy from America under comparable circumstances, as members of a U.S. military that's fighting longer wars and is more isolated at home from the U.S. public than ever.

Combat in Iraq, however, is not entirely like combat in Afghanistan. And research consistently concludes that veterans are returning from Iraq, where the troubled shooter in Wednesday's Fort Hood tragedy served, with what appears to be greater exposure to stressors and higher levels of PTSD. The Fort Hood shooter, an Army truck driver named Ivan Lopez, was reportedly undergoing evaluation for PTSD.

Some numbers from the Department of Veterans Affairs estimate that PTSD affects about 11 percent of veterans of the war in Afghanistan, but 20 percent of veterans who served in Iraq.

There's little data explaining the differences between the two groups partly because of the difficulty of diagnosing PTSD (and identifying veterans who may not know they suffer from it), as well as the fact that the VA itself often combines veterans of the two conflicts together. Over the length of these two wars, though, service members in Iraq have been exposed to more combat, with the kinds of traumas associated with it. This data comes from an early study contrasting soldiers' experiences in the two wars in 2003:

While the nature of both wars has changed over the last decade, that hints at why soldiers fighting in Iraq have consistently remained more likely to report symptoms of PTSD, and why – going forward as veterans – they'll likely need such a deep investment in mental health care.