Using brain scans that illuminated CB1 receptors, the study looked at the brains of 60 volunteers, some with PTSD, some with a history of trauma and some with neither. Women had higher levels of CB1 receptors in brain regions associated with fear and anxiety, consistent with the fact that women are much more likely to develop PTSD than men. According to the U.S. Department of Veterans Affairs' National Center for PTSD, 10 percent of women develop PTSD, compared to 5 percent of men. Scientists can't really explain why yet. Neumeister says it could be a biological phenomenon that women have higher levels of CB1 receptors and are therefore at greater risk, but it's most likely a consequence of trauma.