Studies of trauma victims suggest that shock brought on by violent or life-threatening situations causes damage to the hippocampi, structures in the brain that play a crucial role in learning and memory. Researchers found that in combat veterans suffering from post-traumatic stress symptoms, which include nightmares and vivid flashbacks, the hippocampi were eight percent smaller in volume than in combat veterans who suffered no such symptoms. The researchers concluded that the hippocampi had lost cell mass as a result of trauma.
Which of the following, if true, would most seriously weaken the researchers' conclusion drawn above?
1 In another study, subjects who had experienced the death of a close relative were found to have no reduction in the volume of their hippocampi when compared to those whose close relatives were all still living.
2 In the study, the traumatized veterans were compared with other veterans of similar background, body size, and other characteristics that might have a bearing on brain size.
3 Some individuals are born with hippocampi whose volume is smaller than average, and this reduced volume makes them more susceptible to post¬traumatic stress symptoms.
4 Combat veterans who experience post-traumatic stress symptoms perform significantly worse on tests of verbal memory compared with veterans who suffer no such symptoms.
5 Further study revealed that veterans who had seen more intense combat and had more severe post-traumatic symptoms exhibited even greater reduction in the volume of their hippocampi.

OA C