They found that Dead Space 2 evoked the strongest fear response and discovered some interesting aspects of horror games. The other games they used were Alan Wake, Resident Evil 5 and Condemned.
For one, actual combat is not as scary as the implied threat of combat, and cutscenes are generally not sources of fear for most players. Casual gamers are affected most by them, while core gamers are the least scared. This could be because core games know there's less danger in a cutscene. The more cinematic a sequence, the less fear a core gamer feels, while a casual gamer experiences it like a movie and feels scared.
The first confrontation is almost always the scariest. In every game, across all player experience levels, the first encounters with the enemies are much scarier than later ones. Obviously, being forced to do the same thing over and over removes the threat and mystery.
Gore isn't a source of real fear, just provocative in its implications. However, the grotesque will evoke more fear and gore will increase the impact of a shocking event.
In general, casual players are more easily scared than core players, but also enjoy the games more. Experienced gamers' familiarity with the medium and their existing expectations of it, means they are less likely to become scared than casual gamers.