Some researchers in the 1980's investigated whether there was a possible relationship between personality and having a NDE. In other words they wanted to find out whether NDEs took place in a particular group of people, who may have had a prior predisposition to having the experiences. In one study a comparison was made between those people with and without NDEs, as regards levels of IQ, having an extroverted personality, having a neurotic personality and anxiety. No significant differences were found. In another study in 1984 the researchers looked at measures of attention, hysterical tendencies, death anxiety, danger seeking and psychotic personality. In this study they also found no differences in the two groups, except for an increase in their attention scores. Other researchers have suggested that there may be an increase in having had prior mystical experiences. There is however a limitation associated with these studies, in that they have involved somewhat biased samples of people. Essentially the people used in the studies are a group who have volunteered their experiences and therefore may represent a specific group of people who are more likely to have had prior experiences and therefore more likely to volunteer them to investigators.