Giving new meaning to the oft-uttered exclamation that your friends know you better than you think, researchers from Washington University in St. Louis say close friends know each other well enough to have an idea of how long the other will live.

"You expect your friends to be inclined to see you in a positive manner, but they also are keen observers of the personality traits that could send you to an early grave," says Joshua Jackson, PhD, assistant professor of psychology in Arts & Sciences.

Personality types can predict longevity, even at an early age, according to the study, which was published in the journal Psychological Science and involved data collected in the 1930s about a group of 600 young people in their mid-20s.

In the study, men who were considered open and conscientious by their friends lived long lifespans, and women whose friends said they were emotionally stable and agreeable also benefitted from a long life.

Personality can, indeed, contribute to longevity, says Jackson, who suggests that conscientious men are more likely to maintain a healthy lifestyle, avoiding unnecessary risks, and that women who are emotionally stable could resist the pitfalls of anger, anxiety and depression.

Data on the personality traits of participants was both self-reported and reported as assessed by friends, most of which had been bridesmaids and groomsmen in their wedding parties.

Having obtained dates of death for the grand majority of participants, the team concluded that the friends' assessments of participants' personalities were more accurate predictors of longevity than self-reported personality assessments.

On the effectiveness of self-reported personality assessments, the men were more accurate in predicting their own lifespans than the women, but their friends -- who Jackson suggests could have more insight and less bias -- were the best predictors.

Jackson says he hopes his study could influence the medical community to use personality as an indication of health and longevity and offer personalized, preventative treatment to those at risk.