It's hard to do long-term studies on how personalities change. Besides taking many years, a challenge to overcome is that many experiences that could change personality are self-selected, and thus many social, psychological and economic differences exist between people who had the experience versus those that did not. "It makes a researcher's job tough," jokes Joshua J. Jackson, now at Washington University in St. Louis, "but there are some methods to safeguard against such bias." He cowrote the new study with Felix Thoemmes, Kathrin Jonkmann, Oliver Lüdtke, and Ulrich Trautwein of the University of Tübingen in Germany.

Jackson used data on German men who were in high school at the time the study started. At that time, about 10 years ago, all German men had to either serve in the military for nine months or perform some other kind of civilian service.

First, he looked at the men's personalities before their national service to see if personality predicted the decision to enter the military. Men who chose to serve in the military were less open to experience -- they are less likely to be interested in novel and aesthetic experiences like going to an art museum, for example. They were less neurotic, or inclined to worry. And they were less agreeable -- "less warm and cooperative, interpersonally," Jackson says.

The men were given personality tests again two years later, after they had finished their military or civilian service. Most people's personalities change at this age; it's normal to become more agreeable and more conscientious, and for neuroticism to decrease. Jackson saw those changes in all the men. But men who chose to go into the military, while they were more agreeable two years later than they'd been before, were less agreeable than their peers who didn't do military service. Four years later, after many of the men had gone on to university or into the work force, they were still less agreeable if they'd spent nine months in the military.

How agreeable you are has a lot to do with how well you relate to other people -- "establishing and maintaining positive relationships with friends and romantic partners," Jackson says. "as such, having low levels of agreeableness may be considered a bad thing." On the other hand, some evidence suggests that people who are less agreeable tend to have more career success.

"I cannot say if it's good or bad, but it shows that these individuals -- who, by and large, did not face any combat -- had experiences in basic training that likely shaped the way they approach the world," Jackson says. "The changes in personality were small, but over time, they could have important ramifications for the men's lives," he says.

Note: If no author is given, the source is cited instead.

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