Joshua Jackson of Washington University and a team from the University of Tubingen studied young German men performing their 9 months of military national service (3 in training, 6 on a post), measuring their personality both before training and two years after. A large control group was available thanks to the proportion of German citizens who conscientiously object to military service, opting for civilian duties over the same time period. Those who opt out of the army may differ in terms of personality, so the authors used a smart matching procedure, pairing up budding soldiers with one or two civilians who were similar in terms of personality. This created two comparable samples, matched on pre-training personality, of 241 (soldier) and 628 (civilian) participants.
All participants showed some shifts in personality over time, becoming less neurotic, more conscientious and more agreeable. These trends have been identified elsewhere as a feature of young adulthood, and are often construed as a developing maturity: coping better with setbacks, being more organised and accountable, and having more generosity of spirit toward others. The groups differed in one way only: the effect of increasing agreeableness was one third larger for the civilian than the military group.* This suggests that military training attenuates the upward trajectory of agreeableness seen in early adulthood.
A subset of participants were contacted on two further occasions, two years apart, giving 4 data points with which to examine this trajectory more fully. Across the six years, agreeableness increased year on year for the civilian group in a fairly linear fashion. The military group showed a steady increase, but it was extremely weak (from eyeballing the data, it looks as if the agreeableness increase in this smaller sample may not even be significant, but this isn't directly reported). Agreebleness steadily ticks upwards in young adult years, unless participants undergo military experiences, in which case they see smaller or no changes to this personality trait, with no 'late blooming' of agreeableness to catch them up later.
Research on personality change can be challenging, not least because personality traits tend to be highly consistent. This study's matching procedure enabled it to identify how military experience seems to cause a deviation from the young adult trajectory of growing agreeableness over time. Lower agreeableness matters: it is associated with conflict in relationships and aggression, although it has also been associated with greater occupational attainment. At least as interesting for me, however, are the similarities of change of other traits across both groups. As the authors put it, "the maturation often attributed to military training...may actually be best ascribed to the specific time period of young adulthood."
*Cohen's d of .32 vs .21, both significant to p<.05