Just a small article of interest I found earlier on New Scientist:

Harnessing science to create the ultimate warrior by Linda Geddes – New Scientist

Genetic testing might also enable recruitment officers to determine which soldiers are best for specialist jobs. For example, by combining psychological testing with genetic tests for levels of brain chemicals, a clearer picture of a soldier’s competencies might shine through. “We might say that given this person’s high levels of brain serotonin, they’re going to be calmer under pressure, so they might make a good sniper,” says Paul Zak of Claremont Graduate University in California, who was on the NAS panel. Alternatively, someone with low dopamine might be less likely to take risks, he says, and therefore be better suited as a commanding officer in a civilian area.