It’s more than that though. People who enter the military from privileged backgrounds tend to be less successful because they are more likely to be over-confident in a setting that doesn’t have tolerance for that. That’s from the study that I’ve linked below. It’s a comprehensive look from a variety of perspectives and analyzing a variety of metrics. It’s too long and multifaceted to summarize further here, except to say this:

“Our findings suggest that class-based inequality may also reproduce, in part, because class contexts can imbue advantaged individuals with an exaggerated belief that they are better than others, and outside observers may conflate this miscalibrated confidence with evidence of competence. Thus, our findings suggest that to mitigate inequality, institutions need to establish mechanisms that allow the accurate assessment of competence. For example, many scholars have questioned the value of using interviews in hiring and selection efforts because interviews are highly susceptible to bias (Hunter & Hunter, 1984). Without such mechanisms in place, our society may inadvertently and disproportionately reward those who appear competent, even when they might not necessarily be the most qualified (Anderson, Brion et al., 2012; Anderson & Kilduff, 2009). For example, in Study 4, it is striking that appearing competent was more strongly linked to overconfidence than to actual competence.”