From the NY Times:
Compared to their contemporaries in civilian life, the armed forces have a greater percentage of minorities, a higher proportion of high school graduates and better reading levels. As a group, about 60 percent of enlisted men and women are white; they tend to be married and upwardly mobile, but to come from families without the resources to send them to college.
While blacks make up about 12.7 per cent of the same-age civilian population, they constitute about 22 per cent of enlisted personnel. Perhaps most striking is the number of enlisted women who are black: more than 35 percent, according to Pentagon figures, indicating not only that black women enlist at higher rates, but that they stay in the military longer. In the Army, in fact, half of all enlisted women are black, outnumbering whites, who account for 38 percent.
Most enlisted men and women are slightly more educated than average, so "upwardly mobile, but [] come from families without [] resources" sounds to me a lot like a matter of expediency. They 'place out' of the opportunities they have at home, which is typically rural.
Cf. this very suspect report from the Heritage Found'n (http colon slash slash www-dot-heritage.org/research/reports/2005/11/who-bears-the-burden-demographic-characteristics-of-us-military-recruits-before-and-after-9-11), which uses a questionable "zip code" methodology (when the data doesn't fit, fit the data) to claim that the empirical research of dozens of sociologists is false. In any event, Kane's own figures, contrary to his conclusions (he notes that he didn't do the research at the bottom of the paper), show that the US military recruits almost entirely from people living in areas that earn or below the national median income--median hometown income for those who join the military still ranks below the national median--and from one indecipherable chart if I read it correctly recruits heavily from people who live in areas of least-affordable housing.
Likewise, the disproportionate representation of black men and women speaks volumes about motivations behind military enlistment.
Both the Times piece and Kane report that the typical enlisted military employee is more educated, as I've mentioned, so, so much for being "less intelligent".
Of course, Kane does note that post 11 Sept. 2001, representation of the wealthy in the US military has increased substantially (though not at all proportionally). Likewise there's no mention of the phenomenon of the growing "warrior caste" phenomena that several, notably Charles Moskos, observed, the latter, which strikes me as something that makes these arguments moot. One observation, which seems poorly documented if not necessarily false, is that families with military service histories tend to have later generations serve. Another thing worth noting is that the CIA recruits heavily from college campuses, which suggests a class-based split in the types of military service when broadened to include the intel complex. The most obvious aspect of this is the growth of mercenary firms, which pay on an order of magnitude greater than the Pentagon's payroll dep't.