The U.S. armed forces recruiters have noted that a long time trend, that of more recruits from the middle and upper classes, is continuing. This top 20 percent of the population (in terms of income and social class) was never noted for sending many of its kids into military service during peacetime. And those that did go, usually went, via ROTC programs at college, which enabled students to graduate with an officers commission, and an obligation to serve as such for three or four years. When the draft was made permanent in the late 1940s (the first time that had ever happened in peacetime), the pattern didn't change. The military only needed a fraction of the draft age population, so it was easy to avoid service by staying in college, or getting a job that kept you out of uniform. The upper class kids that did get drafted, were eagerly sought after to fill administrative and technical jobs they already had some education or experience for.
When the draft ended in 1972, the upper class kids stayed away from volunteering. At first. But when the military began raising recruit standards in the 1980s, the military began, mostly via word-of-mouth, losing it's bad reputation. Right after the Vietnam war, the military was full of angry, and often inept, people. Lots of good officers and troops got out. But by the 1980s, that had passed, and by the 1990s, it became more common to see kids from the high end high schools, and colleges, joining. For many, it was for the adventure. The working class kids joined for technical jobs, so they could learn valuable job skills, in addition to earning money for college or technical school when they got out. So the combat units tended to be more upper class than the support units.
At the same time, the old, Vietnam era, myth that only the poor and uneducated joined, persisted, even though it was never true. Even when the draft was in effect, the military only accepted the above average kids. This was even more true as time went by. Thus even the recruits from the poorest families, tended to be healthier, and better educated (had graduated from high school and done well on the standardized test all recruits take) than all of their peers (of the same age and gender). But now, kids from the wealthiest families are edging out those from the poorest ones, when it comes to getting into the military. In the last decade, the recruits from the poorest families has gone from about 20 percent, to about ten percent. Meanwhile, those from the top 20 percent of families (in terms of income) has gone from under 20 percent, to about 25 percent.
The big draw for many middle and upper class kids is the combination of adventure, public service and generous pay and benefits. The educational benefits mean you can save up $100,000 or more if you go to college. Unless your parents are very rich, this kind of benefit is a big deal. Plus, the military pay is good enough, especially if you live and eat on base, that you can get out after four years of active service with at least $20,000 in the bank (especially if you spent some time in a combat zone.) Another edge the upper class kinds have is they tend to be more physically fit (a third of all potential recruits are too fat to serve) and less likely to have been arrested (which makes it more difficult, but not impossible, to enlist.)
The military is well aware of the fact that an important reason for their success in Iraq and Afghanistan has been the quality of the troops. Everyone in uniform wants to keep the quality up.