Over a lifetime, a medical student who becomes a specialist rather than going into primary care can expect to earn $3.5 million more. This is a tempting promise. But, writes Sarah Kliff, the Obama administration — and, arguably, the American health-care system — desperately needs more medical students to choose primary care. Decades of research have confirmed that more specialists leads to more specialty care, which leads to a more expensive system. Now, with the passage of the Affordable Care Act, tens of millions of previously uninsured Americans will be looking for a primary-care doctor. The surprise of the health-care overhaul, at least thus far, is that so many young doctors are cooperating. The number of American medical students matching into primary care residencies jumped 20 percent between 2009 and 2011, according to the Association of American Medical Colleges.