But don’t small businesses at least fuel job growth? Sort of. It’s not small businesses that matter, but new businesses, which by definition create new jobs. Real job creation, though, doesn’t kick in until those small businesses survive and grow into larger operations. In fact, according to path-breaking work by the economist John C. Haltiwanger and his colleagues, once they accounted for the outsize contributions by new and young companies, they found “no systematic relationship” between net job growth and company size.

It’s unlikely such findings will change politicians’ speeches trumpeting small businesses. But if we want to get our job market back on track, they should inform our policy thinking. For example, it’s not only the case that start-ups are of particular importance to robust job growth. They’ve been creating fewer jobs over the last decade. Employment at start-ups fell by almost half, and those losses predated the “Great Recession” — probably one reason job growth was so lackluster over the last decade’s expansion.

Economists do not yet have a good answer as to why start-ups and surviving young companies are creating fewer jobs, but it may have something to do with “allocative inefficiency.” Too many resources flowed to financial engineering in the last decade, and too few went to R & D and innovation outside of the financial sector. The decline of American manufacturing plays a role here as well, as the sector has historically accounted for 70 percent of job-creating private-sector R & D, often in partnership with start-ups and small suppliers.

This isn’t to say that public policy should abandon small businesses. Many face distinctive hurdles compared with large businesses: they have tighter profit margins and thus less room for mistakes, they have diminished access to credit markets and, even with creditworthy borrowing records, many say they’re not getting the loans they need. Small manufacturers often have less access to export markets, and, with emerging economies growing a lot faster than advanced economies, that’s a big disadvantage.

Yet the sector’s primary lobbying group — the National Federation of Independent Business — tends to fight less for these pragmatic policies and more for the standard conservative agenda of lower taxes and deregulation. Indeed, the group has become a purely partisan operation, fighting more for Republican electoral victory than small-business growth. For example, it opposed the president’s jobs bill, even though independent analysts estimated it would significantly increase economic demand, and the federation’s own survey shows that “poor sales” — a k a weak demand — is a much bigger problem for its members than taxes or regulations.