America's entrepreneurs might not be living up to their reputation as job creators. In fact, the median U.S. small business adds fewer than one full-time employee each year, according to new research.

A JPMorgan Chase Institute study of payroll records from more than 45,000 of the bank's small-business customers finds 16 percent lost a full-time employee in 2015, and just 20 percent added more than two.

Small businesses are often credited with providing half of all American workers' jobs. But while 89 percent of U.S. companies with paid employees in 2014 had fewer than 20 on staff, those businesses employed only 17 percent of all workers at companies with paid employees. Real small-business job growth, the study finds, comes from the creation of new companies rather than the expansion of existing ones.

JPMorgan Chase's research focused on customers with total bank funds of less than $20 million, and covered payroll data from February to October to avoid the holiday season.

Payroll is a significant expense for most small-business owners, and their staffing budgets were found to be "very unstable," Diana Farrell, chief executive of the J.P. Morgan Chase Institute, told The Wall Street Journal.

"The reality of it is that most small businesses do not have a steady flow of customers and a steady flow of revenue," Farrell said. "They have goods months and bad months."