Employment-services companies are attracting investors who are betting the U.S. labor market will keep up its steady pace of job creation.

The Standard & Poor’s Supercomposite Human Resources & Employment Services Index — which includes Robert Half International Inc. and Manpower Inc. — has risen 19 percent since Oct. 15, compared with a 1.5 percent increase for the S&P 500 Index. The outperformance coincides with nonfarm payroll data for October and November that exceeded economists’ forecasts.