Illegal workers make up about 5 percent of the U.S. labor force. More than nine in 10 males illegally here are in that labor force, compared with 83 percent of men born in the United States. Illegal immigrants tend to be younger than American workers, which helps explain why they are more likely to hold jobs, researchers said. Female illegal immigrants, however, were less likely to work than their American counterparts, perhaps because most have young children, the Pew analysis said. Illegal immigrants are concentrated in construction, agriculture and cleaning jobs. They make up 36 percent of all insulation workers, 29 percent of agricultural workers and 29 percent of roofers. The Center for Immigration Studies, which advocates restricting immigration, recently issued a report saying that immigrants compete for jobs with less-educated Americans, especially high-school dropouts. About half of adult illegal immigrants have not graduated high school, the Pew analysis found. About a third have less than a ninth-grade education. About a quarter of illegal immigrants have at least some college, with 15 percent holding at least a bachelor's degree.