As measured by their teachers, young adoptive children were more likely than biological ones to get angry easily and to fight with other students. If a 50 percent score represents an average level of this type of “problem behavior,” adopted kindergarteners were higher than average, at 64 percent, while children with two biological parents were at 44 percent. Children in single-parent, step, and foster families all had fewer behavioral issues than adopted kindergarteners, at 58 percent, although this difference was not significant. A similar pattern (63 percent versus 43 percent) emerged for adopted and biological first graders. For his research, Zill examined a longitudinal study of 19,000 students that was conducted by the National Center for Education Statistics beginning in 1998. Zill is the former head of the Child and Family Study Area at Westat, a social-science research corporation.