The number of public schools offering single-sex instruction has risen from fewer than a dozen to 205 since 1997, with classrooms sprouting up in places such as Atlanta, New York, and Philadelphia, says Leonard Sax, a psychologist and physician who directs the National Association for Single Sex Public Education, in Maryland. The increase is directly attributable to the 2001 No Child Left Behind Act, which introduced new flexibility to Title IX of the Education Amendments of 1972. The statute had previously prohibited public schools from segregating classrooms by gender if they wanted to keep receiving federal money. While 205 still represents a tiny fraction of the 96,000 public schools nationwide, Sax foresees growth. He says that there are fundamental, hard-wired, genetically programmed differences in how boys and girls learn. Generalizations are tricky, and other experts have highlighted exceptions, but many boys thrive on motion and noise, while girls often sit still longer and can work better in groups, adds Michael Gurian, author of "The Minds of Boys: Saving Our Sons From Falling Behind in School and Life."