Generally, boys and girls become more polarized through their first years in school. Now, researchers have started to explore how to span that sex divide and are finding that more-equitable coed classrooms can have social and academic benefits for boys and girls alike. While children of both sexes play together as toddlers, by the end of kindergarten, they spend only 9 percent of their playtime with children of the other sex, according to research by Lise S. Eliot, an associate professor of neuroscience at the Chicago Medical School. "Separation is a fact of human childhood and is equally common among young monkeys and apes," Ms. Eliot says in the 2009 Houghton Mifflin Harcourt book "Pink Brain, Blue Brain: How Small Differences Grow Into Troublesome Gaps and What We Can Do About It." That early separation, she says, creates "two separate cultures that persist throughout childhood." But researchers at the American Educational Research Association's annual conference, held in Vancouver, British Columbia, last month, stressed that while all children naturally develop gender identity, classroom demographics and teacher practices can make a big difference in how and whether students develop sex-based stereotypes and prejudices.