According to a joint study by the University of California, San Francisco, and Hiroshima University, of 602 teens (age 15 to 19) surveyed in the Shibuya section of Tokyo recently, 43 per cent said they keep five or more sex friends at a time. In a similar survey of 16-year-olds in two rural prefectures, 20 per cent of boys and 18 per cent of girls said they have at least five sex partners. " To many young Japanese people, everything about sex is casual, says Masako Ono-Kihara, a public-health expert at Hiroshima University School of Medicine. Girls now share their boyfriends like theyfd share chips. Everyone's hand is in the bag.