A 2004 survey by Toronto magazine "NOW" was answered by an unspecified number of thousands. The results show that an overwhelming majority of the males — 81% — began masturbating between the ages of 12 and 15. Among females, the same figure was a more modest majority of 55%. (Note that surveys on sexual practices are prone to "self-selection bias".) It is not uncommon, however, to begin much earlier, and this is more frequent among females: 18% had begun by the time they turned 12, and 6% already by the time they turned 10. Being the main outlet of child sexuality, masturbation has been observed in very young children. In the book "Human Sexuality: Diversity in Contemporary America", by Strong, Devault and Sayad, the authors point out, "A baby boy may laugh in his crib while playing with his erect penis (although he does not ejaculate). Baby girls sometimes move their bodies rhythmically, almost violently, appearing to experience orgasm."