Janell L. Carroll: Sexuality Now: Embracing Diversity, 3. edition, s. 150. Cengage Learning, 2009. ISBN 9780495602743. Google Book. (englanniksi)
Richard D. McAnulty & M. Michele Burnette: Sex and Sexuality: Sexual function and dysfunction, s. 15. Greenwood Publishing Group, 2006. ISBN 9780275985837. Google Book. (englanniksi)
