Masturbation

== Evolutionary utility ==
Masturbation may increase fertility during intercourse. 

Female masturbation alters conditions in the vagina, cervix and uterus, in ways that can alter the chances of conception from intercourse, depending on the timing of the masturbation. A woman's orgasm between one minute before and up to 45 minutes after insemination favors the chances of that sperm reaching her egg. If, for example, she has had intercourse with more than one male, such an orgasm can increase the likelihood of a pregnancy by one of them.<ref>{{cite book |last = Baker |first = Robin |title = Sperm Wars: The Science of Sex |publisher = Diane Books Publishing Company |month=June | year=1996 |isbn = 978-0788160042}}</ref><ref name=Ejaculate_manipulation_by_females>{{cite journal |last = Baker |first = Robin R. |coauthors = Bellis, Mark A. |year = 1993 |month = November |title = Human sperm competition: Ejaculate manipulation by females and a function for the female orgasm. |journal = Animal Behaviour |volume = 46 |issue = 5 |pages = p887, 23p |url = http://cat.inist.fr/?aModele=afficheN&cpsidt=3768816}}</ref> Female masturbation can also provide protection against cervical infections by increasing the acidity of the cervical mucus and by moving debris out of the cervix.<ref name=Ejaculate_manipulation_by_females/>

In males, masturbation flushes out old sperm with low motility from the male's genital tract. The next ejaculate then contains more fresh sperm, which have higher chances of achieving conception during intercourse. If more than one male has intercourse with a female, the sperm with the highest motility will [[Sperm competition|compete]] more effectively.<ref>{{cite paper |author = Thomsen, Ruth |title = Sperm Competition and the Function of Masturbation in Japanese Macaques |publisher = Ludwig-Maximilians-Universität München |date=October 2000 |url = http://edoc.ub.uni-muenchen.de/archive/00000105/}}</ref><ref>{{cite journal |last = Baker |first = Robin R. |coauthors = Bellis, Mark A. |year = 1993 |month = November |title = Human sperm competition: Ejaculate adjustment by males and the function of masturbation. |journal = Animal Behaviour |volume = 46 |issue = 5 |pages = p861, 25p |url = http://cat.inist.fr/?aModele=afficheN&cpsidt=3768815}}</ref><ref>{{cite journal |last = Shackelford |first = Todd K. |coauthors = Goetz, Aaron T. |year = 2007 |month = February |title = Adaptation to Sperm Competition in Humans. |journal = Current Directions in Psychological Science |volume = 16 |issue = 1 |pages = p47-50 |url = http://www.blackwell-synergy.com/doi/abs/10.1111/j.1467-8721.2007.00473.x}}</ref>