Frequency, age, and sex
Frequency of masturbation is determined by many factors, e.g., one's resistance to sexual tension, hormone levels influencing sexual arousal, sexual habits, peer influences, health and one's attitude to masturbation formed by culture; E. Heiby and J. Becker examined the latter. Medical causes have also been associated with masturbation.
Different studies have found that masturbation is frequent in humans. Alfred Kinsey's 1950s studies on US population have shown that 92% of men and 62% of women have masturbated during their lifespan. Similar results have been found in a 2007 British national probability survey. It was found that, between individuals aged 16 to 44, 95% of men and 71% of women masturbated at some point in their lives. 73% of men and 37% of women reported masturbating in the four weeks before their interview, while 53% of men and 18% of women reported masturbating in the previous seven days.
The Merck Manual gives the following statistics: 97% of men and 80% of women have masturbated. It also states that, generally speaking, males masturbate more than females and that the act of masturbation may affect a person negatively due to the disapproving views of others.
Masturbation is considered normal when performed by children, even in early infancy. In 2009, the Sheffield NHS Health Trust issued a pamphlet called "Pleasure" which discussed the health benefits of masturbation. This was done in response to data and experience from the other EU member states to reduce teen pregnancy and STIs (STDs), and to promote healthy habits.
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". "Baby girls sometimes move their bodies rhythmically, almost violently, appearing to experience orgasm." Italian gynecologists Giorgio Giorgi and Marco Siccardi observed via ultrasound a female fetus possibly masturbating and having what appeared to be an orgasm.
Popular belief asserts that individuals of either sex who are not in sexually active relationships tend to masturbate more frequently than those who are; however, much of the time this is not true as masturbation alone or with a partner is often a feature of a relationship. Contrary to conventional wisdom, several studies actually reveal a positive correlation between the frequency of masturbation and the frequency of intercourse. A study has reported a significantly higher rate of masturbation in gay men and women who were in a relationship.
Coon and Mitterer stated: "Approximately 70 percent of married women and men masturbate at least occasionally."
Evolutionary utility
Masturbation may increase fertility during intercourse. A 2009 Australian study found daily ejaculation to be an important factor in sperm health and motility.
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 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. 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.
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 compete more effectively.
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