Premature ejaculation (PE), also known as, rapi ejaculation, rapid climax, premature climax or early ejaculation, is the most common sexual problem in men, affecting 25%-40% of men. It is characterized by a lack of voluntary control over ejaculation. Masters and Johnson stated that a man suffers from premature ejaculation if he ejaculates before his sex partner achieves orgasm in more than fifty percent of their sexual encounters. Other sex researchers have defined premature ejaculation as occurring if the man ejaculates within two minutes of penetration; however, a survey by Alfred Kinsey in the 1950s demonstrated that three quarters of men ejaculate within two minutes of penetration in over half of their sexual encounters. Today, most sex therapists understand premature ejaculation as occurring when a lack of ejaculatory control interferes with sexual or emotional well-being in one or both partners.