elasticity, the anal mucous membrane being thin, and small blood vessels being present directly beneath the mucous membrane, tiny tears and bleeding in the rectum usually result from penetrative anal sex, though the bleeding is usually minor and therefore usually not visible. By contrast to other anal sexual behaviors, anal fisting poses a more serious danger of damage due to the deliberate stretching of the anal and rectal tissues; anal fisting injuries include anal sphincter lacerations and rectal and sigmoid colon (rectosigmoid) perforation, which might result in death. Repetitive penetrative anal sex may result in the anal sphincters becoming weakened, which may cause rectal prolapse or affect the ability to hold in feces (a condition known as fecal incontinence). Rectal prolapse is relatively uncommon, however, especially in men, and its causes are not well understood. Kegel exercises have been used to strengthen the anal sphincters and overall pelvic floor, and may help prevent or remedy fecal incontinence. Cancer Most cases of anal cancer are related to infection with the human papilloma virus (HPV). Anal sex alone does not cause anal cancer; the risk of anal cancer through anal sex is attributed to HPV infection, which is often contracted through unprotected anal sex. Anal cancer is relatively rare, and significantly less common than cancer of the colon or rectum (colorectal cancer); the American Cancer Society states that it affects approximately 7,060 people (4,430 in women and 2,630 in men) and results in approximately 880 deaths (550 in women and 330 in men) in the United States, and that, though anal cancer has been on the rise for many years, it is mainly diagnosed in adults, "with an average age being in the early 60s" and it "affects women somewhat more often than men." Though anal cancer is serious, treatment for it is "often very effective" and most anal cancer patients can be cured of the disease; the American Cancer Society adds that "receptive anal intercourse also increases the risk of anal cancer in both men and women, particularly in those younger than the age of 30. Because of this, men who have sex with men have a high risk of this cancer." Cultural views General Different cultures have had different views on anal sex throughout human history, with some cultures more positive about the activity than others. Historically, anal sex has been restricted or condemned, especially with regard to religious beliefs;