section_path: Sexual behavior/General activities and health
section_name: General activities and health
paragraph_text: General activities and health
  
  In humans, sexual intercourse has been shown to have health benefits, such as an improved sense of smell, reduction in stress and blood pressure, increased immunity, and decreased risk of prostate cancer. Sexual intimacy and orgasms increase levels of oxytocin, which helps people bond and build trust. Some of these benefits, such as stress reduction, also apply to masturbation, as distinct from sexual intercourse with another person. Masturbation is also a healthy element of sexual development in itself.
  
  A long-term study of 3,500 people between ages 30 and 101 by clinical neuropsychologist David Weeks, MD, head of old-age psychology at the Royal Edinburgh Hospital in Scotland, said he found that "sex helps you look between four and seven years younger", according to impartial ratings of the subjects' photographs. Exclusive causation, however, is unclear, and the benefits may be indirectly related to sex and directly related to significant reductions in stress, greater contentment, and better sleep that sex promotes.
  
  Sexual intercourse can also be a disease vector. There are 19 million new cases of sexually transmitted infections (STI) every year in the U.S., and worldwide there are over 340 million sexually transmitted infections each year. More than half of these occur in adolescents and young adults aged 15–24 years. At least one in four US teenage girls has a sexually transmitted infection. In the U.S., about 30% of 15- to 17-year-olds have had sexual intercourse, but only about 80% of 15- to 19-year-olds report using condoms for their first sexual intercourse. In one study, more than 75% of young women age 18–25 years felt they were at low risk of acquiring an STI.