What are the scientific reasons for having sex?

Also in this week's column:

• How does a cross-eyed person's view differ from others?
• What can you learn from the sound of someone's voice?
• What are the most widely practiced religions of the world?

What are the scientific reasons for having sex?

Asked by Jill Howard of Alexandria, Virginia, USA

The reasons for not engaging in sex include transmission of diseases, heart attack due to exertion, and many others. The reasons for engaging in sex are numerous. Among these are:

• Sex helps boost the immune system. According to Dr Carl Charnetski of the Department of Psychology at Wilkes University in Wilkes-Barre, Pennsylvania, people who reported one or two sexual "episodes" per week enjoyed higher levels of Immunoglobin A. This is an antibody that helps fight disease.
• Sex helps boost longevity. In one study cited by Dr Charnetski, men who had more orgasms over a 10 year period boosted their longevity compared with those who had fewer.
• Sex helps ward off cancer. In another study cited by Dr Charnetski, men who had more ejaculations over a 35 year period had 33 per cent less prostate cancer compared to those with fewer ejaculations.
• Sex results in a more youthful appearance. According to a study by Dr David Weeks, a clinical neuropsychologist at the Royal Edinburgh Hospital in Scotland and co-author of Superyoung (1999), men and women who reported having sex an average of four times per week looked approximately 10 years younger than they really were.
• Sex helps reduce stress. Numerous studies show that it does this through lowering anxiety levels, boosting relaxation, and aiding sleeping.
• Sex helps fight depression. A study by Dr Gordon Gallup of the Department of Psychology at the State University of New York at Albany found that women who regularly engage in heterosexual sex in which they come in contact with semen were significantly less depressed than those women that did not. he causal relationship is unclear. Dr Gallup speculates that "possibly because when absorbed through the vagina, semen may have an effect on mood in women". However, Dr Gallup is quick to point out: "Regardless of the findings, this study does not advocate that people abstain from using condoms. Protecting oneself from an unwanted pregnancy or a sexually transmitted disease is far more important."
• Sex helps coping with middle age. This is the inference drawn from research by Dr GA Bachmann at the Robert Wood Johnson Medical School in New Brunswick, New Jersey and published first in 1995 in the International Journal of Fertility and Menopausal Studies and continuing in The Journal of Sexual Medicine in 2006.
• Sex is good exercise. Exercise helps circulation, lowers cholesterol, and releases helpful endorphins.
• Sex helps in losing weight. Well, at least a little. One burns approximately four to five calories per minute or perhaps 300 calories per hour during sex (depending upon how, shall we say, "vigorous" the sex is). About 7,000 to 8,000 excess calories must be burned to lose one kilogram of fat (3,500 to lose one pound). You do the calculations.

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to s.juan@edfac.usyd.edu.au