taken from http://www.newscientist.com/article/mg20127014.300-spanking-brings-couples-together.html
SPANKING is stressful at first, but it could bring consenting couples closer together. That's the implication of two studies of hormonal changes associated with sadomasochistic (S&M) activities including spanking, bondage and flogging.
Brad Sagarin at Northern Illinois University in DeKalb and colleagues measured levels of the stress hormone cortisol in 13 men and women at an S&M party in Arizona, before, during and after participating in activities. During S&M scenes, cortisol rose significantly in those receiving stimulation, but dropped back to normal within 40 minutes if the scene went well. There was no change in those inflicting the activity.
At an S&M event in Colorado, testosterone was measured in 45 men and women. It increased significantly in receiving women only. Donatella Marazziti of the University of Pisa, Italy, says the boost may help women cope with the aggressive nature of S&M activities, or that it could be another sign of stress. In both studies, couples who said the party went well also reported increases in relationship closeness (Archives of Sexual Behavior, DOI: 10.1007/s10508-008-9374-5).
It's important to note that levels of both hormones dropped back down in couples who enjoyed the experience, Marazziti says. "When sexual intercourse is consensual it is not stressful - even if it is extreme sex."
Richard Wiseman, a psychologist at the University of Hertfordshire in Hatfield, UK, adds that almost any shared activity is likely to promote interpersonal closeness. "It doesn't have to be tying up your partner or placing clamps on their nipples, it could be something as simple as cooking a meal together or even doing the housework as a duo," he says.
Nick Neave, a psychologist at the University of Northumbria, UK, says the results are interesting, but future studies should control for whether participants experienced orgasm, which is associated with reduced stress and an increase in hormones associated with partner-bonding and affection.