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.
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To be fair, there may be other ways to explain their conclusions - and you might not need the clamps, harnesses and whips to replicate that kind of closeness (simply cooking a meal together might do the trick as well). Still, there is a stigma associated with S&M that critics are calling into question.
http://www.reuniting.info/science/sex_in_the_brain