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Did you know that there is a whole field of study on kissing called Philematology? Yes, there are courses and scientists who study kissing and its effect on human society. Kissing which is a form of affection that has existed pretty much since mankind was created is also somewhat practiced in the animal kingdom by higher primates. Kissing has been proven to reduce and relieve stress on both partners as the transfer of hormones in the saliva both increases testosterone (which causes desire and bonding/attachment) and reduces oxytocin which is the stress hormone.
A scientific study by Rutgers University showed that fifteen minutes of kissing greatly reduces the amount of stress on an individual and increases the feelings of affection, bonding, nurturing and attraction between couples. It is the exchange of testosterone in the saliva which increases tenderness, romantic interest and promotes that sense of bonding at a primitive level. MRI studies of human brains while kissing have shown wild increases in brain activity just from the act of kissing in various regions of the brain.
One other fact I found was that kissing once was highly hidden in public back in Roman-era times due to a highly contagious facial skin disease but was still widely practiced discreetly. Many paintings and artwork from this time period were discouraged to show kissing as to not promote it at the time.
Some interesting science facts for you.