The penis is designed to remove a rivals semen during intercourse. There is one study by State University of New York that concludes that the penis acts as a "semen displacement device" and evolved in shape in part to displace another man's semen.
The study concluded that the thrust of the penis during love making helps to clear a woman's reproductive system of a previous lover's semen. The results were concluded after experiments using latex phalluses, an artificial vagina and a mixture of starch and water.
Researchers found the coronal ridge of the penis, found where the glans, or head, meets the shaft, could scoop out more than ninety percent of the cornstarch mixture with just a single thrust.
When the same experiment was done with a phallus with out the coronal ridge only "scooped up" around thirty five percent of the cornstarch mixture.
The same research team found that depth of thrust was also very important. A thrust of only three quarters was found to shift only around forty percent of the synthetic semen substitute.
The test were expanded further to study students and found that love making tended to be more energetic if the male believed the women had been cheating or the couple had been apart for a long period. This led researched to conject that me have a sub conscious desire to their partner of any evidence of another man's semen.
The State of New York University lead researcher spoke on the subject saying "We theorize that as a consequence of competition for paternity, human males evolved uniquely configured penises that function to displace semen from the female vagina left by other males".
"Thus, the human penis may enable males to substitute their semen for the semen of their competitors."
"Not only did we obtain data derived from artificial genital models that were consistent with this hypothesis, but we also found that under conditions that raise the threat of female infidelity, college students reported changes in sexual behavior that would serve to purge the vagina of foreign semen prior to ejaculation."
In the intrest of debate there are others that dispute this theory. Mr Derek Machin, a urologist at University Hospital, Aintree, said the theory seemed "far fetched".
"The research might very well be accurate, but I'm not convinced that just because the penis does something like this it was necessarily designed to have that effect."
Dr Colm O'Mahony, chairman of the UK Association for Genito-Urinary Medicine, said the theory seemed flawed.
"If the man continues to thrust after ejaculation he would simply be scooping out his own semen."
"Also does the sexual position matter - I imagine gravity has some role here. Maybe those missionaries knew something about position after all?"
The research is published in the journal Evolution and Human Behaviour.
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