Post-coital Depression Syndrome (PDS)
An evolutionary arms-race defense against sperm-displacement by other males is male quiescence close to the female ( with rapid male aggressive arousal response to threats ) . This easily overbalances into depression (monogamy can be described as an extended mild depression from the male viewpoint) . Stable dual systems evolve where the male quiescence trigger female nurturing instincts . In extreme cases this forms a dual pathology : a repeating cycle of aggressive coitus , male depression , female nursing , recovered male aggressive coitus , etc.
Hominids have had a long period when physically preventing other males from coitus with his lady love was the only way to ensure that only his genes were being transmitted .
Experiments done by G Gallup (shades of nominative determinism !) of New York State University showed that the coronal ridge on human penises are 90% efficient in displacing semen in one deep thrust . (Evolution and Human Behavior vol 24 , p277 or New Scientist 9 Aug 2003 p16 )
Surveys indicated that human males employed deeper thrusts after a separation or if infidelity was suspected .
In the arms race of gene replication , the evolution of the coronal ridge must have accompanied the evolution of counter-actions , the most obvious being that the male must stay with the female until egg fertilization , while fighting off competitors if necessary .
One way of doing this is by reducing certain neurotransmitter concentrations , notably serotonin . This would have the effect of a mild depression ( the storied “little death” ) in males .
Anecdotal evidence : post-coital cigarettes ( nicotine stimulates serotonin production ) , as well as alcohol usage .
Of note here is the evolutionary tie-in between the depression and the rapid aggressive response . One would suspect that a rapid rise in serotonin concentrations would also lead to the easy triggering of extreme aggressive responses . ( Note side effects of Prozac)
Ie: the quiescent post-coital male is challenged and has to go to a fully aware state as quickly as possible . In most cases a blind rage will suffice as a defence , so that is selected for . This is not always sufficient against a canny opponent , so males will also be selected for a balanced aggression response .
The Dual Pathology:
As can be seen from the evolutionary argument above , it would tend to favor depressive males with homicidal rages .
The depressive stage is caused by lowering the serotonin levels below a certain threshold . Feedback mechanisms lower it further : ie trigger a full-blown depression .
The depressive stage triggers the female nursing response . The female nurses the male until the normal sexual aggression is restored , whereupon the cycle repeats . The components of the pathology are genetical and learned response .
The effect is a stable relationship of misery . Each partner fulfills a deep genetic need , but neither is close to their maximum potential .
There is also no evolutionary stopper to prevent too low serotonin levels .
For normal humans:
The male should take some serotonin precursors ( 5HTP or Tryptophan) immediately before or after coitus .
Intervention with serotonin precursors at the start of a depressive episode should prevent most episodes .
Intervention like this would raise general serotonin levels . Additional serotonin stimuli (like alcohol , nicotine ,etc) might trigger a rapid aggression response .