Aglets, I think you originally misinterpreted what Mithra meant by "largest penis". I think he meant that the average penis size in the human male is comparably much larger than the average penis size of any other primate, which is interesting from an evolutionary standpoint.
I would throw out that our larger penis size than other primates may be due to larger vaginas that are needed to pass huge human heads, that unlike about all other mammals come out with unfixed skulls and keep growing after birth to accomodate our huge brains. That is part of why we rear our young for 20 years (instead of one summer as most of the animal kingdom), and the biggest part of why we rule the planet. How big was big enough? Turns out 5.5-7.5".
I could see this as being possible when it comes to penis girth, but not length, and even then, I'm not convinced that it's a good explanation.
I believe you are correct that the circumference of the infant's skull continued increasing because babies with larger brains were probably more intelligent after they matured and out-competed adults who were infants with smaller heads, by natural selection, thus they had more opportunities to reproduce and the only thing that inhibited that feedback loop and prevented baby's heads from simply growing larger and larger with every generation were the limitations of the female's ability to bear a large skulled child. Even now, 4% of women suffer from cephalopelvic disproportion, which means that the baby's skull is too large to pass through her pelvic opening. Before modern cesarians, those women simply died during childbirth. Eighty percent of first time mothers suffer from perineal tears during childbirth, unless they are given an episiotomy, which is simply the cutting of the perineum to prevent a difficult-to-repair perineal tear. Perineal tears occur when the child's head does fit through the cervical opening and the pelvic opening, but it does not fit through the vagina, causing it to rip open during delivery. Before modern medicine, women who suffered from 3rd and 4rth degree tears probably died from blood loss soon after childbirth and most certainly from infection later on, but any perineal tear would have put the birth mother at risk for death from infection. Many mothers died before modern medicine, because of the incredible size of the human newborn's head.
So I would think that natural selection would increase the survival of mothers who have wider vaginas, vaginas that are not as tight, so that they don't tear during childbirth. However, that didn't happen, did it? The majority of women tear during childbirth, putting their lives at grave risk. Why is that? Is it because once the child is born, she has passed on her genes, so that if she dies from an infection
after the baby is born that it has no effect on natural selection? This might be true, and if it's true, then childbirth would also have no affect on the evolution of penises either, since those "selections" were made prior to childbirth, when it mattered. If orphaned babies were always adopted by living mothers who nursed and raised those babies as their own, thus giving the larger-brained-mommy-killing babies an advantage over the smaller-brained-non-mommy killing babies, by natural selection. Of course, we don't know what happened to orphaned babies.
Thinking along another line of reasoning, a shorter wider vagina would probably tear less than a longer skinny vagina. The deeper inside a woman's vagina that the perineal tear extends, the greater the complications, and the greater the likelihood of death from infection or bleeding to death.So if childbirth had a strong influence upon the evolution of large penises and the vaginas that they fit into, then it seems like the circumference of the male's penis should be closer in size to the size of an infant's head, as well as the female's vagina.
In fact, what I wrote above is partially true. The woman's cervix (the opening to her uterus) isn't at the end of the vagina. In most women, it's located 3-5" inside her vagina and longer penis pass by the opening and thrust into the Pouch of Douglas, also known as the cul-de-sac. (In fact, my own cervix is only 2.75" inside, but we're all just a little different.) Since the Pouch of Douglas isn't necessary for fertilization, being deeper inside the woman than her actual cervix, it would seem that it only exists to accommodate penises that are longer than she needs for fertilization. By having a cervix that is closer to her vaginal opening, she's more likely to become pregnant regardless of the length of the man's penis, and during childbirth there is less vagina to tear as the child passes down the vaginal canal.
So there are all my thoughts on the influence of childbirth on the evolution of human genitals. I have to say that after writing out my thoughts, I strongly suspect that all selective forces regarding the mother's vagina and the father's penis probably ended once the child exited the mother's body, and that I believe that means that the sexual selection forces that I wrote about before were probably more influential in selecting for the evolution of women's body characteristics (big breasts at puberty as an indicator of sexual maturity instead of only before she begins nursing, her small frame and relative physical weakness, and the small waist) and social selection forces were probably more influential in selecting for the man's penis size. After all, once we were already at the top of the food chain, natural selection usually begins to give way to other evolutionary forces.
(Just to clarify: I realize that sexual and social selection are types of natural selection, but when I wrote "natural selection" above, I am referring to the "survival of the fittest" model and direct competition type of selection, contrasted against social and sexual selection.)
