Hormones, Monogamy, and Pregnant Sex
Don't read this if you're my brother or brother-in-law.
I'm on the endocrinology portion of my anatomy class. I think it's a little weird that they know that the level of the hormone oxytocin rises in the blood during sex and then spikes at the, uh, end. Most books say it is not known why this is. But they also claim that the same happens in a woman following childbirth. There is, if I remember right, a surge of oxytocin immediately after birth from a mother's hypothalamus, the brain's hormone center, through the pituitary gland and into general blood circulation. According to one book I read, this is a hormonal means to facilitate a psychological bond for the child. I'm not sure how they know that or why it would cause that. But I'm curious if the same hypothesis, if it's indeed realistic for postpartum bonding, could also apply to the oxytocin levels in both genders during sex: that oxytocin facilitates a psychological bond with the person in immediate proximity.* (Incidentally, the hormone vasopressin seems to be used this way as well.)
Oxytocin also causes uterine contractions. It is the primary hormone that instigates labor contractions. It also causes contractions during sex in pregnancy. So I imagine another reason oxytocin is released during sex is to tone and strengthen the pregnant uterus. It is generally recognized that sex can be used to speed along a labor that is initiating slowly.
Another point to consider: the prostaglandins in semen work to soften the cervix. According to our last midwife, single pregnant women typically (but I imagine not always) have more difficult labors.
Now, the last two points (oxytocin as a method of exercising the uterus during pregnancy, and seminal prostaglandins as a method of ripening of the antepartum cervix) indicate that sex doesn't simply initiate a human pregnancy but is intended to accompany it as well. Now, it seems presumable that if a pregnant woman was to have sex consistently throughout her pregnancy, even up to the day of labor, it would most likely be with the same partner. If human males were only intended to impregnate a woman and then go on their way, this elaborate system of sex facilitating childbirth would probably not have evolved. So it seems that human pregnancy revolves around a specific and reliable sexual union, i.e. a monogamous one. And those two theories along with the first (oxytocin as a bond-maker) seem to indicate that the human hypothalamus also has a monogamous sexual relationship in mind.
From this I would venture to say that humans are designed to form monogamous and durable sexual unions. There is a common anthropological assumption that 'early humans' were polyamorous and that monogamy is a late convention, but I don't think this fits with the above. Not even polygamy makes sense of it. There is also a more general idea that humans have a sort of blank sexual slate, or that human physiology has nothing to say about human sexual behavior. Rather this behavior is always explained by exterior forces, like evolutionary struggles, environmental pressures, or social conventions. I don't think this makes sense of the above, either. So I judge that humans are a monogamous species.
*It might be argued that if this is true, then sex should always make people stick together, when clearly it does not. But neither do all women bond well with the children they birth. Some women abandon newborns. As I said, this hormone facilitates a mental bond. It doesn't command the human mind; it influences its emotions and attachments.
I'm on the endocrinology portion of my anatomy class. I think it's a little weird that they know that the level of the hormone oxytocin rises in the blood during sex and then spikes at the, uh, end. Most books say it is not known why this is. But they also claim that the same happens in a woman following childbirth. There is, if I remember right, a surge of oxytocin immediately after birth from a mother's hypothalamus, the brain's hormone center, through the pituitary gland and into general blood circulation. According to one book I read, this is a hormonal means to facilitate a psychological bond for the child. I'm not sure how they know that or why it would cause that. But I'm curious if the same hypothesis, if it's indeed realistic for postpartum bonding, could also apply to the oxytocin levels in both genders during sex: that oxytocin facilitates a psychological bond with the person in immediate proximity.* (Incidentally, the hormone vasopressin seems to be used this way as well.)
Oxytocin also causes uterine contractions. It is the primary hormone that instigates labor contractions. It also causes contractions during sex in pregnancy. So I imagine another reason oxytocin is released during sex is to tone and strengthen the pregnant uterus. It is generally recognized that sex can be used to speed along a labor that is initiating slowly.
Another point to consider: the prostaglandins in semen work to soften the cervix. According to our last midwife, single pregnant women typically (but I imagine not always) have more difficult labors.
Now, the last two points (oxytocin as a method of exercising the uterus during pregnancy, and seminal prostaglandins as a method of ripening of the antepartum cervix) indicate that sex doesn't simply initiate a human pregnancy but is intended to accompany it as well. Now, it seems presumable that if a pregnant woman was to have sex consistently throughout her pregnancy, even up to the day of labor, it would most likely be with the same partner. If human males were only intended to impregnate a woman and then go on their way, this elaborate system of sex facilitating childbirth would probably not have evolved. So it seems that human pregnancy revolves around a specific and reliable sexual union, i.e. a monogamous one. And those two theories along with the first (oxytocin as a bond-maker) seem to indicate that the human hypothalamus also has a monogamous sexual relationship in mind.
From this I would venture to say that humans are designed to form monogamous and durable sexual unions. There is a common anthropological assumption that 'early humans' were polyamorous and that monogamy is a late convention, but I don't think this fits with the above. Not even polygamy makes sense of it. There is also a more general idea that humans have a sort of blank sexual slate, or that human physiology has nothing to say about human sexual behavior. Rather this behavior is always explained by exterior forces, like evolutionary struggles, environmental pressures, or social conventions. I don't think this makes sense of the above, either. So I judge that humans are a monogamous species.
*It might be argued that if this is true, then sex should always make people stick together, when clearly it does not. But neither do all women bond well with the children they birth. Some women abandon newborns. As I said, this hormone facilitates a mental bond. It doesn't command the human mind; it influences its emotions and attachments.