Question of the Week Podcast

Question of the Week episode

Thu, 14th Feb 2013

Are Humans Meant for Monogamy?

Couple (c) Kayugee

Many other species have multiple partners, but are humans meant for monogamy? Plus we ask, do flies have a super sense of smell?

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I believe it is reasonably well established that humans have a strong tendency to  be serially monogamous. The divorce rate would support that notion. Note that divorces could not occur without marriage (blindingly obvious) and that marriage, common as it is in some form in many (most) cultures is a formal societal recognition of the tendency to be monogamous. Thus the divorce rate itself supports the notion of the serial aspect.

The slight sexual dimorphism in body mass, if compared with mate selection in other mammal species, suggests a small tendency to polygamy, which we do see in a minority of cultures. Ophiolite, Mon, 11th Feb 2013

Compared to other primates, we aren't the most promiscuous but not the most loyal, either. Humans apparently fall into the "sort of monogamous" category, with pair bonding behavior and the occasional dalliance.
This is a interesting article that correlates monogamy and testicle size in primates.
cheryl j, Wed, 13th Feb 2013

I would wonder if the question is different for men and women.

Women would achieve the most "reproductive success" by having a stable home environment, and help with providing for and raising the children.  Pregnancy, of course, involves a large investment of energy, and in the past, it was dangerous.

For men, while the above may be good, they would also achieve reproductive success by spreading a few extra wild seeds around the neighborhood. CliffordK, Wed, 13th Feb 2013

With suggestions that within a decade we should have 100% DNA sequencing of babies at birth (or before), the frequently unspoken question/fear about "Who is the father?" will no longer need to be asked.

It will be interesting to see how the social implications of knowing the truth start to dawn on society. evan_au, Wed, 13th Feb 2013

There may be some things that it is better to not be informed of.

Some couples may decide that it is better to not know whether or not the biological father is the husband.  And, of course, ordering a paternity test may be a serious breach of trust.

I believe there is pioneer work on using peripheral maternal blood samples for fetal DNA analysis.  And, for that, one may need, or at least benefit from a paternal blood sample.  CliffordK, Wed, 13th Feb 2013

There are different evolutionary pressures on males and females, because females can have fewer offspring and invest more biological time and energy in producing them. That said, some researchers think the theory that women are programed to be monogamous and more faithful than males has been overstated. There are evolutionary benefits to to not putting all your eggs in one genetic basket so to speak, and maintaining social alliances to unattached males, should your mate get eaten by a tiger. Although females may not be as promiscuous as males, some primatologists say it is common for female primates to have a "back door chimp." cheryl j, Wed, 13th Feb 2013

Historically, marriage might have slowed the spread of diseases such as syphilis, gonorrhea, HPV, and etc, many of which are now treatable or preventable. 

However, if monogamy was practiced in prehistoric times, these diseases would never have evolved. CliffordK, Wed, 13th Feb 2013

I think it's important to note that "evolution" only really cares up to the age of reproduction. Males tend to be able to reproduce successfully for longer than females can (anecdotally in my head, anyway. I wouldn't mind seeing the exact number on that), and therefore the drive to seek out other child bearing females may be stronger (that's no excuse of course!).

I'd like to know the figures for the average age of people when they get divorced. Perhaps biology favours monogamy up to a point, then the curse of longer life spans set in and the "need" for monogamy fades. bizerl, Thu, 14th Feb 2013

The percentage of people living past their mid 40's during prehistoric periods may have been small, so a loss of fertility may not have been that significant.  However, there may still have been evolutionary benefits of selecting younger wives, including finding a wife that was likely to live and contribute until the child was an "adult". CliffordK, Thu, 14th Feb 2013

An analysis of mobile phone records showed that beyond a certain age, a woman tended to shift her main communication channel from a male of similar age (presumably husband/partner) to a woman of younger age.

Perhaps that reflects a woman's transition around the age of menopause from a focus on becoming a mother to being a grandmother?
At this stage, a woman's best strategy for reproductive success may be to help her daughter or niece?

See: evan_au, Thu, 14th Feb 2013

Supposedly the romantic crush phase of a relationship with lots of related circulating hormones lasts about two years, long enough for like 95% of fertile couples to conceive. I've read there is a big spike in divorce rates at about the 4th year of marriage. cheryl j, Fri, 15th Feb 2013

One of the basic lessons of Darwin is that the fate of any species depends on how well it adapts (quality) and on how well it reproduces (quantity).  My observation is that the male and female brains are differently hard-wired: the female primarily for quality issues, such as natural selection, while the male brain is assigned the solitary task of quantity. This is apparent in most mammalian social orders: primates, big cats, livestock, etc. but for the most part suppressed in humans. 

It can be observed that most of our social orders promote marriage & monogamy to the advantage of decidedly female objectives, while only a minority of cultures on this planet still encourage (and reward) polygamy.

My 45 years of marital monogamy notwithstanding, I have observed how much more readily MEN can 'stray' from their marital vows, whenever a 'safe' opportunity presents itself.  I believe the divorce rate most likely reflects this particular male behaviour more often than it supports the notion of serial monogamy. OldBill, Sat, 16th Feb 2013

Supposedly the romantic crush phase of a relationship with lots of related circulating hormones lasts about two years, long enough for like 95% of fertile couples to conceive. I've read there is a big spike in divorce rates at about the 4th year of marriage.

In my humble opinion people get married because they have it written into their DNA. It's always a fight though with another tug on dna. That tug comes from elsewhere and I'm not sure what it is. Probably the thought that being married is like beleif. I think that some religous cirlces call this haging onto an illusion of freedom. Pmb, Sat, 16th Feb 2013

Monogamy is a concept that is only valid in species that reproduce sexually and usually where there are two distinct sexes (i.e. barring hermaphroditic and sex-changing species.) The distinction between nature and nurture/learning as the predominant impetus for monogamy is blurry. Take, for instance, penguins, which are extremely monogamous. In their extreme environment, it may be understood that the rearing of a single offspring may take the lion's share of the work-effort of two parents, and it is the reproductive best interest of both parents to raise the offspring together. A successful pairing is evidence for future a successful pairing with the same partner in the next season. On the other hand, take the case of a stallion. The environments are not as harsh and the populations have the luxury of wide expanses/ranges. In this case, the reproductive strategy for the male would be to mate with as many females as possible in order to assure a higher likelihood that one of the matings will produce their own offspring and to ward off or kill potential cuckolds. In western cultures, this behavior would be construed as promiscuity and (what I would call) ‘sexual-jealousy. The male cannot ever be certain that the product of his pairing with a female will be his own offspring. In the case of the female, she is assured, 100% of the time, a successful mating will forward her genetic heritage. However, during gestation, her physiologic needs and her vulnerability begin to rise as she must nourish the developing fetus and subsequently nurse it and protect it after birth. Any manner in which the female can bond and recruit the assistance of the male would be of tremendous survival value. If the male ‘wanders’ and distributes his attention among several females, the amount of support becomes diluted. Hence, the root of what I believe may be considered among the cognitive beings of western cultures as ‘emotional-jealousy.’ Empirically speaking, I think most men would recognize that they do not necessarily connect sexual relationship with emotional love, at least nowhere as nearly as do women. On balance, it is so that, among humans, the concept of monogamous relationships is largely cultural and learned. It is not natural for human males to be monogamous. As with other herd animals, the only way that a male can be most assured that an offspring is his own, is to pair with more females. For females, there is the assurance that 100% of the time, their offspring carries forth their genetic heritage. Sal Napoli, Sat, 16th Feb 2013

It would be interesting to see a statistical distribution of the answers and comments you get here, between men and women. My bet is that far more women will contend that monogamy is the natural order of things. Sal Napoli, Sat, 16th Feb 2013

Sal - That would be a safe bet, due to the unliklihood that very many female forum members also moonlight as 'Ladies of the Evening'. OldBill, Sat, 16th Feb 2013

I'm seeing statistics all over the ballpark.  But, keep in mind that it isn't only the men that cheat.  And, cheating necessarily involves 2 people.  I.E.  If married men are having affairs, then there are also the women who are having the affairs with them.

19 percent of women and 23 percent of men reported cheating

And another study:
Percentage of men who admit to committing infidelity in any relationship they've had: 57%
Percentage of women who admit to committing infidelity in any relationship they've had: 54%

Percentage of men who say they would have an affair if they knew they would never get caught: 74%
Percentage of women who say they would have an affair if they knew they would never get caught: 68%

Although, perhaps the reason for infidelity is different for men and women. CliffordK, Sat, 16th Feb 2013

Perhaps a better way for the male to ensure that his line is carried on is to enter a monogomous relationship, protecting the female from other males and ensuring that he's the only man for her.

A male cannot be "most assured that an offspring is his own" no matter how many different partners he has sex with, if all the other males are doing the same thing. If he knows that he is the only male who has had sex with a particular female, he is assured of his genes passing on, and in return, the female has acquired a helper and protection during the vulnerable pregnancy and early childhood stage. bizerl, Sun, 17th Feb 2013

What if the spousal selection criteria does not promote monogamy.

So many women seem to be attracted to the "Bad Boy" image, at least in theory.  The thrill & excitement.  And, perhaps men are attracted to women who are self-centered. 

Anyway, it may mean that the men and women are seeking ideals that don't promote stability in a relationship. CliffordK, Mon, 18th Feb 2013

I think the well-intentioned OP began with a false premise about divorces.

Why would nature make an effort to have the sex ratio almost exactly 100:100  (that is, 100 males per 100 females) for humans at the start of their reproductive lives (~20yo)?

First, many more males are conceived than females (roughly 120:100 to 150:100). Due to more spontaneous abortions of male fetuses, the sex ratio at birth is about 105:100, and due to more male deaths than female deaths during childhood, the sex ratio ends up almost exactly 100:100 at the start of their reproductive lives.

As it is, China's one-child policy has caused a shortage of about 30 million women there to date due to aborting female fetuses and committing female infanticide. A couple more generations of this policy, and there will be even more serious consequences. I'm guessing the men have started marrying younger and younger women (marrying them out of future generations), or they will marry they away from other countries. But there's no simple cure for unbalanced sex ratios in adulthood for humans.

My conclusion is that monogamy (of sorts) in humans is optimal even though divorce occurs. A divorce only seems to mean that a particular couple was a monogamy mismatch. Lmnre, Mon, 18th Feb 2013

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