Human Mating Systems: Polygyny (Robert Quinlan, ANTH 468, Washington State U.)

 

[***] indicates alternative links

 

Among animals and humans there are some general patterns in mating that seem regular enough to give them labels:

 

Polyandry [***] [***]:  Females have multiple mates at one time, males do not.

Monogamy [***]:  Females and males only have one mate at a time.

Serial monogamy [***]:  Both sexes have only one mate at a time, but both may have multiple mates over the life-course.

Polygyny [***]  [***]:  Males have multiple mates at one time, females do not. There are some important differences in polygyny between humans and other animals.

If we were forced to classify humans according to our mating system, it would be difficult to say whether we are “somewhat” polygynous or somewhat monogamous.  It’s true that cross-culturally the majority of societies permit polygyny, but it’s also true that the majority of human mating occurs within monogamous unions. Table 1 gives us some sense of the diversity.  What is clear is that our species mating pattern is variable. 

 

Comparison with our closely living relatives, the chimpanzees, suggests that variability in mating may be characteristic of our lineage that we share with our last common ancestor between chimps and humans.  Chimpanzees have a multi-male social organization, meaning that groups include several males and several females.  Within chimp groups there appear to be several variations on mating patterns: The typical pattern is for several related males to dominate the group. This dominant fraternity shares sexual access to females and prevents other males from mating.  A different pattern is for one male and one female to establish a kind of relationship, then when the female enters estrus, she and the male split off from the group for several weeks, when they have sex repeatedly in secluded parts of the forest.  These “consortships” are temporary arrangements, sometimes between male and female friends and sometimes males coerce females into consortships.  Males involved in consortships may or may not be part of the dominant male coalition.  The frequency of consortships varies from one chimp group to the next, hinting at the kind of mating variation we see in humans.  Sometimes a single dominant male chimp can monopolize sexual access to females and the group may be effectively polygynous for a time.

 

Monogamy is very rare among mammals; only 2% of species are monogamous.  Monogamy is more common among primates: about 14% of species.  About 19% of human societies are strictly monogamous (figure 1), but even the data on human polygyny suggest that most marriages are monogamous even though the majority of societies permit polygyny.  Polyandry is very rare among mammals and humans. We return to a discussion of human polyandry later.

 

Figure 1. Frequency of Marriage Types Across Cultures

 

 

Human Polygyny

 

The question is this: Why would women (or females of any species) put up with polygyny?  The standard answer for most animals is that polygyny benefits female fitness.  Where male parental care is not important or where males do not defend resources, then it’s in all female’s interest to mate with the few males that show signs of having the best genes.  This means that some males will have offspring by multiple females and some males will have no offspring at all.  When males provide their mates with resources either directly or indirectly, then those males that are better able to accrue resources attract more mates than less able males.  We can calculate the benefits to females with an example:

 

Suppose it costs 2 units of stuff to produce a viable offspring. One guy has 4 units of stuff and another has 12 units.  Then any woman should prefer to mate with the wealthier guy even if she has to share him with a co-wife, because doing so is better for her fitness than is mating with the poorer guy.  If the wealthier guy divides his resources equally between two wives, then each can have 3 offspring. Whereas if a woman opted to marry the poorer guy rather than share a richer husband, then she could expect only 2 offspring.  The point at which differences among males in resources are great enough for females to prefer to share a mate rather than have one to herself is called the “polygyny threshold” (see Table 1).

 

Table 1. Male and female tradeoffs for polygyny vs. monogamy

 

Units of Stuff

Reproductive Success

Monogamous Man

4

2

Polygynous Man

12

6

Monogamous Woman

 

2

Polygnous woman

 

3

 

For many animals the polygyny threshold model probably explains why females would share a mate: It’s in the interest of their fitness to do so.  The polygyny threshold is especially relevant in the case of “resource defense polygyny” rather than “female defense” or “lekking.”  At first glance resource defense polygyny might seem appropriate for humans too.  After all, the Sultan of Morocco, Mulai Ismail (who we introduced earlier) had 4000 wives and probably had many times greater wealth than did the average guy of his time.  It’s turned out be surprisingly difficult to prove the polygyny threshold hypothesis for humans. 

 

Even if polygyny benefits women, Table 1 shows us that it is much more beneficial for men.  If some particularly powerful men can exclude other men from mating, then powerful men should motivated to coerce women into polygynous unions even if it’s not in his mates best interest.  This alternative to the polygyny threshold model might be called the “male coercion” model.  We design a study to test between the two models.  Within a population we simple compare the reproductive success of polygynous and monogamous women: Results showing that polygynous women have greater reproductive success than monogamous women indicate that we should reject the male coercion model in favor of polygyny threshold. Results showing that monogamous women have greater reproductive success than polygynous women indicate that we should reject the polygyny threshold model in favor of male coercion (Table 2). 

 

Table 2. Women’s predicted reproductive success

 

Polygyny threshold

Male coercion

Polygynous Women

+ (6 offspring)

- (4 offspring)

Monogamous Women

- (4 offspring)

+ (6 offspring)

 

Polygyny among the Dogon of Mali

 

Beverly Strassmann studied polygyny among the Dogon of Mali in West Africa.  The Dogon are farmers who grow millet and other grains for subsistence and onions for a cash crop.  Like the Kipsigis, they are patrilineal and they have arranged marriage.  A man’s parents arrange his first marriage; he arranges subsequent marriages himself.

 

http://mali.pwnet.org/geography/geography_location.htm

 

Dogon society is strongly male dominated. 46% of Dogon men had more than one wife.  This is a high rate of polygyny when considered cross culturally.  Women spend 21% more time working than do men, and men spend 29% more time resting than do women.  Men control agricultural produce: A man gives equal portions of millet to each wife, but he keeps the largest portion of the best quality millet for himself.

 

 

http://www.unc.edu/courses/pre2000fall/anth134/images/dogon%2011.jpg

 

 

http://www.jorgetutor.com/mali/dogon/dogon.htm

 

The Dogon have a strong notion of “menstrual pollution,” meaning that they think that menstrual blood is toxic.  They practice “menstrual seclusion”: When a woman gets her period, she must go to a menstrual hut and wait until her period is over.  Dogon women claim that menstrual seclusion and taboos associated with menstruation are not pleasant.  They interrupt daily routines etc.  Beverly Strassmann has argued that menstrual seclusion is way for Dogon men to track their wives’ reproductive cycle.

 

The Dogon also practice clitoridectomy, the surgical removal of the clitoris at adulthood.  This practice is thought to reduce women’s sexual desire and, hence, control their sexuality, which it may do.  Sex may be quite painful for a “circumcised” woman.

 

First wives don’t have any particular power.  Each wife has her own living space. Men are expected to sleep with their wives on alternate nights.

 

Infant and child mortality are high among the Dogon.  About 20% of babies die in the first year, and about 46% of children die by age 5.  Apparently the child mortality rate has improved somewhat in recent years, but it is still quite high.  Malaria, measles and diarrhea are the main causes of child deaths.  (Did you know that diarrhea is the main cause of child and infant mortality the developing world?  Diarrhea itself does not kill children, but it causes deadly dehydration.  Death from diarrhea is often easily preventable with “oral rehydration therapy”.)

 

 

http://www.jorgetutor.com/mali/dogon/dogon.htm

 

Beverly found that wealthier men had more wives. At first glance this finding might seem to support the polygyny threshold model.  But remember that Dogon women do more work than Dogon men. She also found that additional wives increased the amount of land under cultivation that a man had control over.  The additional labor may explain men’s greater wealth rather than greater wealth explaining men’s ability to have multiple wives.

 

There was not a big difference in the fertility (number of births per woman) of polygynous versus monogamous Dogon women.   Polygynous women tended to have more lives births than did monogamous women (figure 2).  This finding might support the polygyny threshold model.  Recall, however, that there are two determinants of fitness: survival and reproduction. 

 

 

Figure 2. Dogon women’s fertility*

*Fertility was estimated from data on Dogon RS (Strassmann & Gillespie 2002) and child mortality (Strassmann 2000).

 

 

http://www.jorgetutor.com/mali/dogon/dogon.htm

 

In contrast to fertility, there was a huge difference between polygynous and monogamous mothers in their child mortality rates (figure 3).  Polygynous mothers could expect to lose about 1 in 3 of their children in the first five years after birth.  Imagine that! If you had three children you would be almost certain to lose one of them.  Monogamous mothers only lost about 9% of their offspring in the first five years.  So, the fertility data support the polygyny threshold model, but the child mortality data support the male coercion model.  From a quality of life standpoint monogamous women are better off – they have fewer pregnancies over their life (I hear that pregnancy is not that much fun) and they fewer of their children die.  But what about fitness (genetic representation in future generations)?   Reproductive success (number of surviving offspring) is one of the most common measures of fitness available.  Number of surviving grand-children would be a better measure of fitness, but you can imagine how hard it would be study that among living women.

 

Figure 3. Dogon child mortality (% died by before 6th birthday) by mom’s marital status

 

 

Infant and child survival tends to have very consistent pattern across cultures and populations.  The risk of death is almost always highest in the first year of life.  Child mortality tends to remain high until about age 5, then tapers off. In most populations if you make to 5, then you are likely to survive until adulthood. If you make it 10, then you are very likely to make it to adulthood.  Hence human behavioral ecologists tend to measure reproductive success as the number of children that survive to either 5 or 10 years of age.  Among Dogon women monogamous mothers had more offspring surviving to age 10 than did polygynous mothers.  The difference is not huge, but it is “statistically significant” meaning that it is not likely to have occurred by chance (figure 4).

 

http://www.jorgetutor.com/mali/dogon/dogon.htm

 

Figure 4. Dogon women’s reproductive success (offspring surviving to age 10) from Strassmann & Gillespie 2002.

 

 

http://www.ac-versailles.fr/etabliss/lp-corbusier-cormeilles/images/Mali%202001/026.Anciens%20village%20Dogon.Photo%20numerique.JPG

 

 

Table 3. Effect of polygyny on Dogon female fertility, child mortality & reproductive success

 

Fertility

Child mortality

Reproductive Success

Monogamous woman

5.3

9.09%

4.8

Polygynous woman

6.0

31.40%

4.1

 

In sum, both from a fitness standpoint and quality of life standpoint monogamous Dogon women are better off than are polygynous Dogon women.  A polygynous Dogon man, however, can expect to have at least 8 to 12 or more surviving offspring compared with about 5 for a monogamous man.  Based on these data we can “reject” the polygyny threshold hypothesis and accept (or “not reject”) the male coercion model of polygyny at least for the Dogon.  (Note that if a hypothesis is “not rejected,” it does not mean that is proven to be true. It does mean that the hypothesis “survives” to be tested again, and it is provisionally accepted as the best available explanation.) 

 

Despite support for the male coercion hypothesis, the question remains, why do Dogon women put up this situation?  For now the short answer is that Dogon men have perpetuated a culture that allows them to dominate women.  We’ve already discussed several cultural practices apparently geared toward controlling women’s sexuality such as menstrual seclusion and clitoridectomy.  Other Dogon marriage practices put pressure on women to accept polygynous unions.  Dogon men are on average 8 years older than their first wife, and they may be many years older than subsequent wives.  The structure of the Dogon population coupled with the age difference between men and women at marriage creates an artificial shortage of marriageable men.  Figure 5 shows a population pyramid for Mali: The pyramid shows the number of males in each age group on one side and the number of females in each age group on the other.  Age groups are indicated in the center of the graph.  Notice that the 5-9 year old cohort is much smaller than the 0-4 year old cohort. This reflects infant and child mortality for the entire country of Mali. The difference between the 0-4 and 5-9 year old cohorts would be greater for the Dogon alone.  Now compare the number of males in the 30-34 cohort to the number of females in the 20-24 cohort. Notice that there are many more females (about 80% more), which means that women have to share husbands.

 

Figure 5. Mali population pyramid

 

Summary & Conclusion

 

But does the Dogon case for male coercion “generalize” to other populations? Are their differences between human populations in the costs and benefits of polygyny for women? A study of polygyny by Steve Josephson showed that polygyny rarely benefits female fertility (figure 6).  When the fertility of monogamous and polygynous women was compared, in most societies monogamous women had higher fertility than did polygynous women.  In the 9% of societies where polygynous women had higher fertility they may have lower reproductive success like the Dogon.  This cross-cultural data is not conclusive, but it suggests that the polygyny threshold model may not account for human polygyny.  Male coercion appears to be a better explanation for human polygyny.

 

Male coercion may arise from the substantial differences in male and female reproduction that we explored in our discussion of sexual selection.  Male dominance may have roots in biology, but that does not mean we have to accept it.  After all, gonorrhea and cancer have roots in biology, but we want to prevent and cure those biological illnesses.  The situation is not much different with biosocial ills.  We have to understand the interactions of biology and culture for cancer and gonorrhea before we can hope to prevent and treat them—the same is true of social problems associated with male dominance.   

 

Figure 6. Studies of human polygyny and fertility

 

 

 

 

Further reading:

Strassmann, B. & Gillespie, B. 2002. Life-history theory, fertility and reproductive success in humans. Proceedings of the Royal Society: Series B. 269:553-562.

Strssmann, B. 1997 Polygyny as a risk factor for child mortality among the Dogon. Current Anthropology 38:688-695.

Strassmann, B. 2000 Polygyny, family structure and child mortality: A prospective study among the Dogon of Mali. In Adaptation and Human Behavior: An Anthropological Perspective. L. Cronk, N. Chagnon & W. Irons (eds.).

Josephson, S. 2002. Does polygyny reduce fertility. American Journal of Human Biology 14:222-232.