Having discussed who can get married and what they do it for, let’s talk about what shape the marriage can take — by which I mean how many people are in it, and how they’re officially connected to one another.
For most of us, monogamy is the most common structure (mono- meaning “one,” and -gamy meaning “marriage”). That is to say, someone in a monogamous marriage has only one spouse. That person is most commonly of the opposite sex, but not always, especially these days. But it’s worth remembering that unless you come from a religion or a culture that absolutely forbids remarriage after divorce or death, monogamy doesn’t mean having only one spouse, period; it means having only one at a time.
This stands in contrast to polygamy, i.e. many (or rather, multiple) spouses at once. We have a tendency to treat this word as if it means “multiple wives,” but it doesn’t: in fact, there are two more specific words to differentiate multiple wives (polygyny — the second part meaning “woman”) from multiple husbands (polyandry — “many men”).
The confusion probably arises because of the two, polygyny is the much more common structure. There are lots of reasons for this, many of them related to the patriarchal nature of most societies; when men hold power and women are treated as property, then it’s not surprising that the most powerful men will accumulate a larger share of that property to themselves. That doesn’t mean polyandrous societies are matriarchal, though. You see that pattern mostly in the Himalayas, where it’s a solution to the problem of inheritance and limited resources: dividing up the land between all your sons means pretty soon nobody will have enough to live on, so instead you marry all your sons to a single woman — fraternal polyandry. The land stays in one family, and there are fewer children, so you don’t wind up straining your resources. (Other societies accomplish the same end by shipping spare sons off to monasteries instead.)
Polygamy is illegal in the United States and many other Western countries. We tend to have a negative view of it in this country, associating the practice with Mormon fundamentalists in the Mountain West, living isolated from the outside world and engaging in abusive practices including, but not limited to, marrying adolescent girls to older men. It’s also found in some Muslim-majority countries, and used to be legal for Hindus in India until the mid-twentieth century, not to mention historical practices in Judaism — in fact, if you look at history, there are many parts of the world where it was once common.
Is polygynous marriage bad for women? That’s a difficult question to answer. It can be — but then again, so can monogamous marriage. Sometimes the senior wife is cruel and controlling toward the junior wives, but sometimes having co-wives is a source of comfort and support, especially when it comes to tasks like child-rearing. One of the sources I read when researching polyandry for Within the Sanctuary of Wings pointed out that being alone among a group of brothers can be incredibly isolating for women, contrasting that directly with the more female-dominated community of a polygynous marriage. In the end, it’s tough to disentangle the actual effects of polygyny from the surrounding tendencies toward patriarchy, away from female education, etc.
I do want to note, though, that what I’ve been discussing so far is polygamy in its more traditional sense, rather than the modern practice of polyamorous relationships (which in most cases can’t be legally recognized as marriages). The former is very much a setup where multiple people of one sex are married to a single person of the opposite sex, without necessarily implying relationships directly between the same-sex partners; the latter is often (though not always) more communal and egalitarian, and is sometimes termed “group marriage” instead, especially if there are sexual relationships between all the spouses. Line marriage, found in Heinlein’s fiction, is a variant on this where the marriage itself becomes a kind of lineage, adding new partners over time so that the family unit eventually contains none of the original founding spouses.
There’s also a separate-but-related subject in the form of concubinage and the keeping of official mistresses (or, more rarely, male lovers). That one is complex enough that I’ll save it for a later essay, but I want to note here that its more formalized version amounts to a type of polygamy wherein the junior women occupy an inferior status relative to the senior ones, and its less formalized version amounts to a type of polygamy with no legal recognition for anyone other than the official wife. Both have tended to be the province of wealthy and powerful men.
Which brings me around to the social effects of these different structures. I mentioned last week that one of the factors controlling who can get married is money; that’s even more true when multiple spouses enter the picture, because now you need to support even more people. When each wife is expected to maintain her own separate household — one of the methods used to reduce intra-family strife and competition — the outlay is substantial. Men in polygynous societies may take second wives for reasons ranging from status to political alliance to the need for an heir when the first wife is sick or infertile . . . but in such cases, the lament that they’re having trouble saving up enough money to afford that becomes very common.
And then consider supply and demand, so to speak. The human birthrate is basically 50/50 male/female. If some men are claiming multiple wives, that means other men have none. (Unless you have a society that is simultaneously polygynous and polyandrous. I don’t think that’s ever existed in reality, but now I want stories where it’s true.) This can work if you have a warlike society and a decent number of your men are getting killed in battle, skewing the overall gender balance toward women — and there’s a feedback loop there, because if you have a lot of young men running around without families of their own, the odds that they’re going to enter into violent conflicts go up, unless you stow them somewhere safe like a monastery.
It’s almost certainly going to be young men, too. Back when I talked about stages of life, I mentioned that power and influence (and money, though I didn’t say it there) tend to accumulate in the hands of elders. Spouses go hand-in-hand with such things. Couple that with the risk of dying young in childbirth, which has been a serious danger for women through much of history, and you inevitably wind up with lots of situations where older men are marrying much younger women. It doesn’t happen only because of patriarchal tendencies; it’s also the natural consequence of other factors.
So if you want to write about marriage structures other than the type of monogamy that’s common today, think about what factors will support that other structure, and what the effects of it are likely to be.