WrongBot:
There are no economically well-developed modern societies with a social norm other than monogamy, and there are some indications that ubiquitous birth control is a game-changer, so historical evidence may not apply. We're all arguing without large-scale evidence.
That's not really true. In large parts of many rich contemporary societies, monogamous norms have been weakened to the point where a great many people engage in non-monogamous sexual behaviors. Yes, even among those people, the majority seem to consider stable monogamy as a goal to be achieved at some point in the future, but they nevertheless spend significant parts of their lives engaged in casual serial monogamy and promiscuous sex. (And in some lower class environments, even the pretense of monogamous norms has nearly disappeared.)
There is definitely significant large-scale evidence here about what happens (in at least some cases) when monogamous norms break down in a wealthy society. This evidence points quite unambiguously towards female hypergamy, where a minority of exceptionally attractive men account for the overwhelming part of non-monogamous sexual pairings that take place, and women at all levels of attractiveness strive towards men with higher relative status. You can of course dispute the relevance of this evidence, but you definitely can't deny its existence.
(And in some lower class environments, even the pretense of monogamous norms has nearly disappeared.)
You may be assigning too much credence to that news report. It's really just summarizing an argument between two partisan political parties about marriage's declining popularity among the poor. The only quantitative data cited is the number of UK marriages in 1972 and the number of UK marriages in 2009, which are not really enough to settle the claims made in the article or your parenthetical.
There is definitely significant large-scale evidence ...
Wh...
Many of us are familiar with Donald Rumsfeld's famous (and surprisingly useful) taxonomy of knowledge:
But this taxonomy (as originally described) omits an important fourth category: unknown knowns, the things we don't know that we know. This category encompasses the knowledge of many of our own personal beliefs, what I call unquestioned defaults. For example, most modern Americans possess the unquestioned default belief that they have some moral responsibility for their own freely-chosen actions. In the twelfth century, most Europeans possessed the unquestioned default belief that the Christian god existed. And so on. These unknown knowns are largely the products of a particular culture; they require homogeneity of belief to remain unknown.
By definition, we are each completely ignorant of our own unknown knowns. So even when our culture gives us a fairly accurate map of the territory, we'll never notice the Mercator projection's effect. Unless it's pointed out to us or we find contradictory evidence, that is. A single observation can be all it takes, if you're paying attention and asking questions. The answers might not change your mind, but you'll still come out of the process with more knowledge than you went in with.
When I was eighteen I went on a date with a girl I'll call Emma, who conscientiously informed me that she already had two boyfriends: she was, she said, polyamorous. I had previously had some vague awareness that there had been a free love movement in the sixties that encouraged "alternative lifestyles", but that awareness was not a sufficient motivation for me to challenge my default belief that romantic relationships could only be conducted one at a time. Acknowledging default settings is not easy.
The chance to date a pretty girl, though, can be sufficient motivation for a great many things (as is also the case with pretty boys). It was certainly a good enough reason to ask myself, "Self, what's so great about this monogamy thing?"
I couldn't come up with any particularly compelling answers, so I called Emma up and we planned a second date.
Since that fateful day, I've been involved in both polyamorous and monogamous relationships, and I've become quite confident that I am happier, more fulfilled, and a better romantic partner when I am polyamorous. This holds even when I'm dating only one person; polyamorous relationships have a kind of freedom to them that is impossible to obtain any other way, as well as a set of similarly unique responsibilities.
In this discussion I am targeting monogamy because its discovery has had an effect on my life that is orders of magnitude greater than that of any other previously-unknown known. Others I've spoken with have had similar experiences. If you haven't had it before, you now have the same opportunity that I lucked into several years ago, if you choose to exploit it.
This, then, is your exercise: spend five minutes thinking about why your choice of monogamy is preferable to all of the other inhabitants of relationship-style-space, for you. Other options that have been explored and documented include:
These types of polyamory cover many of the available options, but there are others; some are as yet unknown. Some relationship styles are better than others, subject to your ethics, history, and personality. I suspect that monogamy is genuinely the best option for many people, perhaps even most. But it's impossible for you to know that until you know that you have a choice.
If you have a particularly compelling argument for or against a particular relationship style, please share it. But if romantic jealousy is your deciding factor in favor of monogamy, you may want to hold off on forming a belief that will be hard to change; my next post will be about techniques for managing and reducing romantic jealousy.