I want to teach you all a vocabulary word - or a vocabulary phrase, actually:phenotypic plasticityThe reason I want you to know this is that I've been attempting to read "Sex at Dawn," a book that argues (as far as I can tell) that we are not designed for monogamy but rather for, I guess, non-monogamy in whatever form that might take. I can't tell you specifically what it is the book says we're meant for because I'm having a really hard time getting through it. Because it pisses me off.It pisses me off because we live in this moment of cultural awareness, where people are ditching their running shoes and running barefoot, like we did on the savanna, in our earliest evolutionary days; we're ditching bread and eating "paleo," like our pre-agricultural revolution evolutionary forebears; and apparently we're looking to our pre-historic, pre-agriculture ancestors for tips about love and relationships.It makes sense for nutrition and shoes in a way that it does NOT, for love and relationships. It is both lacking in science and hopelessly misguided. Lacking in science: our social structure doesn't leave a fossil record, so anyone who proposes an idea about how we lived on the savannah is basically just making up a plausible story. Misguided: welcome to phenotypic plasticity.Phenotypic. As in phenotype. The observable manifestation of a genotype. The color of your eyes, your height, your immunity to infectious disease, your temperament.Plasticity. Flexibility. Adaptability. Changeability.Phenotypic plasticity, then, is the capacity for a phenotype - the observable expression of a gene - to vary depending on the context in which is develops and expresses itself.Imagine if the color of your eyes were determined not just by your parents' genes but by your level of nutrition early in life. That's the kind of thing.It turns out that, very approximately, the more complex a trait is, the more plastic it is likely to be. Unsurprisingly, human sociosexual systems are MASSIVELY plastic.What influences human sociosexual systems? Christ, a BUNCH of things. Population density and resource abundance are two biggies - and resource abundance appears to be relative rather than absolute, so even the unprecedented abundance of the C21st western world can be interpreted biologically as scarcity, if you're among the "have nots."The result is that for most (about 80%) of human history (and I do mean history), we've been a nominally polygynous species. That's our mid-level abundance structure. When resources became more abundant, we transitioned into a model of nominal monogamy. And in circumstances of extreme resource paucity, we generate polyandrous cultures. (There's only one example that I've read about, the one in Tibet, in which multiple brothers marry one woman.)Before then, what social structure did we have? F*** KNOWS. I enjoy Sarah Blaffer Hrdy's picture of us as tribes of women and children, visited by wandering males. But there's no more reason to believe that theory than any other.And extant pre-literate cultures don't provide one helpful model to follow, they simply VARY. Just one example: the Canela of Brazil include in their wedding vows, "Don't be jealous of your spouse's other sex partners."This is easy to understand in the context of nutrition: traditional Esquimo diet consists almost exclusively of fish and other sea-dwelling animals, because that's what's available, right? Until we developed agriculture, we ate what we could get.With sex and love, it's less directly about what our environment affords and more about how the affordances of the environment shape resource distribution among the population of humans.(Obviously it's all much more complicated than this.)So no. We are not "meant" to be monogamous, nor are we meant to be polygynous or polyandrous or polyamorous or anything else. We are meant to be successful at bearing, birthing, and raising offspring to reproductive age, who then bear us grandchildren. And we, as a species, will do whatever it takes to make that happen.