Proposition 8 overturned

… by the California Supreme Court the Federal Ninth Circuit Court of Appeals. Proposition 8, of course, was a ballot initiative that amended the California state constitution to forbid state recognition of same-sex marriages, passed in 2008 after the state Supreme Court legalized same-sex marriage, and thousands of couples had become married.

From the text of the decision, which is a model of sharp language:

All that Proposition 8 accomplished was to take away from same-sex couples the right to be granted marriage licenses and thus legally to use the designation of ‘marriage,’ which symbolizes state legitimization and societal recognition of their committed relationships. Proposition 8 serves no purpose, and has no effect, other than to lessen the status and human dignity of gays and lesbians in California, and to officially reclassify their relationships and families as inferior to those of opposite-sex couples. The Constitution simply does not allow for “laws of this sort.” [Emphasis added.]

The logic of the decision seems to lean on the fact that California had previously legalized same-sex marriage, and that this right was eliminated by Proposition 8; it doesn’t address the broader question of whether it’s unconstitutional to deny marriage to same-sex couples in states that have never previously recognized such relationships.

Expect an appeal to the U.S. Supreme Court—which, let’s not speculate what they’ll do just yet. ◼

Sex after dawn: Marriage and natural selection

ResearchBlogging.orgThe book Sex at Dawn, by Christopher Ryan and Cacilda Jethá, has had a lot of press in the last month—it first popped up on my radar with Eric Michael Johnson’s review for SEED, and then it became unavoidable (for me, anyway) when Dan Savage devoted a whole column and podcast to it. The thesis of Sex at Dawn is that early humans were highly promiscuous, and that modern expectations of monogamy are probably not consistent with our biology. I haven’t read the book yet, but the discussion surrounding it has largely missed an important detail—human evolution didn’t stop when we invented agriculture.

In fact, we’ve evolved in response to agriculture. My capacity to digest milk proteins at age 28—most other mammals lose this ability as soon as they’re old enough for solid food—is the result of natural selection acting on my northern European ancestors. Sex at Dawn coauthor Christopher Ryan acknowledges exactly this, citing the same example, in a recent response to a question on his blog. I’m not aware of a study that documents human evolution in response to marriage customs, but conveniently enough, an article in the current issue of The American Naturalist does show that a population’s marital customs can shape its response to natural selection [$a].

Vintage postcard via chicks57.

The intensity and nature of natural selection often varies with age—it’s strongest on traits expressed prior to and during the period of life when most reproduction happens, and weaker on traits having to do with life after reproduction is mostly complete.

The new paper examines the effect of marriage on this relationship between age, reproductive activity, and the strength of natural selection. The authors are able to do this thanks to church records of births (via christenings), marriages, and deaths from four Finnish towns during the nineteenth century—a deep multigenerational dataset. The society described is probably as far away from the Sex at Dawn world of communal relationships within hunter-gatherer tribes as Western society has ever gotten—the births recorded are all in the context of monogamous marriages. How monogamous these marriages actually were is debatable; this was also a world before paternity testing. But the study follows women, who would probably have had less opportunity, and certainly had less social leeway, for affairs outside marriage.

Within that society, women’s reproductive success depended strongly on their husbands’ economic status, as approximated by whether or not they owned land. Women who married landowners married almost three years earlier, on average, than those who married non-landowners (between 24 and 25 years old, compared to 27). Women who married at an earlier age generally had more children survive to age 15, the paper’s benchmark for lifetime fitness—and this effect was stronger for women who married landowners.

Vintage wedding portrait via freeparking.

This meant that the intensity of potential natural selection acting on women in the study group peaked around their 30th birthday, declined slowly for around a decade, and then dropped off sharply. By comparison, an estimate of selection intensity based only the women’s probability of survival to a given age (i.e., without accounting for the need to marry before having children) just shows a steady decline with age. So marriage customs probably shaped the way natural selection could act on this population. (The comparison made, though, is a pretty facile one. I’d love to know what the intensity of selection looks like under other post-agricultural mating systems like, say, polygyny.)

Does that mean that these nineteenth-century Finns were evolving in response to the strictures of monogamy? Not necessarily. This study only estimates how strong selection would be on a trait relative to the age at which it’s expressed. That is, traits that reduced (or improved) a woman’s ability to bear children would be more strongly selected against (or favored), if they were expressed while she was between the ages of 30 and 40.

So the fact that marriage customs shape natural selection doesn’t mean that we’ve evolved to be better adapted to current marriage customs than we are to those of pre-agricultural hunter-gatherers. Marriage customs vary considerably among cultures, and over time—I don’t know of any culture that has maintained strict monogamy since the origin of agriculture. Even if a single culture did prefer monogamy that long, natural selection to adapt to that mating system would be working from a pool of genetic variation evolved from hundreds of generations of our earlier polyamorous lifestyle. It doesn’t matter how strongly natural selection would favor a perfectly monogamous person, if such a person doesn’t exist.

In other words, the key insight of Sex at Dawn—which is also a key insight of evolutionary biology in general—is right: What we can become is shaped by what we used to be. That’s certainly an important thing to keep in mind when considering a commitment that lasts till death do you part.

(For example, you might want to makes sure your significant other is of at least the same genus as you. I mean, talk about biological impediments.)


Gillespie, D., Lahdenperä, M., Russell, A., & Lummaa, V. (2010). Pair-bonding modifies the age-specific intensities of natural selection on human female fecundity. The American Naturalist, 176 (2), 159-69 DOI: 10.1086/653668

Get government out of the marrying business

History professor Stephanie Coontz proposes, in an op-ed piece in today’s NT Times, that the U.S. government should give up the business of officially sanctioning marriages:

Perhaps it’s time to revert to a much older marital tradition. Let churches decide which marriages they deem “licit.” But let couples — gay or straight — decide if they want the legal protections and obligations of a committed relationship [emphasis added].

This strikes me as a logical continuation of the principle established by the reformation-era Anabaptist movement. Used to be that the state (via state-sponsored churches) had a hand in baptisms, because they were a handy time to register newly born citizens for taxation and the draft. Then the Anabaptists came along and opposed infant baptism – and five hundred years later no one thinks it at all odd that baptism is a purely religious rite.