The PR aspect of science these days is wild. Here we are in 2017, the head of the EPA pretending we don’t have perfectly substantive data proving it’s our own fault the planet is too warm; simultaneously, doctors can’t say one way or the other whether it’s okay to eat carbs. We thought for sure, though, that one of the narratives we could all agree on was that no good can come from fucking your cousin. To be clear, this taboo is not global culturally, but it is biologically.

New research flies in the face of a whole host of genetic complications, though. Marriages between cross cousins — cousins whose parents are opposite-sex siblings — might have biological benefits after all, and that the bad rap it gets in the public discourse is (slightly) unfair. The research was published Monday in the journal Proceedings of the National Academy of Sciences.

In some populations, siblings of the same sex (the research doesn’t touch on this, but it appears to be considering only heterosexual practices) end up competing against each other for mates. Scientists studying the Amazonian Yanomamö tribe found that while individuals’ fitness — biology-speak for how prolific you are reproductively — suffers if they, their parents, or same-sex siblings marry a relative, they benefit if their children or opposite-sex siblings do.

The researchers analyzed the numbers of total grandchildren ever born among the Yanomamö, as well as number of spouses and impact of spousal relatedness, accounting for factors like patrilocality (when a married couple lives near the husband’s parents). You can see the whole excel chart here.

Benefit: More Children, Stronger Family Ties

They found that both men and women had more children when their opposite-sex siblings married close kin. Brothers in particular reap the benefits when their sisters pair off with cross-cousins. This means that inbreeding literally benefits the patriarchy, which yes, is a little on the nose.

Mate competition between same-sex siblings, especially brothers, can lead to conflict. But when parents arrange a marriage, as is still common in many cultures, they’re sometimes able to call upon female cross-cousins to pair off with unattached males, resulting in greater fitness for those men. This also brings about a strengthening of family ties when that “transaction” is reciprocated in the future, a phenomenon I’m going to call “sexual nepotism,” even though that sounds icky.

“Historically among humans marriage served the purpose of making ‘alliances’ with [other] groups of people,” first author Napoleon A. Chagnon tells Inverse. “Groups bound together by marriage usually also help each other economically, and in situations where [there] are conflicts that lead to deaths, groups defend each other.”

Inbreeding is genetically problematic because it can result in too many recessive genes being passed on and can make for a more vulnerable immune system. Cross-cousin marriages still result in inbreeding depression and reduced fertility, though it does mean that parents will have more grandchildren. But the researchers note that there’s actually an “optimal” level of inbreeding where fitness peaks.

Cousing-fucking is a cheap and easy joke in Western Culture because we use it as shorthand for people who are “backwards,” as a way of illustrating that we, who do not fuck our cousins, are not backwards. The reason we’re given to describing such things as “icky” is because we have been culturally conditioned to think they are so, and because many of us have watched every episode of Law & Order: SVU. But it doesn’t mean that we’re right. Inbreeding is a classic taboo, yet we don’t actually know where the taboo itself comes from. This research indicates that it may in fact have its origin in parent-offspring conflict — a way for individuals to preserve their own fitness levels by discouraging “close kin” pairings that might not benefit them directly. Not to mention, Chagnon tells Inverse, as a safeguard against factors like inbreeding depression.

“Because parents can influence the marriages of their children, these competitive and cooperative relationships can bleed into the next generation and eventually become embedded in the rules governing the marriage exchange system,” the paper reads. “In the Yanomamö, at least, cross-cousin marriage may be a partial resolution to mate competition between co-resident brothers.”

So while we still don’t fully understand what drives these familial dynamics, this research indicates that parental preference and sibling competition might be the root of the taboo. And the next time you’re tempted to make an inbreeding joke, just remember — it’s not all bad.

Abstract

Marriage in many traditional societies often concerns the institutionalized exchange of reproductive partners among groups of kin. Such exchanges most often involve cross-cousins—marriage with the child of a parent’s opposite-sex sibling—but it is unclear who benefits from these exchanges. Here we analyze the fitness consequences of marrying relatives among the Yanomamo¨ from the Amazon. When individuals marry close kin, we find that (i) both husbands and wives have slightly lower fertility; (ii) offspring suffer from inbreeding depression; (iii) parents have more grandchildren; and (iv) siblings, especially brothers, benefit when their opposite-sex siblings marry relatives but not when their same-sex siblings do. Therefore, individuals seem to benefit when their children or opposite-sex siblings marry relatives but suffer costs when they, their parents, or same-sex siblings do. These asymmetric fitness outcomes suggest conflicts between parents and offspring and among siblings over optimal mating strategies. Parental control of marriages is reinforced by cultural norms prescribing cross-cousin marriage. We posit that local mate competition combined with parental control over marriages may escalate conflict between same-sex siblings who compete over mates, while simultaneously forging alliances between opposite-sex siblings. If these relationships are carried forward to subsequent generations, they may drive bilateral cross-cousin marriage rules. This study provides insights into the evolutionary importance of how kinship and reciprocity underlie conflicts over who controls mate choice and the origins of cross-cousin marriage prescriptions.