Kinship explains the I/C Dimension

Part Of: Culture sequence
Followup To: Individualism vs Collectivism
Content Summary: 3000 words, 30 min read

Forager vs Farmer

Homo erectus was a forager. We know this from the fossils: our anatomy diverged from the apes precisely for those features that most support running/hunting (including longer legs, foot arch, fat gluteus maximus, etc). Our species began hunting & gathering some 1,800,000 years ago.

9,000 years ago we switched to farming. Why then? Climate measurements from ice cores shed light on the answer. During the Pleistocene, the climate was extremely variable, with dramatic ecological shifts about every century. In fact, several periods of intense change have been linked to speciation events within the hominin line. 

But the same Milankovitch cycles later introduced periods of climate stability: 12-11 kya, and 9-0 kya. The first stable period correlates with our species first attempt at proto-farming: the Natufian period. Variability returned for another two millennia until the Neolithic revolution led to the true advent of agriculture. 

Why farm? The less nutritious diets of farmers left them shorter, sicker, and more likely to die young. The surplus also generated tremendous inequality, which induced severe warfare between farming communities, whose impact is visible today in the Y-chromosome. 

But farmers did reproduce more quickly than hunter-gatherers. With the “right” set of institutions, farmers could spread across the landscape like an epidemic, driving out foragers in their path. Early farming spread not because it was a better lifestyle, but because farming communities with particular institutions beat mobile hunter-gatherer populations in intergroup competition

Tight vs Loose Kinship

Ever since the Neolithic Revolution, human beings were given in marriage to their cousins & in-laws. The small cost of genetic disorders was more than compensated by social benefits of marriage alliances, creating a “Goldilocks Zone” of optimal mates at intermediate levels of relatedness (second cousins).

These tight (intensive) kinship systems emphasize relatedness-increasing social norms of kin marriages, polygyny, endogamy, and lineal fissions because these behaviors create strongly overlapping networks of kin that often co-reside in the same community. Intensive systems often include marriage alliances between lineages leading to cross-cousin marriages and converging networks of kin (Levi-Strauss, 1949).

In contrast, loose (extensive) kinship lacks most, if not all, of the above relatedness-increasing social norms (Yellen and Harpending, 1972). Marrying unrelated or distantly related individuals increases the total possible numbers of kin by inclusion of a wider cast of individuals to form a more diverse kinship network.  Extensive kinship systems include more geographically-distant marriages in other communities (Fix, 1999) to form alliances with a large number of affines in a diffuse kinship network (Bugos, 1985). 

In contrast to agricultural societies, foraging societies tend to employ loose kinship:

Kinship systems are solutions to economic problems. Loose kinship is useful in unpredictable environments and for nomadic populations given that it provides a plethora of residential options. Hunter-gatherers that exploit a diversity of unevenly distributed food resources may need to hedge their bets by having kin in different places in times of need (Yellen and Harpending, 1972). In contrast, tight kinship reduces the dilution of inheritable family wealth, and may help with kin-based resource defense of such wealth, which is more important for many agropastoral societies as opposed to hunter-gatherer societies (Borgerhoff Mulder et al., 2009). 

Correlates of these kinship systems can be summarized as follows:

Clans and Patriarchy

All pre-modern agricultural societies exhibited tight kinship. To motivate this claim, I want to explain the process by which states evolved.

In all societies, genealogy is the same. But genealogical concepts differ, drawing attention to certain aspects of your family tree, while eliding others. Societies either trace lineages through one parent (unilineal) or both (bilineal). Agriculturalists overwhelmingly use unilineal descent. Why? 

  • This agricultural lifestyle requires sedentism, which fosters community stability. This in turn promotes more elaborate kinship structures, and unilocality. 
  • The agricultural surplus incentivizes resource competition (warfare). In warfare, it is advantageous to construct raiding coalitions with few conflicts of interest. In unilineal systems, every person belongs to just one kinship group. 

There are two forms of unilineal descent: through the mother (matrilineal) or the father (patrilineal). Patrilineal inheritance is reflected in e.g., traditional European practices of the wife taking on the husband’s last name. 

Of these two options, patrilineal social systems are by far the most common. Many explanations for matrilineality have been proposed; one recent proposal suggests they helped building cross-cutting male sodalities for external warfare across meta-ethnic frontiers (Jones 2010). In contrast, Ember et al (1974) found that internal warfare helps predict patrilineality (easier to defend property when all sons live under the same roof) and contiguous lineages (best to keep extended family nearby to help).

Patrilineal social systems are known as clans

Incest aversion is an innate biological response to avoid genetic abnormalities. Known as the Westermarck effect, it is a form of reverse sexual imprinting. In contrast, incest taboos are human-specific norms instilled by culture, which extend the range of incest aversion.  Nearly all clans incest taboos typically forbid within-clan marriage, for at least two reasons. This suppresses sexual competition among men of the same clan and instead focuses their mating efforts outward, on nearby clans. Such exogamy also builds alliances with other clans. (Walter 2000).

Arranged marriages empower patriarchs to strategically use their daughter’s marriages to nourish their clan’s network of alliances. These alliances are reinforced by norms such as levirate marriage (e.g., Deut 25:5-10), which specify that when the husband dies, his surviving wife must marry one of his brothers. This sustains the marital alliance between clans. 

Agriculture won, clans won, consanguineous marriage won. This is why all major world religions (with the exception of the Roman Catholic Church, for reasons we’ll explore next time) are permissive of these intensive kinship strategies including cousin marriage (and, in the Hebrew Bible, also affinal and uncle-niece marriages).

Scaling Up: The Evolution of Statecraft

Agricultural groups compete for resources (surplus). Since group size is a critical determinant of success in intergroup competition, this placed tremendous pressure on groups to scale up. But scaling up is hard. But in the Sepik region of New Guinea, anthropologists have long noted that villages rarely exceeded 300 individuals (3-5 clans, each of which usually consist of several related lineages). If a village strove to grow much larger than this, fissions would erupt, and the village would split – typically along clan lines. Within-clan cooperation is typically not too difficult (aided by the evolutionary logic of Hamilton’s rule). 

However, the relentless logic of cultural group evolution did eventually find two ways to scale up. 

First, age-sets use rituals to better integrate clans. Psychologically potent initiation rituals (rites of terror; roughly, “hazing on steroids”) forge deep bonds between cohorts of males from different kin-groups. The phenomenology of these rituals is paralleled by comradery experienced by the military (band of brothers). After an initiation, norms specify that this cohort is endowed with a new set of privileges, responsibilities, and obligations. Age-sets often work, play and feast together as a unit. Failure to meet their cohort’s collective obligations could threaten to delay their next ritual promotion; which could preclude the entire group from marriage etc etc. 

Second, segmented lineages use lineage myths to bind clans. Norms demand that more closely related clans, who usually control adjacent territories, ally themselves against more distantly related segments. If there is a conflict between brothers, it will be settled by all the brothers, and cousins will not take sides. If the conflict is between cousins, brothers on one side will align against brothers on the other side. However, if the conflict is between a member of a tribe and a non-member, the entire tribe, including distant cousins, could mobilise against the outsider and his or her allies. That tiered mobilisation is traditionally expressed, for example, in the Bedouin saying:

Me and my brothers against my cousins, me and my cousins against the world.

I’ll let Henrich (2020) describe the implications:

This descent-based institution is built around personal and corporate honor. A man’s safety, security, and status—and his family’s—are linked to his reputation. Acts of dishonor can dissolve the reputational shield that protects his property and family from thieves or avengers, and they can reduce his children’s marital prospects and affect the reputation of his entire clan. Hence, relatives closely monitor one another (out of self-interest) and will punish each other in order to restore the honor of their family or clan. Supporting one’s lineage allies is central to each man’s honor. One unfortunate consequence of this is that any particularly aggressive clan could drag the entire maximal lineage into an enduring conflict

Even today, in a world dominated by territorial states, the impact of segmentary lineages can still be felt. In 21st-century Africa, tribal populations with segmentary lineages still experience significantly higher violence and civil war than populations without these institutions. Many familiar cases of chronic conflict in Africa are associated with populations organized by segmentary lineages; for example the two century-long civil war between Dinka and Nuer.  On the other side of the world, the echoes of the culture of honor that were part of Scotland’s segmentary lineages still affect life and death: in counties of the U.S. South, the higher the percentage of Scottish or Scotch-Irish residents in the first U.S. census in 1790, the higher the murder rate is today. The cultural descendants of these migrants still tend to respond aggressively by the honor psychology fermented in segmentary lineages. Boko Haram, Al Shabab, and Al Qaeda, for example, all recruit heavily from populations with segmentary lineages.

Age-sets and segmented lineages allow groups to scale from ~300 to ~3,000; but their success is constrained because they lacked stable, hierarchical authorities. 

Chiefdoms are formed when one clan gains ascendency over sister clans. In the egalitarian multi-clan groups described above, each clan is typically in charge of conducting certain rituals. Ritual ownership often corresponded to real power; e.g. a clan can forbid others from ritual fishing rights. Right to perform these rituals are often grounded in claims about ancestral gods. They can be challenged. Ritual takeovers are one way clans compete within a multi-clan system; given the lack of written language, changes become normalized within a few generations; and then the process may begin anew. In this way, certain clans can emerge as clearly dominant. It is not so much the chief that rules, as the clan (from whose ranks chiefs are elected). 

Recall that arranged marriages serve as an incredibly important social glue; the resultant alliances served to bind clans together and prevent fission. Stratified chiefdoms didn’t emerge until the upper strata (dominant clan) stopped intermarrying with the lower strata. This isolated the upper strata and allowed them to claim that they were fundamentally different from the lower strata—truly divine, superior, and deserving. 

Stratified chiefdoms can evolve into premodern states (kingdoms) as the dominant clan injects new bureaucracies between themselves and the lower strata. These institutions collect taxes, conduct long-distance trade, orchestrate public rites, and marshal armies. They also adjudicate disputes between clans; but premodern states usually left it to the clans to police their own internal affairs, including theft, assault, and even murder. 

Intergroup competition selects for social institutions that scale up. Its relentless Darwinian logic caused societies to scale from 300-person villages to (in the case of the Achaemenid Empire), some 30 million people. But once competition wanes, which often happens when states eliminate their competition, things slowly fall apart. Without the looming threats posed by competing societies, the competition among ruling families within a society will intensify and gradually tear the state-level institutions apart. 

Here, then, is how states evolved from clans:

From Henrich (2020):

At the dawn of agriculture, all societies were built on institutions rooted in family ties, ritual bonds, and enduring interpersonal relationships. New institutional forms were always built on these ancient foundations by variously augmenting, extending, or reinforcing the inherited forms. Social norms related to family, marriage, ritual, and interpersonal relationships became more complex and intensive as societies began to scale up. Crucially, these institutions were always built atop a deep foundation of tight kinship. The fact that people couldn’t simply wipe away their ancient kin-based institutions when building institutions creates what researchers call a strong path-dependence. That is, given that new forms always build on older forms, and these older forms are anchored in our evolved primate psychology, there are a limited number of pathways along which these new institutions can develop. 

Individualism 2.0

Agriculture wins, so we would expect the entire world to employ tight kinship systems. Right?

From 9,000 to 1,000 years ago, this map would indeed have been bright red. However, loose kinship recently made a resurgence in Western (European or European-descended) societies. We’ll explore why this happened next time. 

The civilized world is no longer exclusively dominated by tight kinship. Premodern kin-based states compete with modern impersonal states. 

Kinship Drives the I/C dimension

Last time, we learned about individualism vs collectivism, i.e. the I/C dimension. We adduced lots of data suggesting that cognitive style, social orientation, and moral posture co-vary based on how relationally mobile a society is. 

But why? Why are there systematic differences in relational mobility across nations? To answer this, we turn to our conception of tight vs loose kinship. If you are a male growing up in a clan (with intensive kinship norms), you and your wife will live your entire live in your father’s house. You will be rely on your kin-group for affiliation, for sustenance, for defense. You will not choose your friends or your spouse. In clans, relational rigidity is the norm. We should thus expect tight kinship societies to score highly in the myriad dimensions that comprise the I/C dimension. 

And so it is. Cousin marriage, and the kinship intensity index, are enormously predictive of a holistic cognition, and other-oriented sociality.

You can see how kinship impacts moral systems directly, when investigating rates of blood donations. Tight kinship societies don’t donate blood donations nearly as frequently: why care for strangers (people estranged from your social world) when you can instead invest in the relationships you will keep your entire life? Similarly, they are more likely to trust their in-group than impersonal financial institutions like banks; cousin marriage is enormously predictive of desire to not use checks.

We previously saw how individualistic societies have a dispositional psychology, which features intent as a morally relevant factor. Curtin et al (2020) has shown that kinship intensity explains much of this variance. For low intent scenarios (blue line), looser kinship (moving left) reduced judgment severity.

More generally, and across a suite of features, Schultz et al (2019) show that consanguinity rates explain a dramatic proportion of moral variance, both within-country as seen above, but also between countries. 

In sum, kinship is a very strong determinant of relational mobility, and hence explains most cross-cultural variance along the I/C dimension.

The Role of Pathogens

You may be wondering why sanctity intuitions rely so heavily on disgust, rather than other self-perceptions like dizziness. 

As noted by Schaller & Park (2011), the immune system is a necessary but insufficient protection against disease. The visceral immune system is energetically costly (a 13% increase in metabolic expenditure is required to increase human body temperature by just 1 C), and temporarily debilitating (the syndrome is known as sickness behavior). Surely in addition to a reactive system, animals co-evolved proactive defences against disease: a behavioral immune system.

Disgust evolved to promote pathogen avoidance, and thus contributes to the behavioral immune system. But sometimes people represent contagion risk, even if they aren’t manifesting signs of disease. This is especially true for unfamiliar people, travelling from distant groups with different immunological memory. Is xenophobia related to the behavioral immune system?

The answer appears to be yes. When primed to think about disease, people became much less tolerant of immigration from culturally-dissimilar countries (e.g., in US, immigrants from majority-Muslim countries). 

But the role of diseases extends beyond facultative dispositions. Consistent exposure to disease has been shown in Enke (2019) to increase kinship tightness in societies, along with its characteristic collectivism & ethnocentrism.

Thus, tight kinship is not only an economic response to resource defense, as we saw with agricultural surplus. It also serves to reduce disease transmission. 

The Role of Rice Paddies

We have already discussed how agriculture tends to produce surplus, defense of which strongly incentivizes clan formation. But the specific kind of agriculture matters too! Wheat farming can be conducted by single families on small plots of land. Rice paddy farming, however, due to the economic characteristics of that crop, require multiple families to correlate. This particular cereal, due to the social structures necessary to farm it, induces an even stronger nudge towards allocentric psychology than wheat.

Together with the discussion of pathogens, we have shown evidence to suggest that ecological and economic factors are important antecedents to kinship structure. Ecology doesn’t determine kinship, but it does bias it.


We have seen human societies tend to organize themself using tight or loose kinship. Each kinship system creates its own social worlds, which in turn incentivizes its own suite of psychological adaptations (cognitive, social, and moral). 

One of the virtues of this anthropological theory is that it explains the individualism/collectivism dimension as a function of kinship, and explains why so many moral, social, and cognitive factors covary along this single continuum. 


  1. Barrett et al (2016). Small scale societies exhibit fundamental variation in the role of intentions in moral judgment.
  2. Borgerhoff Mulder et al. (2009). Intergenerational Wealth Transmission and the Dynamics of Inequality in Small-Scale Societies
  3. Bowles & Choi (2013). Coevolution of farming and private property during the early Holocene
  4. Bugos (1985). An evolutionary ecological analysis of the social organization of the Ayoreo of the Northern Gran Chaco
  5. Curtin et al (2020). Kinship intensity and the use of mental states in moral judgment across societies
  6. Dohmen et al (2018). Patience and comparative development.
  7. Ember et al (2014). On the Development of Unilineal Descent
  8. Enke (2019). Kinship, Cooperation, and the Evolution of Moral Systems. 
  9. Fix & Shepherd (1999). Migration and colonization in human microevolution.
  10. Helgason et al (2008). An Association Between the Kinship and Fertility of Human Couples
  11. Henrich (2020). The WEIRDest people in the world: how the West became psychologically peculiar and particularly prosperous
  12. Levi-Strauss (1949). The elementary structures of kinship. 
  13. Jones (2011). The Matrilocal Tribe: An Organization of Demic Expansion
  14. Korotayev (2004) Unilocal Residence and Unilineal Descent: A Reconsideration
  15. Schaller & Park (2011). The Behavioral Immune System (and Why It Matters)
  16. Talhelm et al (2014). Large-Scale Psychological Differences Within China Explained by Rice Versus Wheat Agriculture
  17. Walker & Bailey (2014). Marrying Kin in Small-Scale Societies 
  18. Walter (2000). From Westermarck’s Effect to Fox’s Law: paradox and principle in the relationship between incest taboos and exogamy.
  19. Yellen and Harpending (1972). Hunter‐gatherer populations and archaeological inference

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