Kinship & Family Structures

Descent rules on one family tree, from patrilineal to bilateral

Patrilineal, matrilineal, double, bilateral and ambilineal descent worked on one family tree, with lineages, clans and kindreds defined and residence rules.

Portrait of Ines ValderaInes Valdera · · 9 min read
Descent rules on one family tree, from patrilineal to bilateral

Everyone has two parents, four grandparents and eight great-grandparents. A descent rule is a society's answer to a narrower question: which of those links put a person into a group, and which of them pass on land, office or a name? George Peter Murdock, in Social Structure (1949), described a rule of descent as the thing that attaches a person at birth to a particular set of relatives, the ones from whom certain services can be expected. He also made a point that still trips up first readers. Descent is about social allocation, not about who people think their biological relatives are. Matrilineal societies such as the Hopi recognize a child's tie to the father perfectly well, and often forbid marriage with relatives on his side too.

This piece takes one hypothetical person and runs the main rules over the same family tree. Kin types use the usual letters, read outward from the person at the center (Ego): F father, M mother, B brother, Z sister, S son, D daughter. FFM is the father's father's mother.

Eight great-grandparents, five rules

Ego's eight great-grandparents are FFF, FFM, FMF, FMM, MFF, MFM, MMF and MMM. Each rule picks out a different subset of them as the founders of the groups Ego belongs to.

Rule Great-grandparents whose group Ego joins Number of descent groups Ego belongs to
Patrilineal FFF (through F and FF) one
Matrilineal MMM (through M and MM) one
Double FFF for one kind of group, MMM for another two, of different kinds
Bilateral all eight, equally none in the strict sense; a personal kindred instead
Ambilineal any one of the eight, chosen link by link usually one active membership

Some notes on reading the table.

Under patrilineal descent Ego belongs with F, FF and FFF, and with everyone else who traces back to FFF through men only: FB and his children, FFB and his son's children. Ego's sister belongs too, but her children will join their own father's group. FFM married into the line; by the rule itself her group is her own father's.

Matrilineal descent is the mirror image with one consequence that surprises people. A man belongs to his mother's group, but his own children belong to their mother's. If property and office stay inside the group and pass between men, a man's heir within his group is his sister's son, not his own son.

Double descent is not the same as bilateral descent, and Murdock spelled out the difference with grandparents. Under double descent Ego shares one group with FF and another with MM, but no group at all with FM or MF. Under bilateral descent all four grandparents count the same.

Ambilineal descent lets the link be through either parent, generation by generation. Three choices of "father or mother" give 2 × 2 × 2 = 8 possible routes back three generations, one ending at each great-grandparent. The rule allows any of them; residence, inheritance or preference usually settles which one a person activates.

What a single line leaves out

At generation n back Ego has 2^n ancestral places. A unilineal rule keeps exactly one of them per generation. The share is 1/2^n:

  • three generations back: 1 of 8, or 12.5%
  • five generations back: 1 of 32, about 3.1%
  • ten generations back: 1 of 1,024, under 0.1%

Counting all generations from parents up to the fourth: 2 + 4 + 8 + 16 = 30 ancestral places, of which 4 lie in the patriline. A surname inherited from the father follows exactly that thread through a tree that doubles at every step, which is why pedigree collapse and surname history tell such different stories about the same family.

Lineage, clan, kindred: what the groups look like

Descent rules produce groups, and the groups come in sizes. The Ethnographic Atlas, the large comparative dataset Murdock built and that D-PLACE now publishes, uses these grades for unilineal groups:

  • lineage: a descent group whose core membership normally lives in a single community or part of one
  • sib, which British writers call a clan: a group whose core membership spans more than one community
  • phratry: a grouping of sibs when there are more than two such top-level groups
  • moiety: the top-level group when there are exactly two of them, so that the whole society falls into halves

In everyday anthropological usage a lineage is also the group whose members can trace the actual genealogical steps to their founder, while a clan claims common descent without being able to list every step. Diagrams make the difference easy to see; our guide on how to draw a kinship diagram covers the symbols.

A kindred is a different shape

Bilateral descent rarely makes groups that last. What it makes is the kindred: everyone related to a given person up to some limit, often second cousins, on all sides. Murdock's observation was structural. No two people have the same kindred unless they are full siblings, so kindreds overlap instead of dividing a community into separate blocks, and a kindred can hold no land and outlive no one.

Here is the overlap worked out for a hypothetical family. Suppose every couple in the parents' generation has two children, every parent has exactly two siblings, and kindreds stop at first cousins. Ego's kindred, counting Ego:

  • Ego and one sibling: 2
  • parents: 2
  • grandparents: 4
  • aunts and uncles: 4
  • first cousins, two for each aunt or uncle: 8

That is 20 people. Now take one of Ego's cousins on his father's side, a child of FB. By the same counting his kindred is also 20 people. They share FF, FM, F, FB, FZ, Ego and his sibling, the cousin and his sibling, and FZ's two children: 11 people. The other 9 on each side belong to one kindred and not the other. Ego's mother and her whole family sit in his kindred and outside his cousin's; the cousin's mother's family is the reverse. Extend the rule to second cousins and the overlapping only grows.

Residence: where a couple lives

Descent says which group a child joins. Residence says where a newly married couple sets up home, and so which relatives end up living side by side. The Atlas codes (definitions condensed from D-PLACE):

  • patrilocal: with or near the husband's patrilineal kin; virilocal is the same when those kin do not form patrilineal groups
  • matrilocal: with or near the wife's matrilineal kin; uxorilocal is the same without such groups
  • avunculocal: with or near the husband's mother's brother or other men of his matrilineage
  • ambilocal: with either set of parents, by choice or circumstance
  • neolocal: apart from both sets of relatives

Murdock argued that residence rules respond first when economic and social conditions change, and that a shift in residence sets off adjustments that can end by reorganizing descent itself. That is a claim about sequence, and it predicts that descent and residence should line up. They do. The table below is our own cross-tabulation of the Ethnographic Atlas data as released by D-PLACE: 1,252 societies coded for both the derived descent variable (EA043) and the prevailing residence pattern (EA012). Two small descent categories, mixed and quasi-lineages, are left out of the table.

Descent Societies Patri- or virilocal Matri- or uxorilocal Avunculocal Ambilocal Neolocal Spouses live apart
Patrilineal 582 566 2 0 4 6 4
Matrilineal 157 27 54 62 4 6 4
Double 51 48 0 3 0 0 0
Bilateral 352 169 74 0 62 47 0
Ambilineal 48 33 4 2 8 1 0

The avunculocal column includes the Atlas codes for optionally avunculocal residence.

Three readings:

  1. Patrilineal descent goes with patrilocal or virilocal residence almost without exception: 566 of 582, or 97.3%. Fathers, sons and brothers stay put; wives move in.
  2. Matrilineal descent is the interesting row. The single largest category is avunculocal, 62 of 157 (39.5%), ahead of matrilocal or uxorilocal at 54 (34.4%). Under matrilocal residence the women of a matrilineage live together and its men are scattered among their wives' homes. Avunculocal residence brings a young man to his mother's brother, so the men of the matrilineage end up together instead. The 27 matrilineal societies with patrilocal or virilocal residence are the case where the group's members are spread out on both counts.
  3. Bilateral societies are the only ones with substantial neolocal residence (47 of 352), which fits a rule that builds no corporate descent groups for a couple to live next to.

How common is each rule?

Across the 1,274 societies D-PLACE codes for descent, the counts are theirs and the percentages ours:

Descent type (EA043) Societies Share
Patrilineal 590 46.3%
Bilateral 362 28.4%
Matrilineal 160 12.6%
Duolateral (double) 52 4.1%
Mixed 50 3.9%
Ambilineal 48 3.8%
Quasi-lineages 12 0.9%

Read these as counts within a sample, not as a census of humanity. The Atlas describes 1,291 societies, each pinned to a focal time and place, and its own documentation notes especially good coverage of Africa and western North America. Neighboring societies also share history, so they are not independent observations; that problem has a name, and our explainer on Galton's problem shows why it matters for any table like the two above.

A check you can run

Which first cousins share a descent group with Ego? Take the four male first cousins and apply each rule.

Cousin Patrilineal Matrilineal Double Bilateral
FBS same group different same patri-group same kindred
FZS different different different same kindred
MBS different different different same kindred
MZS different same group same matri-group same kindred

Check one row by hand. MBS takes his group from his father, MB, under patrilineal descent; MB belongs to MF's patrilineal group, which is Ego's mother's group, not Ego's. Under matrilineal descent MBS takes his mother's group, and she married into MB's family from another group. Different both times, as long as people marry outside their own descent group, which is the usual rule.

Whatever unilineal rule a society follows, the fellow members among Ego's first cousins are parallel cousins, and the two cross cousins, FZS and MBS, are always outsiders. That single fact does a lot of work in marriage rules, which is where our explainer on cross and parallel cousins picks up.

† Links to books are affiliate links. As an Amazon Associate I earn from qualifying purchases.

Sources and further reading

More in Kinship & Family Structures