Descent rules define socially recognized kin groups by tracing connections through chains of parent-child ties. A society may focus exclusively on connections traced through the male parent (patrilineal) or through the female parent (matrilineal). In either case, the culture is employing a unilineal, or single-line, descent system.
When descent is patrilineal, the descent group is composed of people of either sex whose fathers belong to the group. Siblings belong to the descent group of their father, but their mother belongs to a different descent group, the group to which her father belongs. Therefore, a man's children will belong to his descent group, but a woman's children will not belong to her descent group. Analogously, if descent is matrilineal, siblings belong to the mother's group but their father does not. A woman's children will belong to her descent group, but a man's children will not belong to his. Sometimes a society will assign individuals to one unilineal descent group for one purpose and to the other for another purpose, resulting in a system of double descent. For example, the person's patrilineal descent group may be in charge of political functions, while inheritance operates through the matrilineal descent group.
In contrast to societies that trace descent unilineally, individuals in some cultures such as the United States are characterized by bilateral descent rules, tracing relationships through both parents. In these societies, other institutions, such as governments, churches, businesses, and voluntary organizations, provide the structure and perform the functions of other societies' kin-based groups. In some societies, descent is traced through one parent for some people and through the other parent for other people; this is ambilineal descent. For instance, males may trace descent through their fathers, and females may trace descent through their mothers.
Because unilineal descent rules produce bounded and nonoverlapping groups, unilineal descent is a more powerful organizing principle than bilateral descent in that unilineal descent groups are able to act as corporate groups on behalf of their members in a way that bilateral descent groups cannot. Each patrilineal descent group in a society that traces descent through the father has a particular identity and membership that is entirely different from the identity and membership of any other patrilineal descent group in the same society. Where descent is traced bilaterally, by contrast, only full siblings belong to precisely the same descent group because only full siblings have the same parents. Where descent is reckoned bilaterally, a person tends to single out some relatives within his or her kin group as more important than others. This close circle of kin is referred to as one's kindred. Who is included in one's kindred and who is not is a matter of individual choice based upon individual preference and sentiment. What is more, the definition of kindred shifts, depending upon circumstances. For instance, people in the United States are likely to count a smaller number of relatives as close when planning the guest list for Christmas dinner than when they are writing wedding invitations. In either case, because bilateral descent groups fan out indefinitely, it becomes hard to decide where to draw the line between kin who are close and kin who are not. Since each person belongs to a unique descent group and different bilateral descent groups in the same society have somewhat overlapping but also somewhat different memberships, these groups cannot function effectively as representatives of their members.
Unilineal descent, specifically patrilineal descent, is the most common system of reckoning (Ember and Ember 1988). Therefore, the majority of cultures around the world exploit blood and marriage connections to maximize the power and effectiveness of the kin group in supervising a wide variety of activities in which individuals participate. Unilineal descent groups are important sources of political power in many societies. The leaders can arbitrate disputes between individuals within the descent group or between different descent groups. They can go to war in support of a group member and retaliate for wrongs done to one of their own. Unilineal descent groups can delegate land rights and often act as a kind of government vis-à-vis the members. Unilineal descent groups also have important economic roles. Such groups can own land, money, houses, religious places and objects, songs, economic capital, and even personal names. Property is often inherited through the unilineal descent group. Unilineal descent groups can lend money and maintain members who have no other means of support. The unilineal descent group is also commonly the center of religious activity. Often a descent group is identified with supernatural beings who may be ancestors or claimed ancestors of members of the group. Supernaturals may be believed to protect and otherwise affect the members of the group, and the members may, in turn, be required to engage in particular activities in an effort to influence the actions of the supernatural.
Particular descent groups can also be associated with particular sets of taboos that the members of the group are obligated to honor. Marriages, often regulated by the unilineal descent group, may be prohibited or preferred between members of the same descent group, depending upon the norms of the group. Unilineal descent groups may also take over the burden of providing what are sometimes very costly payments to the bride or bride's family when a member of the group is married.