Final answer:
Bilateral descent is the practice of tracing one's lineage through both the mother's and father's lines equally, as opposed to unilineal descent, which traces through only one parent's line (either patrilineal or matrilineal). This form of descent is common in modernized nations and affects societal structures like inheritance and marriage.
Step-by-step explanation:
The principle that a descent group is formed by individuals who believe they are related to each other through both their mothers' and fathers' lines is known as bilateral descent. In such cultures, tracing one's lineage is not limited to a single parent's side, but rather includes both paternal and maternal ancestors. This approach to kinship is typical of modernized nations, where both sides of a person's family are considered equally important in determining their familial connections and social obligations.
In contrast, unilineal descent traces kinship through just one parent's line. This can either be patrilineal, which follows the father's side, or matrilineal, which follows the mother's side. Different societies may adopt these various descent systems based on their cultural practices and values, which in turn influence societal structures such as inheritance, social alliances, and marriage customs.