Final answer:
Parallel cousins are the children of EGO's same-sex siblings, while cross cousins are the children of EGO's opposite-sex siblings. In tribal societies, EGO typically chooses their marriage partner from their cross cousins to maintain family unity.
Step-by-step explanation:
Parallel cousins are the children of EGO's parents' same-sex siblings, while cross cousins are the children of EGO's parents' opposite-sex siblings. In many tribal societies, EGO would choose their marriage partner from among their cross cousins. This practice helps maintain a stable and significant presence of the family unit across generations. For example, if EGO's father has a sister and his mother has a brother, their children would be EGO's cross cousins.