What type of research assesses genetic links among family members?

The correct choice is a family study, which specifically focuses on examining genetic links and the heritability of traits or disorders among relatives. Through this method, researchers analyze the similarities and differences in characteristics between family members to understand how genetics may influence these traits or conditions. By studying families, especially those that are biologically related, scientists can identify patterns of inheritance and the extent to which genetic predispositions contribute to various behaviors or psychiatric disorders.

Other research methods listed serve different purposes. Longitudinal studies track the same subjects over time to observe changes and developments, while cross-sectional studies compare different subjects at a single point in time. Case studies involve an in-depth analysis of an individual or a small group, focusing on unique characteristics or experiences rather than broader genetic linkages within families. These distinctions highlight why family studies are particularly tailored to investigate genetic relationships among relatives.