What term describes the multigenerational families forming due to declining birthrates and rising life expectancies?

The term "beanpole families" accurately describes multigenerational families that arise from a combination of declining birthrates and increasing life expectancies. This concept illustrates families that, while they may have fewer members in each generation (due to lower birth rates), span several generations — typically including great-grandparents, grandparents, parents, and children.

In beanpole families, the structure is vertical rather than horizontal, meaning there is a strong connection across generations, but fewer siblings within each generation. Individuals are often connected to multiple generations, enhancing support networks and relationships across ages despite the overall smaller family size.

The other terms do not encapsulate this specific familial structure. Nuclear families refer to a structure consisting of two parents and their children, without extended members. Extended families include additional relatives, such as aunts, uncles, or grandparents, but do not specifically address the dynamics brought about by declining birthrates and increased longevity. Blended families involve the merging of two family units, typically following remarriage scenarios, without direct reference to generational aspects or demographic shifts. Thus, beanpole families best captures the phenomenon described in the question.