"Bowling Alone" describes how Americans have become less connected socially in various ways over the past few decades. One interesting point from the book is that there are two ways that we can change in the aggregate.
Suppose that bowling has become less popular in recent times. This could be caused by social change: mostly all of the same people decided that they would rather do other things instead. Bowling could be less popular among all age groups than it has been in years past, even comparing the attitudes of today's 50-year-olds to how they felt about bowling two decades ago, when they were in their 30s.
Or the shift could be caused by generational change: the bowling fanatics have been growing old and dying off over the years, only to be replaced by their younger, bowling-apathetic counterparts.