Everyone's close-focus eyesight worsens as we age, but the aggregate demand for reading glasses changes little. Period effects affect all people who live through a given era, regardless of their age. Period effects can produce both individual and aggregate change, often quickly and enduringly, without any age-related differences. The sharp drop in trust in government between 1965 and 1975, for example, was almost entirely this sort of period effect, as Americans of all ages changed their minds about their leaders' trustworthiness. Similarly, as just noted, a modest portion of the decline in social capital during the 1980s appears to be a period effect.
Generational effects affect all people born at the same time.
In pure generational effects, no individual ever changes, but society does. The key to this paradox, as David Butler and Donald Stokes observed in another context, is to ask, not how old people are, but when they were young. In effect, the figure on the bottom of page 43 lines up Americans from left to right according to their date of birth, beginning with those born in the last third of the nineteenth century and continuing across to the generation of their great-grandchildren, born in the last third of the twentieth century. Then rather abruptly, however, we encounter signs of reduced community involvement, starting with men and women born in the early 1930s. Remarkably, this downward trend in joining, trusting, voting, and newspaper reading continues almost uninterruptedly for nearly 40 years.