FIGURE 12.6 Age Differences in Intellectual Abilities Cross-sectional data on intellectual abilities at various ages would show much steeper declines. Longitudinal research, in contrast, would show more notable rises. Because Schaie’s research is cross-sequential, the trajectories it depicts are more revealing: None of the average scores for the five abilities at any age are above 55 or below 35, which means that most people of every age are able to function intellectually.