Figure 1 graphically illustrates such trends for a number of selected countries. Taking the Seattle Longitudinal Study as a reference point, Desjardins and Warnke (2012) conclude that cross-sectional age-skill profiles for fluid intelligence measures tend to start from a higher level and decline more steeply than in longitudinal profiles for the same measure. Longitudinal studies are intuitively attractive, because they allow controlling for unobserved within-person characteristics that are time-invariant.