Discrepancies Between Findings of Longitudinal and Cross-Sectional Studies in Adult Life: Physique and Physiology

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Abstract
Whereas cross-sectional surveys suggest that height, grip strength, and vital capacity should all decrease between the ages of 18-19 and 56 or later, no such decrease was observed in longitudinal studies. Possible explanations for the apparent decrease shown cross-sectionally are, in addition to the usual assumption of individual changes during aging, the secular trend toward increasing height and selective survival of small persons. In addition to such discrepancies, there are other problems which can be settled only by longitudinal study: changes known to occur with individual aging but with unknown time of onset, rate, and magnitude (e. g., stature and the anteroposterior chest diameter); and contradictory findings of several cross-sectional studies (e. g., skinfold thickness). Longitudinal studies of aging are not merely desirable - they are mandatory.