| 43 sciences, such that when two of the components are known the third is determined, even though all three maybe of interest to the investigator. For the researcher of aging the major issue is that in age difference research (cross-sectional studies) age is confounded with cohort, and in age change research (longitudinal studies) age is confounded with period or time of measurement. Except in animal research, convergence of findings from cross-sectional and longitudinal study only occurs under very special circumstances (also see Ferrer & Ghisletta, Chapter 2; Zelinski et al., 2009 ). A number of research designs have been proposed that allow at least partial unconfounding of these components by means of replicated cross-sectional or longitudinal sequences (cf. Schaie, 1977, a,
2007; Willis, 1989 ). Less attention, however, has been given to provide alternate solutions by freeing the confounded components from their rigid identification by calendar time ( Schaie, 1986 ). This could be accomplished by treating age as a dependent rather than an independent variable to determine the age at which a behavior or event of interest is first observed or the span of ages over which a given behavior endures (cf. Wohlwill, 1970). On the other hand cohort would need to be defined as referring to the common entry into a similar environment at a common point in time, whereas time could be redefined as the event density characterizing a particular period cf. Schaie, 1984, 1986, a ).
When these issues were first identified for students of the psychology of aging, the immediate concerns were to determine how they might confound our understanding of age changes and differences overlarge periods of the life standing. More recently researchers in the psychology of aging have become more interested in how historical changes in societal influences