Experimental gerontopsychology attempts to test causal hypotheses about old age-related behavioral changes by the manipulation of age-differences. An experimental treatment is introduced with the purpose of equating different age-groups with respect to a potentially relevant function. If the treatment results in an assimilation of the behavior of the different age-groups (age by-treatment interaction), differences in this function are con sidered as causal for the normally observed behavioral differ ence. If it does not result in an assimilation of the behavior of the different age-groups...