One limitation of most research investigating interaction effects is lack of detail regarding the predicted form of the interaction. Researchers could specify whether an ordinal or disordinal interaction is predicted. For example, educational researchers might want to estimate the age at which one early intervention treatment becomes more effective than another, so policy makers can tailor interventions to children of appropriate ages. Or, Lynn (1999) offered a controversial maturational theory of intellectual development that holds that earlier maturation in females will lead to higher performance relative to males on intelligence tests at early ages. But, by mid to late adolescence, males will begin to outperform females due to their later maturation and larger brain size. Research contexts such as these suggest that interactions should be disordinal, with a cross-over point at some point on age.
