5. Cross-species generalizations: External validity is also in question when
research is conducted with nonhumans and presumed to be readily
applicable to humans. Before we can consider whether the results obtained
with one species can be generalized to another species, we must note the
parallels and differences between the two species on the mechanism or
process of interest. For example, rats are an excellent species to use for
research on eating. Rat eating is similar to human eating both physically
and behaviorally (rats and humans have similar digestive systems, eating
patterns, and food preferences). As a result, researchers can confidently
generalize the results of research with rats to humans. In contrast, the
blowfly is not a good species to use to generalize results to humans’ eating
because, unlike that of humans, the blowfly’s eating behavior is purely
reflexive and not learned (Logue, 1991). All of this is not to imply that
nonhuman research is worthless and not applicable to humans; many
major scientific advances in understanding humans have been made from
research conducted with nonhumans. We must be careful not to presume,
however, that all nonhuman research is directly applicable to humans.