Caveats of this research include the possibility of subjects "using grammatical gender as a strategy for performing the task", and the fact that even for inanimate objects the gender of nouns is not always random. For example, in Spanish, female gender is often attributed to objects that are "used by women, natural, round, or light" and male gender to objects "used by men, artificial, angular, or heavy." Apparent failures to reproduce the effect for German speakers has also led to a proposal that the effect is restricted to languages with a two-gender system, possibly because such languages are inclined towards a greater correspondence between grammatical and natural gender.