Cage space requirements for non-human primates in the United States of America are less than those in European countries. Studies in support of the assumption that the US legal minimum cage size provides adequate space have limited value because they only tested cages without structural enhancement. It is not surprising that non-human primates cannot be animated to be more active or to behave in more species-typical manners by only providing them with extra barren space. Explicitly stipulating that all cages have to be equipped with properly installed, elevated structures appropriate to each species and age category would make the US standards more adequate. Such structures would no longer restrict the caged primate to an unnatural, permanent terrestrial lifestyle but would allow the animal to make use of the arboreal, 'safe' dimension to which she/he is biologically adapted. Minimal height requirements will have to be upgraded in the US to accommodate these ethological considerations.