Among humans, race has no taxonomic significance; all people belong to the same hominid subspecies, Homo sapiens sapiens.[7][8] Regardless of the extent to which race exists, the word "race" is problematic and may carry negative connotations.[9] Social conceptions and groupings of races vary over time, involving folk taxonomies [10] that define essential types of individuals based on perceived sets of traits. Scientists consider biological essentialism obsolete,[11] and generally discourage racial explanations for collective differentiation in both physical and behavioral traits.[6][12]