Taxa above the species level are typically based on the judgments of taxonomists. The present paper reports an objective correlate of these judgments. The stem length of any node in a dated phylogenetic tree can be defined as the interval between two dates: the date when the stem leading to the given node diverges from the rest of the tree; and the date when the node splits into its descendant branches if there are at least two, or else the present time if the node consists of a single species. In published dated phylogenies of six large clades, families tend to have longer stems than genera, which tend to have longer stems than unnamed nodes. Longer stems allow more time for the ancestors of taxa to develop their distinctive characters before splitting into species that share those characters.

Sources of data and methods of analysis are in the file called 'description' in the dataset.                                                                                                        

This information is in the file called 'description'.