Most are small, but an unnamed form dating to 15–16 mya is comparable in size to the largest living slow lorises.
They are thought to have reached the islands of Sundaland when the Sunda Shelf was exposed at times of low sea level, creating a land bridge between the mainland and islands off the coast of Southeast Asia.
The earliest known mention of a slow loris in scientific literature is from 1770, when Dutchman Arnout Vosmaer (1720–1799) described a specimen of what we know today as N.
bengalensis that he had received two years earlier. In 2008, Groves and Ibnu Maryanto confirmed the promotion of the fifth species, the Javan slow loris, to species status, a move that had been suggested in previous studies from 2000.
The group's closest relatives are the slender lorises of southern India and Sri Lanka.