This is generally considered to have originated somewhere in central or eastern North America, but there is much uncertainty since it is now a cosmopolitan weed (FNA 4, W; Ladd & Thomas 2015). Seeds of this species and probably many other Amaranthaceae may be widely dispersed in the guts of some birds and mammals (Janzen 1984, Orłowski & Czarnecka 2009, Green et al. 2016). There has been much confusion between this species (2n = 34) and hybridus (2n = 32); see notes under the latter. FNA 4 stated: "Occasional forms morphologically intermediate between Amaranthus retroflexus and taxa of the A. hybridus aggregate (e.g., A. powellii and A. hybridus, in the strict sense) are known both in the Americas and the Old World. Usually such plants are treated as hybrids; in many cases they are probably just extremes of the natural variability of A. retroflexus." Further checking of identifications is needed among retroflexus and hybridus, also comparing powellii and arenicola. Some keys are difficult to use, especially if distinguishing reportedly monoecious species (including the retroflexus-hybridus group) from strictly dioecious species (subgenus Acnida). It is often impossible to find male florets in fruiting collections of retroflexus and hybridus; F stated that Amaranthus in general is "monoecious, dioecious or polygamous."