Neoteny (niː.ɒ.tə.niː) is the retention, by adults in a species, of traits previously seen only in juveniles (pedomorphosis/paedomorphosis), and is a subject studied in the field of developmental biology. In neoteny, the physiological (or somatic) development of an animal or organism is slowed or delayed. Ultimately this process results in the retention, in the adults of a species, of juvenile physical characteristics well into maturity. The English word neoteny is borrowed from the German Neotenie, the latter constructed from the Greek νέος (young) and τείνειν (to extend). The standard adjectival form is "neotenous", although "neotenic" is often used.
In vertebrate biology, neoteny is most easily identified when sexually mature, completely viable juveniles or larvae are found.
Specific individual traits that differ in descendant organisms, when compared to ancestors, are sometimes called neotenies; humans, for example, appear to have several neotenies in comparison to chimpanzees.
Neoteny in evolution
Neoteny plays a role in evolution, as a means by which, over generations, a species can undergo a significant physical change. In such cases, a species’ neotenous form becomes its “normal” mature form, no longer dependent upon environmental triggers to inhibit maturity. The mechanism for this could be a mutation in, or interactions between: genes involved in maturation, changing their function to impede this process.
Neoteny is not the only contributing factor affecting maturation in species that may have undergone neotenous changes over the course of their evolution, and its actual involvement in the following examples is not well understood:
- flightless birds—physical proportions resemble those of the chicks of flighted birds;
- humans—with traits such as sparse body hair and enlarged heads reminiscent of baby primates. Lactose tolerance in adults is a form of neoteny now considered normal in some European populations and their descendants while most other humans are lactose intolerant as adults. It corresponds to a mutation that permits the digestion of lactose beyond the lactation period.
- pets, such as dogs—which share many physical features with the immature wolf (these same traits were found during the development of the tame silver fox). Such puppy-like traits may have made early dogs seem "cute" and less threatening than wolves, leading to both natural and artificial selection of such dogs.
It is possible that the origin of the chordates (the phylum including all vertebrates) was the result of an instance of neoteny. Molecular evidence suggests that the nearest relatives of the chordates are the tunicates, marine filter feeders. Although sessile in their adult, sexually mature form, tunicates have a motile larval dispersal form, which has a notochord similar to that found in chordates. At some point, the motile larvae of the tunicate became sexually mature before metamorphosis. As a sexually active pelagic organism it had considerable feeding and habitat colonization advantages over the sessile form, so was at an evolutionary advantage. However, the alternative - that the sessile form developed later and the pelagic form was ancestral - is also thought possible.
Neoteny in humans
Neoteny in humans can be seen in different aspects. It can be compared with other great ape species, between the sexes and between individuals.
There is controversy over whether adult humans exhibit certain neotenous features, or juvenile characteristics, that are not evidenced in other great ape species. Stephen Jay Gould was an advocate of the view that humans are a neotenous species of chimpanzee; the argument being that juvenile chimpanzees have an almost-identical bone structure to humans, and that the chimpanzee’s ability to learn seems to be cut off upon reaching maturity.
While neoteny is not necessarily a physical state experienced by humans, paedomorphic characteristics in women are widely acknowledged as desirable by men . For instance, vellus hair is a juvenile characteristic. However, while men develop longer, coarser, thicker, and darker terminal hair through sexual differentiation, women do not, leaving their vellus hair visible.
Paedomorphic variations not only exist between the sexes, but also between individuals, with some people displaying more characteristics of neoteny than others. This trend carries over to variations among ethnic groups as well.
Bruce Charlton, Reader in evolutionary psychiatry at Newcastle University U.K., suggests that there may be such a thing as "psychological neoteny."  Due to recent changes in culture, he says, “In a psychological sense, some contemporary individuals never actually become adults.” Delayed maturity may be a consequence of later parenthood, itself caused by more prolonged formal education - especially among women 
One example of a neotenic trait in vertebrates is the salamander species axolotl, which usually remains fully aquatic as it matures. Other salamanders, such as the widespread tiger salamander of North America, may retain the external gills usually only present in immature individuals, as adults in some populations in marginal habitats.
Neoteny and progenesis
Neoteny and progenesis are both mechanisms that result in paedomorphosis. Neoteny delays physiological, but not sexual, maturity. Comparatively, progenesis speeds up sexual, but not physiological, maturity. Progenetic organisms achieve sexual maturity in their juvenile state. This is most commonly found among certain amphibians and insects.
- See Charlton BG, The rise of the boy-genius: psychological neoteny, science and modern life. Medical Hypotheses. 2006; 67: 679-81. The article is also discussed by Matthew Hutson in The Peter Pan-demic, Psychology Today magazine, Nov/Dec 2006, and by Clay Risen in Psychological Neoteny, The New York Times Magazine, December 10, 2006
- Charlton BG, Psychological neoteny and higher education: Associations with delayed parenthood Medical Hypotheses. 2007; 69: 237-40.
- DannyReviews.com—Ontogeny and Phylogeny, Stephen Jay Gould (book review), Danny Yee (October 16, 1992)
- DavidBrin.com—“Neoteny and Two-Way Sexual Selection in Human Evolution: A Paleo-Anthropological Speculation on the Origins of Secondary-Sexual Traits, Male Nurturing and the Child as a Sexual Image,” David Brin, PhD
- Neoteny.org—“Neoteny: The multidisciplinary implications of heterochronic theory”
- NIH.gov—“Ontogenetic study of the skull in modern humans and the common chimpanzees: neotenic hypothesis reconsidered with a tridimensional Procrustes analysis,” X. Penin, C. Berge, M. Baylac, American Journal of Physical Anthropology, vol 118, no 1, p 50-62 (May, 2002)
- NHM.org—“Neoteny/Juvenilization: Some dogs look and act forever young,” Natural History Museum of Los Angeles County
- Trut, Lyudmila N. (1999). "Early Canid Domestication: The Farm-Fox Experiment" American Scientist. 87(2), 160-169. (A Russian study of pedomorphosis in a 40-year breeding program to domesticate silver foxes.)