Your hair and fingernails continue to grow after you die.

The story appeared in enough places to acquire the feel of established fact: exhumed bodies found with longer nails and longer hair than they had in life. The detail showed up in literature, in horror films, in casual references to the uncanny persistence of biological processes after death. From Erich Maria Remarque's All Quiet on the Western Front to contemporary horror fiction, hair and nails growing after death functioned as a marker of how the body's mechanisms continued past the point of consciousness, a disturbing suggestion that the boundary between life and death was not perfectly sharp.

The mechanism people imagined, continued cellular growth driven by some residual biological process, was physiologically impossible. Hair and nail growth require a continuous supply of glucose and oxygen, delivered by active circulation to dividing cells in the hair follicle matrix and in the nail matrix at the base of each nail plate. When cardiac activity stops, circulation halts, and these cells lose their energy supply within minutes. Division ceases. Hair and nails, in any meaningful biological sense, stop growing the instant a person dies.

What changes, in a manner of speaking, is what is visible, and it changes only in apparent terms. The mechanism is dehydration.

After death, the body loses water through evaporation at the skin surface without any corresponding intake or replenishment. Soft tissue dehydrates and contracts. The skin around hair follicles pulls back, drawing away from the base of each hair shaft and exposing a greater length of hair than was visible in life. At the fingertips and toes, the skin retracts around the nail bed, pulling away from the hyponychium, the tissue junction at the base of each nail, and exposing the lower portion of the nail that had previously been covered. The nails appear longer because more of each nail has been revealed, not because any growth has occurred.

The illusion is most pronounced in bodies that have undergone significant dehydration before examination: bodies exhumed from dry soils, bodies refrigerated before burial, bodies that went undiscovered for some time. To an observer who did not understand the mechanism, the visual impression of lengthened hair and elongated nails was entirely convincing.

Historians of medicine have traced similar observations in the historical literature on premature burial. Many accounts of corpses displaying signs of life, movement, color, extended hair and nails, were almost certainly records of normal postmortem changes observed by people without the physiological framework to interpret them. The fear of being buried alive drove grave openings in seventeenth- and eighteenth-century Europe; what mourners found when they looked confirmed what they feared, because they did not know what they were seeing.

The correct explanation, dehydration and tissue retraction, was not a difficult one once anyone examined the physiology directly. Forensic anthropologist William Maples, who spent decades on forensic casework in the United States, described the mechanism in his writings on forensic pathology as straightforward: cells that no longer receive oxygen cannot divide; tissue that dehydrates contracts; what appeared to be growth was the revelation of already-existing material. A 2007 paper in the British Medical Journal surveying common medical myths included this one among those whose scientific refutation was clear and longstanding, noting that the apparent growth was an artifact of postmortem tissue retraction.

The persistence of the myth had less to do with evidence than with narrative appeal. A body continuing to produce visible change after death suggested some obscure residual vitality. It was a more interesting story than dehydration. But interesting and true are different categories, and in this case they did not overlap.