[William Bennett Bean (1909–80198935ya; research overview/bio); previously: 1953, 1963a/1963b, 1968, 1974; there appears to have been no followup before his death 9 years later in 1989.] A 35-year observation of the growth of my nails indicates the slowing of growth with increasing age.
The average daily growth of the left thumbnail, for instance, has varied from 0.123 mm a day during the first part of the study when I was 32 years of age to 0.095 mm a day at the age of 67.
…More than 37 years ago, rather casually, I began to study the rate of growth of my nails, fingernails and toenails. The stimulus that set me on this course I have related in several articles, the first of which appeared in 1953.
Methods …I make an indentation with the little file commonly employed to open small glass vials. On the first day of each month, I file a transverse groove just at the edge of the free margin of the cuticle, being careful not to push it back or interfere with it within a week or two after marking the nail. The end is recorded when the mark has just reached the free margin of the nail, exactly 1.45 cm from the start.
Early in my observations, I measured nail clippings by linear growth and by weight. With careful calculations I found that anywhere from 25% to more than 50% of the nail had been used up by unnoticed attrition. Not only does the length of the nail wear away but the dorsal surface also wears down. If a fingernail is trimmed with scissors and not filed, sharp angles can be felt, since scissors simply takes away bites. Without filing, these sharp points disappear in a day or two from unnoticed wear and t
Findings …When I first began to measure the rate of nail growth, I scored marks on all my nails. Within a few months I found that each nail had its own pace. This was clearly distinguishable even by the rather crude method that I used. Some nails grew rapidly; some, in an intermediate phase, less rapidly; and some, slowly. The differences were small but regular. There was consistency in the variation, so if I applied a ratio I could tell by measuring one nail what the others were doing, and this I did on several occasions. In simple terms, toenails grow more slowly than nails of the hand, and the nail of the middle finger grows more rapidly than the nails of either the thumb or the little finger or the other two middle fingers. By measuring one nail, the rate of growth can be calculated for all. Since I am right-handed, I settled on the measurement of the rate of growth of my left thumbnail. Out of ~450 months, I have forgotten to mark the nail only twice.
…I made a little tattoo in my thumbnail to use as a benchmark in case the cuticle seems to recede or advance. The rate of growth of the nail is determined by the number of days the mark takes to arrive at the free edge. The nail still measures the same distance from cuticle to free edge, 1.45 cm, as it did 37 years ago. I have not observed one common sign of aging, the recession of the pulp from the nail bed, which, if large, would give an appearance of a shortened growth time.
…In an article published in September 1974, I made a fairly extensive review of studies of nail growth and made some observations on other deciduous tissues. The following statements hold true. Nails grow faster in children than in adults. Rate of nail growth diminishes with progressing age. Hyperaemia or a substantially warm environment makes nail growth accelerate. Biting the fingernails increases the rate of growth ~20%, probably because the frequent manipulation of the nail stimulates the circulation of the germinal area in the nail root. Immobilization, such as that caused by hemiplegia or having a limb or digit in a plaster cast, greatly slows growth. Pregnancy increases it, perhaps by as much as a third. Those who live in climates with sharp changes of temperature tend to have a faster rate of growth during the warm seasons and a slower rate during the cold seasons. Ischemia is associated with a marked slowdown in the rate of nail growth.14
…I have added 5 more years to my record of nail growth. It had remained fairly steady through 1975, 1976, and 1977, taking ~153 days from start to finish. During the incomplete year of 1978, the rate has slowed down a trifle, perhaps by a day.