How often do I cut my fingernails?
Feat. my toenails
Abstract
By logging basic qualitative data when I cut my nails throughout 2024, I use many dubious methods to turn this small amount of data into a large amount of data. By "inspection comparative derivatives" we extrapolate the lengths of each nail that've been respectively removed, either through permissible means (cutting) or impermissible means (picking and biting). The aggregate data for each nail are given, finding, among others, that certain digits are likelier to be picked (thumbs and big toes; the fourth and fifth digit of each hand or foot escaped least scathed this year) and that fingernail pickage was correlated with fingernail growth.
Introduction
Around this time last year, I asked my friend how often they cut their fingernails. To my surprise, they gave an answer (a surprising one at that - once a week!), which I wasn't able to do. I began writing this stuff down to satiate this answer: each time I cut my nails, I'd write the date down and roughly how much I cut off each nail, qualitatively. Before we begin with the results, here's a fact-of-the-day-style rundown of human fingernails; I would assume that most of the following also applies to toenails:
Fingernails are known to grow at roughly 4cm/year each, and toenails at roughly half that. The middle finger generally grows fastest unprovoked. However, when a finger is damaged, the nail bed is stimulated and more blood is supplied thither; that nail then grows more quickly to repair itself. This means that nails that are more frequently damaged (by impremissible shortenings, i.e. picking and biting) also grow more.
The parts of the nail that pertain to its growth sit in the nail sinus, a small area (labelled D) underneath the eponychium, which is a fold of skin that protects the cuticle. The germinal matrix is the tissue that generates keratinous cells that become the nailplate itself. The cells harden as they extend outward, but remain attached strongly to the nail bed, which is made up of a thin layer of skin. The nail bed has grooves that attach the epidermis, dermis and nailplate. It also has a high density of nerves and blood capillaries. There's actually also a layer of skin on your nail, which is part of the cuticle. The thing that I thought was called the cuticle (and sometimes is called the cuticle) is the alive part of skin, called the eponychium, that forms a seal between the dead cuticle and your insides. There's a similar seal between the nail and the fingertip called the hyponychium (labelled G on the diagramme) - cutting your fingernails too much hurts because of this component. The other part of interest is the lunala (B on the diagramme) which is almost like a second nail that grows under the main one. If you remove your nail, the lunala will remain; if you damage the lunala, it won't repair itself as well. The area of nail above the lunala (the bit that's actually white) is an important diagnostic tool for some diseases like kidney failure or anaemia because it's right next to the source of the cells, so the nail there is relatively young.
Nails are roughly a tenth water, and are actually more permeable than skin. Getting common substances that would usually be fine on your skin onto your nails can pose unexpected risks, for example garden herbicides, fungicides (for example in athlete's foot treatment), bleach and urea (which is often found in hand lotions). This is why nails stain more easily than skin, by intention or not.
Methods
If I told you my methods you'd laugh at me.
The only thing I will describe is the notation I used during this project and in the proceeding section: my fingers were numbered 1-5 thumb-pinkie and then the same order for 6-A (A in place of "10" as it's single-character; think of it as elevenary) on my feet. Thus, my right-hand ring finger is R4 and my left-foot pinkie is LA.
Results and Discussion
Let's just get into the fun stuff.
To answer the question that forms the title for this article and the inspiration for my datalog, it depends. My left hand I cut 15 times during 2024, and my right hand just 14 times - this disparity is because I need my left hand to have shorter nails than my right so I can play my viola properly. Once during the middle of the year I must've decided that my right hand didn't need doing quite yet, so they got out of synch for a few months. My toenails I cut 8 times during the year, save R9 and RA which were only cut 7 times for some inexplicable reason.
Well, all those numbers are actually the answer to a slightly different question: how often do I intend to cut my nails; how often do I sit down with the clippers with the intention of severing fragments from my body, which I guess is what I actually meant when I asked the question in the title; but "How often do I cut my nails?" is slightly different. The data in the paragraph above are just counts of the number of data that I penned, at all. However, sometimes I'd cut one nail and not another, perhaps because that nail was recently cut or bitten off. This would be noted as "LEFT" in the log. Ignoring these, we get slightly more varied results:
My fingers were each cut 13-14 times; this means that only R4 and R5 were cut every single time (as the least bitten fingers, assumedly). R1 (the thumb) was the exception: it was cut just 10 times, meaning around a third of times I tried to cut my R1, it was already too short to cut - assumedly as the most bitten finger. All toes were 6-7 times.
So, to summarise, I cut (most of) my fingers every 24-28 days and my toes every 45-60 days. And now the fun stuff:
I turned my qualitative log into a quantitative estimate of the length of fingernail grown, bitten/picked off and cut off, through some rather mathematically dubious means (see ##Methods).
Wow! Pretty chart!! If you'll excuse my horribly manicured hands. There are a couple of interesting conclusions to take: the right hand grew more (possibly due to being more used in day-to-day (I'm right-handed) and thus more stimulated and having more blood supplied), as did the thumbs and forefingers (possibly for similar reasons). Of course, the error bars are so large that on the whole we can't actually draw any proper conclusions hence, but we don't care about proper statistics do we.
No surprises in this slightly less pretty chart. (It's just the same but we can see that the toenails grow at around 65% speed - roughly in line with what the internet tells me). Now, onto the dangerous stuff, the stuff that gets you a scolding from Grandma:
This one is a bit juicier. The words "pick" and "bite" are used interchangably - on the whole it's picking (i.e. with other fingers) but there is some biting that's impossible to distinguish out. Similar error bars to the previous two apply, but they've been taken out to make the data look more impressive. There's a clear favour to pick the thumbs - perhaps because they're the most pickable or because they're the most delicious. Middles, rings and pinkies were least scathed during 2024, with L5 managing to escape entirely (or at least, small enough not to be picked up on by my monthly qualitative logging).
This was a very interesting result: throughout the first half of the year, I got better at not picking and biting my nails. I started wearing nail varnish in June, and there was a marked decrease in picking shortly thereafter, but around September, I started picking more. This implies that nail polish may have helped my efforts to stop picking my nails, but wasn't a cure-all and I grew sloppy in the recent months. Of course, this isn't very conclusive either way as the graph takes data from both my fingers and toes, but I only varnish my fingernails.
This is yet another interesting result that confirms the commonly-held and widespread beliefs: fingernails that get picked more also grow more (p<0.05). This result doesn't prove the causation (which is supported by the blood supply argument) but it does put strong evidence towards it. Alternative causations may be that the most used nails tend to grow more quickly because it's evolutionarily advantageous to replenish the more frequently used and more frequently broken ones, and these are also the nails that I most frequently pick (possibly because they're the most used and thus the likeliest to have developed cracks to pick, or perhaps because they're most convenient to pick).
The final segment that I thought would be fun to calculate was the mass I grew and lopped off throughout the year. First, I had to calculate the mass of nail, which I did by dropping various nail clippings into saline solution of different densities. Modelling it as travelling at terminal velocity through the water, I recorded how the velocity varied with the density of the solution, and thusly gained an estimate for the acceleration due to buoyancy and thence the density of nail. Then, the cross-sectional area of each nail was measured and multiplied through to give the following results:
I grew 152g of nail in 2024. 107g (70%) of that was severed through legitimate means (cut off) and 45g (30%) through illegitimate means (bitten or torn off). Assuming that the protein-rich keratinous nail was fully recycled back into keratin, I "wasted" 107g of protein on my nails.