Hold a section of hair up to a window and look at the last few centimeters. Somewhere in there, one strand will have forked into a tiny Y. Look long enough and you'll find one that has split into a paintbrush, and another with a pale speck partway along it that you hadn't noticed before.
These are all the same event at different stages: a crack, in a fiber that has no means of closing it. For decades the explanation stayed vague, partly because nobody could make a split end on demand in a lab. Now they can, and the results are more specific than most of the advice.
A crack that ran the wrong way
Most of a hair strand is the cortex, the inner core, built largely from keratin (a tough structural protein) arranged in fibers that run lengthwise. Those bundles are held together by a layer called the cell membrane complex, a sort of mortar. Around the outside sits the cuticle, six to eight layers of overlapping scales laid like tiles on a roof.[1]
That construction explains a split. Hair is strong along its length and weaker across it, the way wood splits more easily along the grain than against it. A crack that starts inside a strand can run along the mortar between those fibers, so it travels lengthwise instead of cutting across.[1]
Healthy hair tends to break with a flat fracture near the root. Further along, and in hair weakened by the environment or by treatments like bleaching, the same fibers fail in stepped, frayed or split patterns.[1] The material has changed by the time it gets there. A paired comparison of root and tip sections from the same strands backs this up: the tips were about twice as irregular in diameter, with more surface cavities.[3]
It's the tangle, not the tug
Pulling on a strand is not how this happens. Snapping a hair in straight tension takes so much force that it would come out at the root first. Ordinary combing through free-moving hair creates very little stress too.[1]
Tangles are different. In a tangle, one strand bends sharply over another, and when a comb drags that loop along, the point of extreme bending travels down the hair. The stresses involved include shear, a sideways sliding force inside the strand, which is thought to drive cracks along the grain. It happens again every time you brush, which makes fatigue the likely way hair fails: the same way a paper clip eventually snaps from being bent back and forth.[1]
Researchers at Trinity College Dublin, working with L'Oréal scientists, built a machine to do exactly this, looping two hairs together and dragging one through the other.[1] Hair from a person prone to split ends failed in about 207 cycles at a light load, against about 531 for hair from someone who never got them. The failures also looked different. Split-prone hair usually cracked near the center of the strand, with splits running more than 5 millimeters. In the other hair, splits started at the surface and most stayed under a millimeter.
The honest limit: those were two donors, both with straight hair. It's a strong demonstration of a mechanism, and it can't tell you how common each pattern is. Follow-up work with more hair types found cracks starting at three different boundaries inside the strand, with those deep in the cortex running lengthwise and those near the surface cutting across.[2]
The pale specks have the same origin. In lab rubbing tests, white dots along the strand appeared when hairs were pressed together and bent over one another, and in hair from a person with split ends those dots were linked to lengthwise splits.[4] That work is preliminary, but it points to the same culprit: localized stress where strands meet.
What weakens the mortar
Chemical treatments do their damage in the gaps between fibers, which is why they make splitting worse specifically. Bleach, perms and relaxers break cysteine bonds, strong links in the hair's protein, and the loss of strength is permanent unless they're rebuilt chemically.[1]
Time on the bleach mattered in the Dublin tests. Twenty minutes made no significant difference to how many cycles hair survived. Forty-five and 80 minutes did, though only some of the samples were affected, and the authors say more testing would be needed to confirm any shift in how the splits formed.[1]
Lipids matter here too. Bleach, dye, straightening and sun all speed up the loss of the fats that protect the strand and keep it flexible.[5] Heat has its own damage pathway, which the heat protection guide covers. And wet hair needs extra care, since water weakens hydrogen bonds and leaves the strand more prone to physical damage until it dries.[1]
“A split end is a crack that took the easy route. Hair is strong along its length and weaker across it.”
What no product can do
The visible part of a hair is a thin tube of dead, fully keratinized cells.[6] Nothing in it is alive to heal anything, so a split stays split.
Products can glue the two sides together for a while, and the numbers are humbling. One lab study reported that a commercial leave-on held split ends together for 1.5 hours, against more than 72 hours for experimental silk fibroin particles that aren't a product you can buy.[7] Even chemical straightening, which significantly improved broken cuticle in a study of 30 women, made no significant difference to split ends.[8]
So a smoother end after a serum is real, but it's cosmetic and it washes out. Treat any "repairs split ends" claim as "hides them until your next wash."
Cutting is the only way to remove one. In the lab, a split that started inside a strand kept extending under repeated stress,[1] so if you trim, cut above where the fork begins. You may have heard that hair needs a trim every eight to twelve weeks. We found no study that sets a schedule, so let your own ends decide.
What actually slows them down
The mechanism tells you where to aim: fewer tangles, less friction when they happen, and less weakening of the mortar.
Conditioner comes first, because it works on exactly the right variable. Friction between strands rises as hair ages and loses its natural surface lipids, and conditioners can bring it back down to levels close to untreated hair, for as long as the deposit lasts.[9] A 2010 study found that nearly all commercial products with anti-breakage claims combined silicones with cationic polymers (positively charged conditioning ingredients that cling to hair), which is worth knowing when you read a label.[10] The silicones guide covers the trade-offs.
Detangle like the tangle is the problem, because it is. Start at the ends, work upward in sections, and use something with slip. In the Dublin tests most surface splits ran toward the root, against the direction of normal brushing, and the authors note that moving hair the wrong way may encourage them.[1] That's a good reason to go easy on backcombing. The breakage guide has more on technique.
Space out the chemistry and keep bleach to the time it actually needs. Use a heat protectant, and lower the temperature first.
Regular oiling is worth a mention, with a caveat. Women who used coconut oil once or twice a week had about 65% less diameter irregularity along their strands than women who didn't,[3] but comparing two groups can't show the oil caused it. The oils guide explains which oils are worth trying.
Reading your own ends
Splits close to the tips on long hair are mostly age. Splits appearing higher up, especially after a color or a run of hot styling, are a sign that the mortar is going, and the guide to color-treated hair is the better next stop. The guide to hair concerns helps if you're not sure what you're looking at.
How much conditioning your hair can take before it goes limp depends on the rest of it: strand thickness, porosity, damage history. The Roots quiz reads those together and matches products at the ingredient level, so you can see which conditioners suit your hair rather than whichever bottle promises the most repair. If you haven't taken it, it's the fastest place to start.