Have you ever found a huge bruise on your leg and thought, "Where the hell did that come from?" I have. And so has every woman I have ever met.
"I get them all the time," says LA comedian Melinda Kashner. "My boss even had an 'is everything okay at home?' conversation with me about it."
Kashner has at least one bruise of unknown provenance on her body at any given time. "It's both legs and arms—almost exclusively limb-based," she says. Kashner is not alone.
On tumblr, a title-only post that reads "team mysterious leg bruise" has received more than 350,000 notes. The rebloggers are overwhelmingly female-identified. When I put the word out on Facebook that I was looking for other members of Team Mysterious Leg Bruise, the responses I received were exclusively from women. One woman sent me a picture of her current array of leg bruises; together, they formed a frowny face. The one man who commented on my post did so only to tag a woman he knew.
"I remember being a kid in fourth grade, when I had to wear a jumper, the other kids would always ask where the bruises on my legs came from," says Jackie, owner of the frowny face of bruises, "but I never had any clue."
Jackie still doesn't have a clue. "It's probably just that I'm clumsier than I even realize? But I don't think I walk into that much stuff."
Skin is made of three layers: the epidermis (topmost layer, what you can see), the dermis (where capillaries, sweat glands, and hair follicles hang out), and the hypodermis (mostly fat, with some blood vessels and other structural supports). A bruise forms when blood vessels in the dermis or hypodermis break, causing blood to leak into the surrounding tissue. A bruise doesn't show up immediately; they usually appear a day or two after the injury. The color of the bruise depends on the depth of the injury.
"There's something called the Tindell effect," explains Mayo Clinic dermatologist Dawn Davis, M.D., "where colors can look different depending on what layer of skin they're in. Things look darker the deeper they go in the skin." A bruise that would be brown or reddish near the surface of the skin would look purple or even black if the injury is hypodermal.
Is there something about women's skin that makes us so easy to bruise? Dermatologist Jeffrey Benabio writes on his blog that women bruise more easily than men because our skin has more fat and less collagen. "The dense collagen layer is thicker in men and the blood vessels are held more securely," he writes. "Similarly, structural differences between men's and women's skin can be seen in things like cellulite."
According to Dr. Joel Cohen of AboutSkin Dermatology, collagen is "the main structural building block in the skin." In the dermis, collagen forms a network of fibers that hold the rest of the skin together like a net. The collagen supports blood vessels so they are more protected from blunt force. Subcutaneous fat, on the other hand, does not structurally support the blood vessels—it's padding. "It's part of our buffer before we get down to the bone," says Cohen. "It protects our bones and muscles from the outside world."
Imagine your skin is a truck full of peaches.
Imagine your skin is a truck full of peaches. In collagen-heavy skin, the peaches are in crates. They're kept firmly in place and are less likely to bruise on the journey. In fattier skin, the peaches are floating in pudding. They're more likely to be jostled around and generally tempered with. If those trucks full of peaches are driven down the same road, and pudding truck will likely have more bruised peaches than the non-pudding truck.
There's only one problem with this theory: Not every woman has more subcutaneous fat than every man. It's a common belief that women have an extra layer of fat—one that pops up on weight loss and bodybuilding blogs and even in an episode of Cheers.
As is often the case, gender differences are complicated. "It's hard to make such sweeping generalizations," says Cohen. "It really depends on somebody's BMI." BMI stands for Body Mass Index, a number derived from your total body mass divided by your height. Like gender, humans have an extremely wide range when it comes to BMI.
"What we can say is that men and women have different BMI ranges," says Davis. "And where they store fat, that's due to estrogen and testosterone. Men and women can bruise different places." Men tend to accumulate fat "above the belt," whereas women store junk in their "thighs, hips, and buttocks."
Davis points out one more reason why women may bruise more easily and often than men. "Estrogen weakens blood vessel walls," he says. Scientists are still unsure how exactly, but estrogen prevents blood vessel walls from building. Estrogen is also a vasodilator, meaning it keeps blood vessels open, which could increase bruising. Dilated blood vessels mean more blood leaking out before clotting. "It's why women get varicose veins," says Davis, "and why women's blood vessels change after menopause."
This article originally appeared on VICE US.