A downturned mouth, sometimes called an “upside down smile” or “resting frown,” usually traces back to one specific muscle: the depressor anguli oris, which pulls the corners of your mouth downward. When this muscle is naturally strong, overactive, or working against weakened opposing muscles, the result is mouth corners that slope down even when you feel perfectly content. But the reasons behind this vary widely, from the smile type you were born with to age-related tissue changes, nerve conditions, and even the shape of your jaw. Understanding which cause applies to you matters, because the fix for each is different.
The Muscle Behind the Downturned Mouth
The depressor anguli oris (DAO) is a fan-shaped muscle that runs from the lower edge of your jawbone up to the corner of your mouth. Its job is exactly what the name implies: it pulls the mouth corner down and slightly outward. Everyone has this muscle, and it fires when you frown, grimace, or express sadness. The trouble starts when the DAO is hyperactive or disproportionately strong compared to the muscles that pull the mouth corners up. When that imbalance exists, even a relaxed face can look like it’s frowning.1Plastic & Reconstructive Surgery. Anatomical Considerations Regarding the Location and Boundary of the Depressor Anguli Oris Muscle with Reference to Botulinum Toxin Injection
What makes this muscle especially interesting is that it appears to be uniquely developed in humans. Research into the evolutionary history of the DAO suggests it plays a role in emotional signaling that goes beyond what other primates can express.2PubMed Central. Anatomical Evidence for a Uniquely Human Depressor Anguli Oris and a Novel Helplessness Signaling Hypothesis The ability to pull the mouth corners down sharply may have evolved as a distress or helplessness signal. That is useful when you actually feel sad, but less helpful when the muscle’s resting tone gives you a permanent “sad face” you didn’t ask for.
Not Everyone’s Smile Works the Same Way
People often assume a smile means the corners of the mouth go up. That is one smile type, but it is not the only one. Research on smile dynamics identifies at least three distinct patterns. In a “commissure smile,” the zygomaticus major muscle lifts the mouth corners upward, and this is the version most people picture. In a “cuspid smile,” the upper lip rises uniformly without much upward pull at the corners, more like a window shade lifting. And in a “complex smile,” the upper lip rises while the lower lip simultaneously drops.3PubMed Central. Analysis of dynamic smile and upper lip curvature in young Chinese
If your natural smile pattern leans toward the cuspid or complex type, the corners of your mouth may not lift much or at all when you smile. This isn’t a malfunction. It’s simply which muscles dominate when your brain tells your face to express happiness. People with a strong DAO and a weaker zygomaticus major may find that even their best effort at a smile looks flat or slightly downturned to others. This is a normal anatomical variation, not a medical condition, though it can feel frustrating when people constantly ask if you’re upset.
How Aging Pulls Your Mouth Corners Down
Even if you had perfectly upturned mouth corners at twenty, several forces conspire to drag them down over the decades. The skin around the mouth is among the most active real estate on the face. Years of speaking, chewing, and making expressions gradually deepen the creases that run from the corners of the mouth toward the chin, known as marionette lines. Meanwhile, the fat pads in the midface thin out and descend, the jawbone slowly loses volume, and collagen in the skin fragments and weakens. The combined effect is a mouth that looks like it’s frowning even when you’re at rest.4PubMed Central. Natural and sun-induced aging of human skin
Sun exposure accelerates this process substantially. UV radiation breaks down the collagen matrix that gives skin its firmness, and the perioral region is particularly vulnerable because it’s rarely covered or generously sunscreened. Smoking also plays a measurable role: the repetitive pursing motion and the vascular constriction from nicotine both contribute to deeper lines and more downward pull at the mouth corners. Gravitational effects, bone resorption, and changes in the sphincteric muscles all pile on over time, making downturned corners one of the most common complaints patients bring to facial rejuvenation specialists.5Thieme / PubMed Central. Anatomy and Aging of the Perioral Region
Why Women Often Notice It More
If you’re a woman who feels like your perioral area aged faster than your male friends’, there’s a structural reason for that. Histologic studies have found that men have significantly more sebaceous glands and sweat glands in the skin around their mouths, along with a denser blood vessel network in the dermis. These differences appear to give male perioral skin more support and resilience. In direct comparisons, female skin replicas showed more numerous and deeper wrinkles around the mouth.6PubMed. Perioral wrinkles: histologic differences between men and women
A validated photonumeric scale for perioral wrinkling confirmed that age, gender, and years of smoking are the strongest predictors of how severe these lines become.7Journal of the American Academy of Dermatology. Development and validation of a photonumeric scale for perioral wrinkling This doesn’t mean men are immune, but it helps explain why many women develop noticeable downturned mouth corners and marionette lines at an earlier age than they might expect.
When a Nerve Condition Is Involved
Sometimes a downturned mouth isn’t about muscle strength or skin elasticity at all. It’s about the nerve signals reaching those muscles. The facial nerve (cranial nerve VII) controls the muscles of facial expression, and damage to it, whether from Bell’s palsy, surgery, trauma, or infection, can leave one or both sides of the mouth drooping. After the acute phase resolves, many people don’t fully recover. Instead, the nerve regrows along slightly misdirected pathways, creating a phenomenon called synkinesis, where trying to move one set of facial muscles accidentally fires another.
Research on patients with incomplete recovery from facial paralysis found that smile movements are among the most frequently impaired, and the most common form of synkinesis was unintended eye closure when trying to smile.8PubMed Central. Features of facial asymmetry following incomplete recovery from facial paralysis After recovery from facial palsy, abnormal DAO activity can also develop and actively restrict movement of the mouth corner, pulling it down even during attempted smiling.9PubMed. Effect of Weakening of Ipsilateral Depressor Anguli Oris on Smile Symmetry in Postparalysis Facial Palsy If your downturned smile appeared after a neurological event or gradually worsened on one side, this is worth discussing with a specialist, because the treatment approach for nerve-driven asymmetry differs from cosmetic solutions for aging.
Born With It: Congenital Asymmetry
Some people have had an uneven or downturned mouth since birth. One recognized cause is congenital hypoplasia of the depressor anguli oris muscle (CHDAOM), a condition affecting roughly 3 to 6 out of every 1,000 newborns. In this condition, the DAO on one side is underdeveloped or absent. The result is that when the baby cries, the working DAO on the normal side pulls that mouth corner down, while the affected side stays still, creating an asymmetric crying face.10BMJ Case Reports CP. Congenital hypoplasia of depressor anguli oris muscle (CHDAOM): an uncommon cause of asymmetric crying facies in childhood
CHDAOM is generally considered benign, and the asymmetry often becomes less noticeable as the child grows and gains more voluntary control over facial expression. However, it can initially mimic facial nerve palsy, and pediatricians sometimes investigate to rule out more serious causes. In adults who have always had one side of their mouth that turns down differently from the other, CHDAOM or a similar congenital muscular variation may be the underlying explanation, even if it was never formally diagnosed in childhood.
How Others Read Your Downturned Mouth
One of the most frustrating aspects of an upside-down smile is how other people interpret it. The DAO and the surrounding lower-face muscles are directly tied to the facial action units that communicate sadness, contempt, and displeasure. Research into how lower-face muscles contribute to perceived emotion found that the DAO participates in expressions linked to socially significant emotions and influences how approachable or dominant a person appears.11PubMed Central. A Psychological Guide to Lower Face Botulinum Toxin Injections: Baseline Emotional Functions of Facial Expressions In other words, when your mouth corners sit low, people around you may unconsciously read you as unhappy, unapproachable, or even annoyed, regardless of how you actually feel.
This perception gap has real social consequences. People report being told to “cheer up” or asked “what’s wrong?” dozens of times a day, which can be isolating. The science confirms that their experience isn’t imagined: the lower face is a primary zone humans scan to gauge another person’s emotional state, and a resting downward curve sends a strong signal that overrides context. Awareness of this dynamic is part of why so many people seek treatment, not because their mouth is medically problematic, but because the social mismatch between their inner state and their external expression is exhausting.
Botulinum Toxin for the Mouth Corners
The most common nonsurgical treatment for downturned mouth corners is a small injection of botulinum toxin (Botox, Dysport, or similar) directly into the DAO. The logic is straightforward: by weakening the muscle that pulls the corners down, you allow the opposing elevator muscles to lift them slightly upward. The effect is subtle, typically shifting the resting mouth position from a frown to neutral, and lasts around three to four months before needing a repeat treatment.
Getting the injection site right matters enormously. The DAO sits close to several other muscles, and injecting too low, too medially, or too deeply can cause toxin to spread into adjacent muscles, producing an asymmetric smile or difficulty controlling the lower lip. A retrospective anatomical study found that a three-point injection technique performed in the upper half of the DAO, rather than lower near the jawline, was associated with good outcomes and fewer side effects from toxin spreading to neighboring muscles.12PubMed. A Retrospective and Anatomical Study Describing the Injection of Botulinum Neurotoxins in the Depressor Anguli Oris Other researchers have proposed a simpler single-point injection per side using anatomical landmarks, with doses of about 2 to 3 units per point, emphasizing that the injection should stay shallow (intradermal or subdermal) and should not be placed too close to the mouth corner to avoid affecting smile muscles.13Anatomy & Cell Biology. Novel anatomical proposal for botulinum neurotoxin injection targeting depressor anguli oris for treating drooping mouth corner
Beyond cosmetics, this same approach has therapeutic applications. In patients with post-paralysis synkinesis, weakening an overactive DAO on the affected side can restore smile symmetry by allowing the mouth corner to move more freely during attempted smiling.9PubMed. Effect of Weakening of Ipsilateral Depressor Anguli Oris on Smile Symmetry in Postparalysis Facial Palsy
Fillers, Surgery, and Other Interventions
When the downturned appearance is driven more by volume loss and sagging tissue than by pure muscle overactivity, dermal fillers become relevant. Hyaluronic acid, calcium hydroxyapatite, and poly-L-lactic acid fillers can restore volume in the marionette line area, prejowl sulcus, and chin to physically support the mouth corners from below.14PubMed. The Role of Toxins and Fillers in Optimizing Perioral Rejuvenation For best results, many practitioners combine fillers with botulinum toxin: the filler replaces lost structural volume while the toxin reduces the downward muscle pull. Cannula-based injection into the deeper layers around the marionette lines has been shown to be effective, with neurotoxin to the DAO enhancing outcomes by addressing both the static and dynamic components of the droop.15PubMed. Anatomical-based diagnosis and filler injection techniques: marionette line (static labiomandibular fold)
For more severe cases, surgical options exist. A “corner mouth lift” involves removing a small amount of skin at the commissure to physically reposition the corner upward. Several techniques have been developed, including direct excision, modified anguloplasty using superimposed triangle designs, lentiform excision, and rhomboid excision. Some surgeons use a staged approach that begins with toxin injections and fillers and progresses to autologous fat transfer and DAO resection only when less invasive methods aren’t sufficient.16PubMed Central. Surgical Method of a Corner Mouth Lift The trade-off with surgery is a small scar at the mouth corner, though experienced surgeons place incisions along the natural lip line to minimize visibility.
Facial Exercises and Neuromuscular Retraining
If your downturned mouth relates to muscle coordination problems rather than structural loss, facial neuromuscular retraining (fNMR) may help. This is a specialized form of physical therapy originally developed for patients recovering from facial nerve injuries. Rather than trying to “strengthen” the muscles, which can actually worsen imbalances, fNMR focuses on teaching the brain to fire the right muscles in the right patterns. The goal is improving coordination, reducing unwanted co-contraction, and gradually restoring more symmetrical movement.17PubMed. Facial rehabilitation: a neuromuscular reeducation, patient-centered approach
The approach involves practicing small, precisely controlled movements, often using a mirror or surface EMG feedback, to retrain the brain’s motor maps for the face. For people with synkinesis after Bell’s palsy or other facial nerve conditions, the evidence is encouraging: patients can learn to decouple unwanted muscle contractions and produce more intentional facial expressions over time.18Management of Post-Facial Paralysis Synkinesis. Facial Neuromuscular Retraining for Synkinesis More general facial exercise programs have also been explored as a way to counterbalance aging effects on the perioral region, though the evidence here is thinner and the results are more modest.19PubMed Central. Enhancing facial aesthetics with muscle retraining exercises-a review
The Evolutionary Angle
There’s one more layer to why your face does this, and it goes back a long way. The human smile is widely believed to have evolved from the “silent bared teeth” display seen in other primates. In most primate species, baring the teeth is a signal of appeasement or fear, typically directed from a subordinate animal toward a dominant one. In species with more relaxed social hierarchies, the same display shifted to become a signal of reassurance between equals, and in humans it eventually became the smile we associate with friendliness and warmth.20Evolutionary Human Sciences. Revisiting Darwin’s comparisons between human and non-human primate facial signals
The downward pull of the mouth corners, by contrast, appears to have its own distinct evolutionary trajectory. The DAO’s development in humans may represent an adaptation for communicating helplessness or distress, a signal that would have been valuable for eliciting caregiving or social support.2PubMed Central. Anatomical Evidence for a Uniquely Human Depressor Anguli Oris and a Novel Helplessness Signaling Hypothesis So when your resting face sends a “sad” signal to the people around you, it’s tapping into a very old communication channel, one that predates language and operates below conscious awareness for both sender and receiver. Your mouth corners aren’t malfunctioning. They’re speaking a language your face evolved to speak, just at a volume you might prefer to turn down.