Sigmatism is the clinical term for what most people call a lisp. It refers to the distorted production of sibilant sounds, the hissing or buzzing consonants like /s/, /z/, “sh,” “ch,” and “zh.” While lisping is extremely common in young children learning to speak, it becomes a clinical concern when it persists beyond the age when these sounds are typically mastered, usually around 4 to 5 years old for most sibilants.
Which Sounds Are Affected
Sigmatism doesn’t always stop at “s” and “z.” English has six sibilant sounds: /s/, /z/, “sh,” “zh,” “ch,” and the “j” sound as in “judge.” Some languages have even more. A child or adult with sigmatism may distort just one or two of these sounds, or struggle with the entire group. The core problem is the same across all of them: the tongue isn’t directing airflow precisely enough to produce the sharp, focused sound these consonants require.
Types of Sigmatism
Not all lisps sound the same, and speech-language pathologists classify sigmatism by where the tongue goes wrong.
- Interdental (frontal) sigmatism: The most recognizable type. The tongue pushes forward between the upper and lower teeth during “s” and “z” sounds, producing something closer to a “th.” This is the version most people picture when they think of a lisp, and it’s also the most common in young children.
- Lateral sigmatism: Instead of air flowing in a narrow stream over the center of the tongue, it escapes over the sides. The tongue position closely resembles the one used for the /l/ sound, and the result is a wet, slushy quality. Lateral lisps are generally considered more complex to treat than frontal ones.
- Dental sigmatism: The tongue presses against the back of the upper front teeth rather than sliding between them. The sound comes out muffled or dull rather than crisp.
- Palatal sigmatism: The tongue makes contact with the roof of the mouth (the hard palate) during the sound, producing a distortion that can sound similar to “sh” when the person is aiming for “s.”
What Causes It
Sigmatism can have structural, developmental, or habitual origins, and sometimes more than one factor is at play.
Tongue-tie (ankyloglossia) is one well-documented contributor. When the strip of tissue connecting the underside of the tongue to the floor of the mouth is unusually short or tight, it restricts tongue movement. Research shows that people with tongue-tie experience roughly 5.5 times more restrictions in tongue mobility and atypical muscle patterns compared to those with a normally attached frenulum. That limited range of motion can make it difficult to position the tongue precisely enough for sibilant sounds.
Dental alignment also matters. Missing front teeth, gaps, crowding, or an overbite can change the space the tongue has to work with. Children who lose baby teeth early or have orthodontic issues sometimes develop compensatory tongue placements that persist as habits even after their teeth shift. Malocclusion, where the upper and lower teeth don’t meet correctly, has been linked to changes in tongue posture that can affect sibilant production.
In many cases, though, there’s no structural abnormality at all. The child simply hasn’t learned the precise motor pattern for these sounds. Prolonged pacifier use or thumb-sucking can reinforce a forward tongue position that carries over into speech. And some children develop a lateral or palatal tongue placement for sibilants without any identifiable cause.
When Sigmatism Is Developmentally Normal
Young children routinely substitute, distort, or drop sounds as they learn to talk, and sibilants are among the trickier ones to master. According to developmental guidelines from speech-language pathology, the /s/, /z/, “sh,” and “ch” sounds fall into the “middle developing” category, generally expected to emerge between ages 4 and about 4 years 11 months. The voiced “zh” sound (as in “equation”) develops even later, typically between ages 5 and 7.
A 3-year-old who says “thun” instead of “sun” is following a completely typical pattern. A 6-year-old doing the same thing is more likely to benefit from intervention. The distinction matters because early referral for a child who’s simply on a normal developmental timeline can cause unnecessary anxiety, while waiting too long for a child who genuinely needs help allows the pattern to become more entrenched.
How It’s Diagnosed
A speech-language pathologist evaluates sigmatism through a combination of listening, observation, and structured testing. The assessment typically includes a review of the person’s medical, dental, and developmental history, along with standardized articulation tests that sample specific sounds in different word positions (beginning, middle, and end of words). The clinician listens for which sounds are affected, how they’re distorted, and whether the errors are consistent or variable.
Beyond just identifying the lisp, the evaluation looks at oral-motor function: how well the tongue, lips, and jaw move and coordinate. The clinician may check for structural issues like tongue-tie, high palate, or dental irregularities. They’ll also assess whether the distortion affects the person’s ability to be understood in conversation and how it impacts their daily communication, whether that’s classroom participation for a child or professional interactions for an adult.
How Sigmatism Is Treated
Speech therapy for sigmatism focuses on teaching the tongue a new placement and helping the person build it into a habit. The specific approach depends on the type of lisp.
For interdental sigmatism, the core goal is getting the tongue behind the teeth. A therapist might use a mirror so the person can see their tongue position, or place a small tool along the midline of the tongue to help guide airflow into the correct narrow stream. These techniques, called phonetic placement methods, give concrete physical cues rather than relying on vague instructions like “say it differently.”
Lateral sigmatism often requires more intensive work because the airflow pattern is fundamentally different from the target. Therapy typically starts by establishing a central airflow pattern, sometimes by starting from a sound the person already produces correctly (like “t” or “ts”) and gradually shaping it toward a clean “s.”
For both types, therapy moves through predictable stages: producing the sound in isolation, then in syllables, then in words, then in sentences, and finally in spontaneous conversation. Each stage builds on the last, and the transition to natural speech is usually the longest part. Younger children often progress faster because they’ve had less time to reinforce the habit. Adults can absolutely correct sigmatism too, but it typically takes more conscious practice to override years of muscle memory.
When a structural issue is involved, treatment may require coordination with other professionals. Tongue-tie can be addressed with a minor surgical release (frenotomy), after which speech therapy helps the person take advantage of their new range of motion. Orthodontic treatment can resolve dental spacing issues that contribute to the problem.
Sigmatism in Adults
While sigmatism is most often discussed in the context of children, plenty of adults live with it. Some were never referred for speech therapy as children. Others went through therapy but didn’t fully resolve the pattern. And some developed a lisp later due to dental changes, injury, or poorly fitting dental work.
Adults seeking treatment often do so because their speech affects their confidence in professional settings, public speaking, or social situations. The process is the same as for children: retraining tongue placement and airflow, then practicing until the new pattern becomes automatic. Most adults who commit to regular practice see meaningful improvement within a few months, though full carryover into everyday conversation can take longer.