Cheiloscopy: Lip Prints and Their Forensic Value

The vermilion zone of the lips is covered by a stratified squamous epithelium that is thinner and less keratinised than the facial skin surrounding it. This surface is crossed by a series of grooves and furrows that run in various directions and depths. They are not uniform across the lip surface: patterns differ between the upper and lower lip, between the central and commissural zones, and between individuals of different ancestry. The grooves are visible with the naked eye on close inspection and can be enhanced photographically with oblique lighting.

The lips are functionally complex: they seal the oral cavity, participate in speech articulation, and contribute to facial expression. The grooves on the vermilion zone are believed to arise during fetal development along lines of differential epithelial growth. Unlike wrinkles on aged skin, which form under mechanical stress, the vermilion grooves are present from birth and reflect an underlying structural pattern in the connective tissue. This developmental origin is what supports the stability claim: they do not appear gradually or change with use in the way that mechanical creases do.

The lip print deposited on a surface includes the pattern from both the upper and lower lip. Cosmetic products such as lipstick or lip balm make the print immediately visible. Without them, the print is latent, left by the thin film of sebaceous secretion and sweat on the lip surface. Lipstick prints can often be photographed directly or lifted with transparent adhesive tape. Latent prints require chemical development before they are visible enough to classify.

Suzuki and Tsuchihashi's 1970 paper in the Journal of Forensic Medicine established the classification system that most subsequent research and casework has built on. They examined lip prints from a Japanese population and proposed that the grooves on the vermilion border could be reliably grouped into a small number of pattern types. The system codes each lip print by dividing the lip surface into four quadrants and assigning a pattern type to each.

The full lip print code is expressed as a four-quadrant string: upper left, upper right, lower left, lower right. An individual might code as I-II-I'-III, for example. Studies across different populations have found variation in the distribution of types: Type I and Type I' tend to be more common, and population differences exist between Japanese, Brazilian, Indian, and European samples, suggesting that the system captures real biological variation. Some researchers have proposed modifications, including the Renaud (1973) and Kasprzak systems, but Suzuki-Tsuchihashi remains the reference standard.

Stability studies have followed individuals over periods of months to several years and compared lip prints taken at different time points. Suzuki and Tsuchihashi's original paper included a longitudinal component showing that the groove pattern did not change over the study period. Subsequent researchers have tested stability through seasonal changes in lip condition, deliberate cosmetic treatment, and recovery from minor lip injuries. The consistent finding is that the fundamental groove pattern is preserved, though superficial texture may vary with hydration, temperature, and cosmetic products.

The uniqueness claim rests on population studies showing no two individuals in the sample shared an identical four-quadrant code. A 1997 study by Renaud and colleagues examining several hundred French individuals found no repeated patterns. Indian studies in the 2000s and 2010s, examining samples of 50 to 200 individuals, similarly found no duplicates. Twin studies are of particular interest: in the three published reports examining identical twins, the twins' lip prints were distinguishable from each other, supporting the claim that the pattern reflects developmental individuality rather than simply genotype.

The impact of the menstrual cycle, pregnancy, and hormonal changes on lip morphology has also been examined, and current evidence does not show pattern-altering effects. Chapping, herpes labialis (cold sore) lesions, and surgical procedures on the lip can temporarily obscure the pattern during the active phase, but studies of recovered lips post-healing have found the underlying groove pattern restored. This is consistent with the developmental origin hypothesis: the groove pattern reflects the underlying connective tissue structure, not just the surface epithelium.

Lip prints are most commonly found on glass surfaces (drinking glasses, wine glasses, windows, mirrors), polished ceramics, lacquered surfaces, and occasionally paper. The substrate matters enormously both for the clarity of the deposited print and for the collection method used. Glass and non-porous polished surfaces give the best results because they provide a stable base for lipstick or sebaceous material and do not absorb the print into the matrix.

1. Scene documentation
   Before touching the item, photograph the lip print in situ with scale and oblique lighting. The photography stage may reveal enough pattern detail for preliminary classification even before collection. Document the orientation and position of the print relative to the object.
2. Visible (lipstick) prints
   On non-porous surfaces, transparent adhesive lifting tape or gelatin lifters can transfer a visible lipstick print cleanly. Alternatively, the whole object is submitted to the laboratory. Photographic enhancement under ultraviolet or infrared illumination can improve contrast for faint coloured prints.
3. Latent prints on non-porous surfaces
   Techniques borrowed from fingerprint development work well: aluminium powder brushing, cyanoacrylate fuming followed by fluorescent staining, or vacuum metal deposition for delicate surfaces. Each method has different sensitivity and different risks of damaging the substrate; the choice follows the same decision logic as fingerprint development on the same surface type.
4. Latent prints on porous surfaces
   Paper and some fabrics absorb the lip deposit. Ninhydrin reacts with amino acids in the sebaceous and sweat fraction and produces a purple Ruhemann's adduct visible under visible or fluorescent light. DFO (1,8-diazafluoren-9-one) gives better contrast on some paper types. Results on porous surfaces are generally lower quality than on glass.

Once developed and photographed, the crime scene print is examined for completeness. A partial print covering only one or two quadrants is less discriminating than a full four-quadrant print. The examiner notes whether the print represents the whole lip or only part, whether any distortion from smearing is present, and which Suzuki-Tsuchihashi types are identifiable in each readable quadrant.

Cheiloscopy has been used in criminal investigations in Japan, India, Brazil, Spain, and elsewhere, and lip print comparisons have been presented in court in several countries. In India, lip print evidence has been admitted in state courts and has been discussed in forensic odontology proceedings as a supporting identification tool. The technique is referenced in Interpol DVI guidelines as a method for supplementary comparison. These facts establish that the technique is operationally real and has been accepted by some judicial systems.

The scientific critique is distinct from the operational history. The core criticisms are three. First, there is no agreed minimum concordance criterion: examiners use different thresholds for how many quadrants must match, or what degree of partial match is sufficient, before reporting a positive identification. Second, inter-examiner reliability is moderate at best. Studies testing multiple examiners on the same print set have found disagreement rates of 15 to 30 percent on groove type classification. Third, and most fundamentally, no population database large enough to generate validated probability statistics exists.

Proponents respond that cheiloscopy does have a body of supportive research, that no confirmed false positive has been documented in the published case literature, and that the technique is used in combination with other evidence rather than as a stand-alone identifier. This is a fair point, and it describes the responsible use model: lip print comparison as one layer of corroborating evidence, explicitly framed as such in the expert report, with the examiner acknowledging the absence of a population-level probability model when cross-examined.

The forensic odontologist's identification toolkit now includes dental charting and radiographs, DNA from pulp tissue, rugoscopy, and cheiloscopy. Each has different requirements for ante-mortem records, different survival profiles in catastrophic scenarios, and different evidential weights. A competent report describes what each available method found and presents the combined picture rather than relying on any single technique.

Cheiloscopy is most useful in a scenario where a suspect is alive and a known lip print sample can be taken for comparison against a scene print. This is analogous to fingerprint comparison in a burglary investigation: a latent print on a glass, a reference print from a suspect, and a side-by-side comparison. The technique is less useful for post-mortem identification of the unknown dead, because the deposited scene print is rarely available to compare against a recovered body.

• Scene-to-suspect comparison: the primary forensic use; a crime scene print is compared against reference prints from a named suspect. Glass-contact offences (assault, theft) are the most common scenario.
• Mass-casualty identification: a secondary use where lip condition in a deceased is compared against a clinical photograph taken during life. Rarely decisive on its own.
• Ante-mortem records: clinical photographs taken by dental or cosmetic practitioners sometimes capture sufficient lip detail for comparison if a reference print cannot be taken from the living person.