Why Laboratory Mice Are White: Genetics and Breeding

The Uniqueness of Laboratory Mice

Historical Context of Mouse Domestication

The domestication of mice began with the capture of wild Mus musculus specimens for agricultural pest control in the 17th century. Farmers observed that captured individuals adapted quickly to stored grain environments, prompting intentional breeding for reduced aggression and increased fecundity. By the early 19th century, naturalists such as François Magendie incorporated captive mice into experimental physiology, establishing the first systematic colonies.

The transition from wild to laboratory stock accelerated after the discovery of the albino mutation in the late 1800s. Albino mice provided visual uniformity, facilitating anatomical observation and genetic analysis. Selective breeding programs at institutions like the University of Cambridge and the Jackson Laboratory refined this phenotype, producing genetically homogeneous strains used for biomedical research.

Key historical milestones:

1800s: Identification of the coat‑color mutation (albino) in laboratory colonies.
1902: Development of the “Swiss” (C57) inbred line, emphasizing genetic stability.
1911: Initiation of the Jackson Laboratory, focusing on standardized mouse strains.
1920s–1930s: Expansion of inbreeding techniques, resulting in fully homozygous white lines.
1940s: Adoption of white mice as the default model for genetics, immunology, and pharmacology.

These events established the white mouse as the predominant laboratory organism, linking its historical domestication to contemporary genetic and breeding practices.

Early Observations of Mouse Coloration

Early European naturalists recorded mouse coat colors during the mid‑1800s, noting that most laboratory strains exhibited a uniform white phenotype while wild populations displayed a range of pigments. Observations by Karl Pearson and William Castle in the 1890s documented the prevalence of albinism among colonies bred for experimental reproducibility. Their reports emphasized that white individuals were easier to identify, facilitated genetic crosses, and reduced visual interference in physiological studies.

Key milestones in the documentation of mouse coloration include:

1868 – John Graham published a catalog of mouse specimens, distinguishing albino specimens from pigmented varieties.
1892 – William Castle’s breeding experiments highlighted the inheritance of the white coat as a single‑gene trait, later identified as the c (albino) allele.
1902 – Karl Pearson’s statistical analysis of coat color frequencies confirmed Mendelian ratios in controlled matings, reinforcing the notion that white coat dominance could be quantified.

These early records established a baseline for later genetic investigations, linking the observed white phenotype to a heritable mutation and setting the stage for systematic breeding programs that prioritize the albino strain for laboratory use.

The Genetic Basis of White Fur

Melanin Production Pathway

Tyrosinase and Albinism

Tyrosinase is a copper‑containing oxidase that initiates melanin production by converting L‑tyrosine to L‑DOPA and then to DOPAquinone. These reactions generate the pigment precursors that polymerize into eumelanin and pheomelanin, the pigments responsible for dark fur, eye color, and skin tone in rodents.

Albinism arises when mutations disrupt the TYR gene, eliminating or severely reducing enzymatic activity. The loss of melanin synthesis yields a phenotype characterized by white coat, pink eyes, and lack of pigment in the retina and inner ear. In laboratory mice, several alleles—such as albino (c), c‑h, and c‑c—are documented; each allele carries a distinct point mutation, insertion, or deletion that impairs tyrosinase folding, copper binding, or catalytic function.

Breeding strategies exploit these mutations to produce uniform white colonies. Maintaining an albino line requires:

Homozygous recessive genotype (tyr/tyr) to ensure complete loss of pigment.
Regular genotyping to detect accidental heterozygous carriers that could reintroduce pigment.
Controlled matings that avoid outcrossing with pigmented strains, preventing the reappearance of functional TYR alleles.

The white phenotype offers practical advantages: ease of visual assessment for transgenic markers, reduced background coloration in imaging studies, and consistent baseline for behavioral experiments. However, albinism also associates with visual deficits and altered auditory function, factors that must be considered when interpreting experimental outcomes.

Other Genes Affecting Pigmentation

Laboratory mice exhibit a predominantly white coat because mutations in several pigment‑related genes suppress melanin synthesis or distribution. Beyond the classical albino allele of the tyrosinase gene, additional loci contribute to color variation.

Kit (c‑Kit) – loss‑of‑function mutations disrupt melanocyte migration during embryogenesis, resulting in reduced pigment cells and a white phenotype.
Mitf (Microphthalmia‑associated transcription factor) – haploinsufficiency diminishes transcription of melanogenic enzymes, leading to hypopigmented coats.
Mc1r (Melanocortin‑1 receptor) – null alleles prevent signaling that normally shifts melanin production toward eumelanin; mice with inactive Mc1r display lighter fur.
Asip (Agouti signaling protein) – overexpression antagonizes melanocortin receptors, favoring pheomelanin synthesis; strong Asip activity can mask darker pigments.
Oca2 (Oculocutaneous albinism II) – deletions reduce melanosomal pH, impairing tyrosinase activity and producing pale fur.
Slc24a5 (NCKX5) – allelic variants alter ion transport in melanosomes, affecting melanin maturation and resulting in lighter coat colors.
Hsp90 (Heat shock protein 90) – reduced chaperone function destabilizes client proteins such as Kit, indirectly influencing melanocyte survival.

Breeding programs that aim for uniform white coats often combine these alleles to ensure complete suppression of melanin. Selecting for homozygous null mutations in Kit and Mitf, together with a recessive albino tyrosinase allele, guarantees a consistent lack of pigment across generations. Conversely, maintaining heterozygosity at Mc1r or Asip can introduce subtle shade variations useful for experimental controls.

Genetic Mutations Leading to White Phenotype

Tyrosinase-Deficient Albinism (c/c genotype)

Tyrosinase-deficient albinism results from two loss‑of‑function alleles at the Tyr locus (c/c genotype). The mutation eliminates enzymatic activity required for the conversion of tyrosine to DOPA and subsequently to melanin, halting pigment production in melanocytes. Consequently, skin, hair, and ocular tissues remain unpigmented, giving laboratory mice their characteristic white appearance.

Inheritance follows an autosomal recessive pattern. Homozygous c/c individuals display the albino phenotype, while heterozygous carriers (c/+) retain normal coloration but transmit the allele to offspring. Breeding strategies that maintain a pure c/c line involve:

Selecting only c/c parents for mating to ensure 100 % albino progeny.
Periodic genotyping to confirm the absence of wild‑type Tyr alleles.
Avoiding inadvertent introduction of pigmented strains, which could reintroduce functional Tyr alleles through outcrossing.

The c/c genotype influences experimental variables. Lack of pigment reduces light absorption in the eye, affecting visual‑system studies, and eliminates background coloration that can interfere with imaging of fluorescent reporters. Moreover, the uniform white coat simplifies phenotypic scoring in genetic screens and behavioral assays where visual cues must be minimized.

From a breeding perspective, the albino line provides a stable genetic background for transgenic and knockout models. Maintaining the c/c allele requires strict colony management, regular health monitoring, and documentation of pedigree to prevent genetic drift that could reintroduce functional Tyr variants.

Other Hypopigmentation Genes

Laboratory mouse strains exhibit a range of hypopigmented phenotypes that arise from mutations in genes distinct from the classic albino allele. These additional hypopigmentation genes affect melanin synthesis, melanosome formation, or melanocyte development, producing coats that range from cream to slate-gray and facilitating specific experimental designs.

Oca2 (p) – encodes a melanosomal membrane protein; loss‑of‑function alleles reduce melanin deposition, yielding a pale brown coat. Homozygous p mice retain functional vision but display markedly lighter fur.
Slc45a2 (underwhite, uw) – transports substrates required for melanin production; the uw allele diminishes pigment intensity without compromising retinal function.
Kit (dominant white spotting, W) – a receptor tyrosine kinase essential for melanocyte migration; heterozygous W mice present extensive white patches, while homozygotes are embryonic lethal.
Kitl (steel, sl) – ligand for Kit; sl alleles impair melanocyte survival, producing a diluted coat and occasional white spotting.
Mitf (microphthalmia, mi) – transcription factor regulating melanocyte differentiation; mi alleles cause severe pigment loss and ocular abnormalities, useful for studying pigment cell lineage.
Hps1 and Hps4 (cocoa, cp; mouse coat color 3, mc3) – components of the biogenesis complex for melanosomes; mutations generate a subtle gray‑white coat and serve as models for Hermansky‑Pudlak syndrome.
Tyrp1 (brown, b) – modifies melanin type; the b allele shifts pigment from black to brown, often combined with other hypopigmentation alleles to produce lighter phenotypes.

Each gene follows a Mendelian inheritance pattern, typically recessive, except for dominant alleles such as Kit‑W. Breeders exploit these alleles to generate strains with defined pigmentation levels, enabling visual tracking of cells, assessment of gene‑editing efficiency, and evaluation of skin‑related phenotypes. Understanding the functional impact of these hypopigmentation genes expands the genetic toolkit for mouse model development.

Breeding for Uniformity

Inbreeding and Strain Development

Maintaining Genetic Homogeneity

Maintaining genetic homogeneity is essential for reproducible research using white laboratory mice. Uniform genetic backgrounds eliminate confounding variables, allowing investigators to attribute observed phenotypes directly to experimental manipulations rather than hidden genetic variation.

Achieving homogeneity relies on controlled breeding strategies. Inbred strains are produced by successive brother‑sister matings for at least 20 generations, resulting in individuals that are nearly genetically identical. Backcrossing to a defined parental line further purges unwanted alleles while preserving the white coat genotype. Marker‑assisted selection accelerates this process by confirming the presence of specific loci associated with coat color and other traits.

Colony management practices reinforce stability. Routine genotyping verifies that each generation retains the intended allelic composition. Segregated housing of breeding pairs prevents accidental outcrossing. Cryopreservation of embryos or sperm provides a genetic backup, enabling the re‑establishment of a line if drift occurs.

Key actions for preserving homogeneity:

Perform regular genotypic screening of breeding stock.
Limit breeding pairs to a defined number to reduce genetic drift.
Rotate breeders according to a documented pedigree to avoid accidental inbreeding depression.
Store germplasm from core individuals for long‑term reference.
Document all breeding events in a centralized database.

By integrating these measures, research facilities sustain the white phenotype and the genetic uniformity required for high‑precision biomedical studies.

Benefits for Research Reproducibility

White laboratory mice provide a genetically uniform population that minimizes variability unrelated to experimental manipulation. Controlled breeding schemes produce offspring with identical alleles at most loci, ensuring that observed differences stem from the intended interventions rather than hidden genetic background.

Consistent genotype reduces intra‑experiment noise.
Uniform coat color simplifies visual identification and automated imaging.
Standardized breeding protocols allow laboratories to exchange animals without altering genetic composition.
Predictable phenotypic baseline facilitates calibration of equipment and assays.

These attributes enable researchers to replicate findings across different facilities. When multiple groups employ the same strain, statistical comparisons rely on equivalent biological material, increasing confidence in pooled analyses. The reduction of confounding variables shortens the sample size required to achieve a given power, conserving resources while preserving scientific rigor.

Overall, the genetic and breeding characteristics of white mice create a reproducible platform that supports reliable data generation, cross‑lab validation, and cumulative knowledge building.

Selective Breeding for Albinism

Early Breeding Practices

Early 20th‑century researchers required mice with easily distinguishable phenotypes for experimental consistency. Albino individuals, displaying a uniform white coat and pink eyes, offered visual clarity and reduced background variation. Consequently, breeding programs deliberately introduced the recessive albino allele (c) into emerging colonies.

Breeders employed several systematic methods:

Selection of albino founders from natural populations or spontaneous mutants.
Repeated backcrossing of heterozygous offspring to albino parents to increase homozygosity for the c allele.
Establishment of inbred lines through sibling mating for at least 20 generations, fixing the white phenotype and associated genetic background.

Institutions such as the Jackson Laboratory formalized these practices. They maintained strict pedigree records, isolated colonies to prevent outcrossing, and distributed standardized white strains to other laboratories. The resulting uniformity in coat color facilitated comparative studies, immunological assays, and genetic manipulation across research facilities worldwide.

The Advantage of a Visible Trait

White laboratory mice provide a readily observable phenotype that simplifies experimental design and colony management. The conspicuous coat color enables rapid identification of individuals, reduction of handling time, and reliable segregation of genetic lines.

Visual screening: Researchers can distinguish homozygous, heterozygous, and wild‑type animals without molecular assays, accelerating genotype verification.
Standardization: Uniform coloration eliminates background variation that could interfere with phenotypic measurements, such as behavioral scoring or imaging studies.
Breeding efficiency: Visible traits allow immediate selection of breeding pairs, decreasing the number of litters required to achieve desired genotypes.
Error mitigation: Misidentification of mice is a common source of experimental bias; a distinct coat reduces this risk by providing a clear, non‑invasive marker.

The white coat also facilitates integration with automated tracking systems, which rely on contrast detection for monitoring locomotion, social interaction, and physiological responses. Consequently, the visible trait enhances reproducibility, lowers costs, and improves data quality across biomedical research that utilizes murine models.

Research Implications of White Mice

Standardization in Scientific Experiments

Reducing Variables in Drug Testing

White laboratory mice dominate pre‑clinical studies because their uniform coat color reflects a genetically homogeneous background. This homogeneity simplifies experimental design, allowing researchers to focus on pharmacological effects rather than genetic noise. Reducing variables in drug testing therefore relies on controlling three principal sources of variability: genetic background, environmental conditions, and procedural consistency.

Select a single inbred strain with a stable albino phenotype to eliminate coat‑color‑linked metabolic differences.
Maintain constant temperature, humidity, and light‑dark cycle throughout the study period.
Standardize handling protocols, including cage changes, feeding schedules, and injection techniques.

Genetic uniformity minimizes inter‑animal differences in drug metabolism, receptor expression, and immune response. Environmental control prevents stress‑induced hormonal fluctuations that can alter pharmacokinetics. Consistent procedures reduce operator bias and measurement error. Together, these measures increase statistical power, lower required sample sizes, and improve reproducibility of efficacy and safety assessments.

Consistent Immunological Responses

White laboratory mice provide a genetically uniform platform for immunological studies. The albino phenotype arises from mutations in the Tyrosinase (Tyrc) gene, which eliminates melanin production. Breeding programs maintain homozygous Tyrc alleles, ensuring that all offspring share the same coat color and, importantly, the same underlying genetic background. This uniformity reduces phenotypic variability that could affect immune parameters.

Consistent immunological responses in white mice result from several factors:

Stable major histocompatibility complex (MHC) expression: Inbred strains such as C57BL/6 and BALB/c carry defined MHC haplotypes, allowing predictable antigen presentation.
Uniform cytokine profiles: Genetic homogeneity yields reproducible baseline levels of key cytokines (e.g., IL‑2, IFN‑γ), facilitating comparison across experiments.
Controlled microbiota exposure: Standardized housing and diet for white mice limit microbial influences on immune development, preserving assay reliability.
Predictable antibody responses: Fixed IgG subclass distributions and affinity maturation patterns simplify vaccine efficacy assessments.

Breeding strategies reinforce these attributes. Pairings are performed between individuals with confirmed genotype and phenotype, and offspring are screened for coat color and genetic markers before inclusion in research colonies. This practice eliminates inadvertent introduction of genetic drift, which could otherwise alter immune responsiveness.

Overall, the white coat serves as a visual indicator of a tightly regulated genetic framework. By preserving consistent immunological traits, white laboratory mice enable reproducible experiments, accurate interpretation of immune mechanisms, and reliable translation of findings to broader biomedical contexts.

Ethical Considerations

Animal Welfare and Genetic Modification

White coat in research mice results from targeted disruption of pigment‑producing genes, most commonly the tyrosinase (Tyr) mutation. The mutation is introduced through embryonic stem cell manipulation or CRISPR‑based editing, followed by selective breeding to establish stable, homozygous lines. This genetic uniformity reduces variability in experiments that rely on visual markers, but it also raises specific welfare issues.

Genetic alteration eliminates melanin, increasing susceptibility to ultraviolet radiation and skin lesions. Mice with a lack of pigment exhibit higher rates of ocular abnormalities, such as corneal opacity and cataract formation, because melanin normally protects retinal tissue. Breeding programs must therefore incorporate protective housing conditions—low‑intensity lighting, UV‑filtering cages, and regular ophthalmic examinations—to mitigate these risks.

Breeding strategies that prioritize rapid line establishment can amplify stress. Repeated backcrossing and large litter sizes often lead to overcrowding, competition for nursing, and higher mortality. Ethical protocols require:

Monitoring of litter size and weaning rates to prevent excessive sibling competition.
Provision of enrichment items that compensate for reduced visual cues, such as textured nesting material and olfactory stimulants.
Routine health assessments focusing on skin integrity, eye health, and body condition scores.

Regulatory frameworks, including the Guide for the Care and Use of Laboratory Animals and national animal welfare statutes, classify the creation of albino lines as a procedure that demands justification based on scientific necessity and a detailed mitigation plan. Institutional Animal Care and Use Committees (IACUCs) evaluate proposals for genetic modification, ensuring that the anticipated benefits outweigh the increased risk of dermatological and ocular complications.

The welfare impact of white‑coat mice extends to experimental outcomes. Stress‑induced physiological changes, such as altered hormone levels, can confound data interpretation. Researchers must account for these variables by including appropriate control groups and by documenting any welfare‑related interventions. Transparent reporting of genetic background, housing conditions, and health monitoring practices supports reproducibility and ethical accountability.

The Role of Color in Perceived Vulnerability

Laboratory mice are predominantly white because selective breeding has fixed alleles that suppress pigment production. This uniform appearance simplifies visual monitoring, but it also shapes how researchers and observers judge the animals’ condition.

Human observers tend to link light coloration with fragility. Experiments measuring perceived vulnerability show that participants rate white rodents as more delicate, more likely to experience pain, and more in need of protection than darker‑coated counterparts. The association arises from cultural symbols of purity and from evolutionary cues that associate pale fur with sick or young individuals.

Gentle handling increases when staff view a mouse as vulnerable.
Staff report higher empathy scores for white subjects, influencing interaction style.
Data on stress markers can be skewed because heightened caretaker attention elevates baseline cortisol.
Preference for white strains persists, reinforcing the visual bias across generations of breeding.

Designing studies that involve mouse behavior or physiology must account for color‑based perception. Strategies include blind assessment of animal condition, inclusion of pigmented strains as controls, and training personnel to evaluate health indicators independently of coat color. Controlling for this bias improves reproducibility and reduces systematic error linked to the visual impression of vulnerability.

Beyond White: Other Laboratory Mouse Strains

Genetically Engineered Mice

Knockout and Transgenic Models

Laboratory mice are predominantly white because the dominant allele for the non‑pigmented coat (commonly the C57BL/6 background) is selected for its ease of observation and compatibility with genetic manipulation. Knockout and transgenic approaches exploit this uniform background to dissect the genetic mechanisms governing pigmentation.

Knockout models eliminate specific genes involved in melanin synthesis or melanosome transport. Typical examples include:

Tyrosinase (Tyr) knockout – abolishes the enzyme catalyzing the first step of melanin production, resulting in albino phenotypes.
Melanocortin‑1 receptor (Mc1r) knockout – disrupts signaling that determines eumelanin versus pheomelanin ratios, altering coat shade.
Oca2 and Slc45a2 knockouts – impair melanosomal pH regulation, reducing pigment deposition.

Transgenic models introduce exogenous DNA to overexpress or ectopically express pigment‑related genes. Representative lines are:

Tyrosinase‑related protein 1 (Tyrp1) overexpression – enhances melanin polymerization, producing darker coats on otherwise white strains.
Human MC1R transgene – allows comparative studies of human pigmentation variants within a mouse system.
CRISPR‑mediated knock‑in of pigmentation regulators – creates precise allelic substitutions to mimic natural coat color mutations.

Both strategies rely on the white genetic background to provide a clear visual readout of phenotypic changes. By introducing or removing pigment genes, researchers can quantify the contribution of each factor to coat coloration, assess epistatic interactions, and validate therapeutic targets for pigment disorders. The consistency of the white phenotype also simplifies breeding schemes, reducing the need for extensive backcrossing and accelerating experimental timelines.

Fluorescent Proteins and Reporters

Fluorescent proteins and reporter constructs are essential tools for visualizing cellular processes in white laboratory mice, whose lack of pigmented fur eliminates background autofluorescence and enhances signal detection. By inserting genes encoding green fluorescent protein (GFP), mCherry, or newer far‑red variants into the mouse genome, researchers create strains in which specific cell types or physiological events emit light detectable through non‑invasive imaging.

The white genetic background contributes to consistent expression levels because the albino mutation (commonly the Tyrc allele) removes melanin synthesis, reducing light absorption and scattering. Consequently, fluorescence intensity measured in vivo more accurately reflects reporter activity rather than tissue opacity. This advantage is particularly valuable for longitudinal studies of tumor growth, neuronal activity, or immune cell trafficking, where repeated imaging of the same animal is required.

Key considerations when designing fluorescent reporter lines for albino mice include:

Promoter selection – tissue‑specific promoters (e.g., Synapsin for neurons, CD4 for T cells) restrict expression to the desired cell population.
Insertion site – targeting safe‑harbor loci such as Rosa26 ensures stable, ubiquitous expression without disrupting endogenous genes.
Spectral compatibility – pairing far‑red fluorophores (e.g., iRFP) with the white coat minimizes overlap with residual tissue autofluorescence.
Copy number – homozygous integration typically doubles fluorescence output, but may affect viability; heterozygous lines balance brightness and health.

Breeding strategies leverage the albino genotype to maintain a uniform background across generations. Crosses between reporter carriers and albino strains are performed using standard Mendelian ratios, with genotyping confirming both the Tyrc allele and the reporter transgene. Maintaining homozygosity for the albino mutation simplifies colony management and guarantees the optical clarity required for high‑resolution imaging.

In summary, fluorescent protein reporters exploit the optical advantages of white laboratory mice, providing reliable, quantifiable readouts of biological activity while genetic manipulation and breeding protocols preserve the albino phenotype essential for optimal imaging performance.

Diverse Coat Colors and Their Research Applications

Black (C57BL/6) and Its Immunological Profile

The C57BL/6 strain, commonly referred to as black mice, is a cornerstone of immunological research. Its genome carries the dominant allele at the coat‑color locus (a), which masks the recessive albino allele (c) present in many laboratory colonies. Breeding programs maintain the black phenotype by selecting for homozygous a/a individuals, ensuring a stable genetic background for experimental reproducibility.

Immunologically, C57BL/6 mice exhibit a Th1‑biased response, characterized by elevated interferon‑γ production and robust cytotoxic T‑cell activity. This bias influences the outcome of infection models, tumor studies, and vaccine trials. Key features include:

High natural killer (NK) cell cytotoxicity
Strong CD8⁺ T‑cell expansion upon antigen exposure
Predominant IgG2c antibody isotype
Efficient class I major histocompatibility complex (MHC‑I) presentation

The strain’s susceptibility to certain pathogens, such as Listeria monocytogenes and Plasmodium spp., reflects its immune profile. Conversely, resistance to others, like Leishmania major, demonstrates the specificity of its Th1 orientation. These traits are encoded by multiple loci, including Ifng, Tbx21, and Stat1, which are routinely examined in knockout and transgenic experiments.

When black mice are crossed with albino strains, the resulting F1 generation typically displays the dominant black coat, while the underlying immune characteristics may shift depending on the genetic contribution of the non‑C57BL/6 parent. Careful breeding strategies preserve the desired immunological phenotype, allowing researchers to isolate the effects of coat color genetics from immune function in comparative studies.

Agouti and Behavioral Studies

The agouti allele produces a banded hair pigmentation pattern that yields a brownish‑gray coat, contrasting sharply with the albino phenotype commonly selected for laboratory colonies. In most inbred strains, the agouti locus is either mutated or silenced, allowing the recessive albino allele (c) to dominate and produce a uniform white coat. This genetic manipulation simplifies visual identification, reduces variability in optical imaging, and eliminates background coloration that can interfere with phenotypic assays.

Behavioral research frequently exploits the agouti genotype as a natural comparator because the agouti pigment gene is linked to neuronal pathways influencing anxiety, learning, and social interaction. Studies have documented that mice carrying functional agouti alleles exhibit:

Reduced baseline anxiety in elevated plus‑maze tests.
Enhanced spatial learning performance in Morris water‑maze trials.
Altered social dominance hierarchies in group housing.

These behavioral differences arise from pleiotropic effects of the agouti signaling protein (ASP), which modulates melanocortin receptors in the central nervous system. ASP activity influences melanocortin‑4 receptor signaling, a pathway implicated in stress response and reward processing. Consequently, the presence or absence of agouti pigmentation can serve as an indirect marker for neurochemical status in experimental cohorts.

When breeding white laboratory mice, researchers often introgress the agouti allele into otherwise albino backgrounds to create controlled phenotypic contrasts. Such breeding schemes enable direct assessment of how coat‑color genetics intersect with behavioral phenotypes, providing insight into gene‑environment interactions without introducing additional genetic confounds.