DZ Twins Research Articles

The relative contribution of environmental and genetic factors to phenotypic variation in familial Mediterranean fever (FMF)

Introduction Familial Mediterranean fever (FMF) is an autosomal recessive disease, caused by mutations in the FMF gene MEFV (MEditerranean FeVer). It has a large phenotypic diversity even in patients with similar genotypes. Despite evidence that environmental factors (EFs) and genetic factors, including MEFV mutations (such as M694V, E148Q) and background modifier genes (MGs), affect the clinical manifestations of FMF, the relative contribution of each remains unknown. Methods To investigate the relative contribution of environmental and genetic factors to the phenotype of FMF, we compared the intra-pair clinical concordance of 10 mono and 7 dizygotic twins with FMF. The part played by EFs was determined by the phenotypic discordance of the monozygous twins, and the MGs effect was determined by deducing the environmental effect, computed for MZ twins, from the phenotypic discordance of the dizygous twins. Results The mean ± SD of intra-pair concordance was higher in the MZ than in DZ twin group (88.1 ± 13.2 vs. 70.7 ± 14.1 respectively, P value < 0.05). Based on the concordance in clinical manifestations in MZ and DZ twins, the environmental effect on the phenotype of FMF is estimated as 11.9% ± 6.6% and the MGs effect as 17.4% ± 15.5% in average. Conclusions In FMF the phenotype is affected by MEFV mutations, MGs and EFs in an estimated ratio of about 6:1.5:1 respectively.

Conflict, Closeness and Comfort: The Inter-Twin Relationship as a Risk Factor for Behavioral Difficulties

There is currently little evidence concerning the impact of the inter-twin relationship on behavioral outcomes and yet the twin relationship is frequently hypothesized to be a unique source of psychopathology in twins. The current study asked whether the inter-twin relationship is a predictor of behavioral difficulties and whether there are zygosity differences in this relationship. An Australian sample of same sex twins (N = 356, 164 MZ and 192 DZ) was studied in the year prior to school (Time 1) and again in first year of school (Time 2). Associations between twin-relationship and behavior problems were examined via path-analysis, and Satorra-Bentler chi-square difference tests were used to compare twins across zygosity. Results show that both conflict and lower levels of warmth at Time 1 were predictive of hyperactivity and conduct disorder at Time 2, but they were not associated with emotional difficulties or peer problems. While DZ twins shared less warmth than MZ twins, there were no differences in behavior problems. Conflict and lower levels of warmth in the inter-twin relationship are associated with hyperactivity and conduct disorder and may serve as important considerations when making decisions regarding class separation at entry to school.

Discussion on ‘Heritability in the Era of Molecular Genetics: Some Thoughts for Understanding Genetic Influences on Behavioural Traits’ Wendy Johnson, Lars Penke, and Frank M. Spinath

The target paper of Johnson, Penke, and Spinath (2011) provides a broad up‐to‐date summary of the many reasons that limit the usefulness of the concept of heritability for understanding genetic contributions to personality differences. My comment focuses on one main reason why heritability is relatively silent about the roles of specific genes in creating personality differences: heritability refers to the outcome of personality development, not to its molecular basis, and therefore, the effects of nonshared genes can be transformed over development into effects that appear genetic in heritability estimates, and the shared genes can, over development, produce effects that appear to be nonshared environmental. Just as (non)shared environmental effects tell us little about (non)shared environments, (non)shared genetic effects tell us little about (non)shared genes. Copyright © 2011 John Wiley & Sons, Ltd. In the context of personality research, heritability refers to the proportion of observed variance in a personality trait in a particular population that is attributed to trait‐relevant genetic differences in this population. Because the attribution to genetic differences is based on assumptions about the genetic relatedness of relatives (MZ versus DZ twins, biological versus adopted siblings, etc.) that are ultimately based on the probability of sharing genes at the molecular‐genetic level (more precisely: sharing alleles of genes), the concept of heritability seems to refer directly to the molecular‐genetic level. However, heritability is estimated on the basis of the phenotypic similarity of relatives and the extent to which they share genes overall. It thus refers to the similarity of trait‐ relevant overall gene expression, not to the similarity of particular trait‐relevant specific genes. Inferences from the similarity in trait‐relevant gene expression to the similarity in trait‐relevant specific genes rest on the assumption of a 1–1 correspondence between genes and effects of these genes. Such a 1–1 correspondence is a tacit assumption underlying the reasoning of many geneticists and psychologists concerned with genetically informed explanations of personality differences. For example, this tacit assumption drives gene association studies, both single allele and genome‐wide; that is, the search for gene loci at which particular alleles are correlated with trait scores (trait‐relevant genes). More specifically, it is assumed that for a trait with high heritability such as height, it is easier to find genes involved in the trait than for a trait with lower heritability (e.g. neuroticism), and the correlations are expected to be higher. The failure to account for more than 5% of trait heritability by effects of specific genes even in large genome‐wide association studies for very accurately measurable traits such as height led to the announcement of a ‘missing heritability problem’ (Maher, 2008), a bad choice of label because what is missing is not heritability—missing are trait‐relevant alleles. My argument here is that expectations to find trait‐ relevant alleles based on high heritability are misguided because the contribution of each gene to the trait may be too small to be detectable with reasonable sample sizes (one of the standard arguments for explaining ‘the missing heritability’) and because they rest on the assumption of 1–1 correspondences between genes and genetic effects, and this assumption is faulty. Figure 1 illustrates the lack of correspondence between the molecular‐genetic level of genes and the quantitative‐genetic level of genetic effects. Assume that the two genomes G1 and G2 share particular alleles because they are genetically related and do not share other alleles because they are not MZ twins. A 1–1 correspondence view assumes that for any trait, the similarity of the trait‐relevant genetic effects P1 and P2 is due only to the similarities between G1 and G2 (the solid lines in Figure 1). However, there may be cross‐effects indicated by the dotted lines in Figure 1 such that alleles of G1 European Journal of Personality, Eur. J. Pers. 25: 267–286 (2011) Published online (wileyonlinelibrary.com) DOI: 10.1002/per.834

Twin-Singleton Differences in Neonatal Brain Structure

Twin studies suggest that global and regional brain volumes are highly heritable. However, estimates of heritability vary across development. Given that all twin studies are open to the potential criticism of non-generalizability due to differences in intrauterine environment between twins and singletons, these age effects may reflect the influence of perinatal environmental factors, which are unique to twins and which may be especially evident early in life. To address this question, we compared brain volumes and the relationship of brain volumes to gestational age in 136 singletons (67 male, 69 female) and 154 twins (75 male, 79 female; 82 DZ, 72 MZ) who had received high resolution MRI scans of the brain in the first month of life. Intracranial volume, total white matter, and ventricle volumes did not differ between twins and singletons. However, cerebrospinal fluid and frontal white matter volume was greater in twins compared to singletons. While gray matter volumes at MRI did not differ between groups, the slope of the relationship between total and cortical gray matter and gestational age at the MRI scan was steeper in MZ twins compared to DZ twins. Post-hoc analyses suggested that gray matter development is delayed in MZ twins in utero and that they experience 'catch-up' growth in the first month of life. These differences should be taken into account when interpreting and designing studies in the early postnatal period.

Psychosexual Study of Communist Era Hungarian Twins

Our aim in this study is to describe the characteristics of sexual development in twins and estimate the role of heritability and environmental factors as causes of certain sexual disorders. Two hundred and ten adult same-sex twin pairs (92 monozygotic [MZ] female, 41 MZ male, 55 dizygotic [DZ] female and 22 DZ male pairs) were involved in the study. Data were collected in 1982 by self-administered questionnaires that included items on sexual maturation, sexual life, contraception, mutual sexual activity within twin pairs and alcohol use. The ratio of married to unmarried twins was nearly the same in MZs and DZs, with the exception that the divorce rate was higher in MZ female twins (14%), and DZ and male twins were slightly more likely to be single. Menarche was later in twins compared to non-twin Hungarian women. 57% of MZs experienced menarche within 3 months of each other, 77% within 6 months while it occurred for 30% and 43% respectively in DZs. The first seminal emission indicated some delay in male twins compared with the Hungarian general population sample. MZ first kisses occurred later than DZ's first kisses. The same was true for the first petting, masturbation and first sexual intercourse. Anorgasmy is 27% heritable but the estimate is not statistically significant. Concordance rate for premature ejaculation in MZs was greater than in DZs but the structural equation model showed significant misfit. Age at menarche appeared to be strongly heritable.

Genes contribute to the switching dynamics of bistable perception

Ordinarily, the visual system provides an unambiguous representation of the world. However, at times alternative plausible interpretations of a given stimulus arise, resulting in a dynamic perceptual alternation of the differing interpretations, commonly referred to as bistable or rivalrous perception. Recent research suggests that common neural mechanisms may be involved in the dynamics of very different types of bistable phenomena. Further, evidence has emerged that genetic factors may be involved in determining the rate of switch for at least one form of bistable perception, known as binocular rivalry. The current study evaluated whether genetic factors contribute to the switching dynamics for distinctly different variants of bistable perception in the same participant sample. Switching rates were recorded for MZ and DZ twin participants in two different bistable perception tasks, binocular rivalry and the Necker Cube. Strong concordance in switching rates across both tasks was evident for MZ but not DZ twins, indicating that genetic factors indeed contribute to the dynamics of multiple forms of bistable perception.

Inheritance of 18 quantitative dermatoglyphic traits based on Factors in MZ and DZ twins

18 quantitative finger and palmar dermatoglyphic traits were analyzed with the aim of determining genetic effects and common familial environmental influences on a large (358 nuclear pedigrees) number of twins (MZ and DZ). Genetic analysis based on principal factors includes variance and bivariate variance decomposition analysis. Especially, Factor 1 (digital pattern size) is remarkable, due to its degree of universality. The results of genetic analysis revealed all three extracted factors have significant proportion of additive genetic variance (93.5% to 72.9%). The main results of bivariate variance decomposition analysis appears significant correlation in residual variance between digital pattern size factor (Factor 1) versus finger pattern intensity factor (Factor 4), and palmar main lines factor (Factor 2) verses a-b ridge count (Factor 3), but there was no significant correlation in the genetic variance of factors.

DeFries–Fulker Analysis of Longitudinal Reading Performance Data from Twin Pairs Ascertained for Reading Difficulties and from Their Nontwin Siblings

Reading difficulties are both heritable and stable; however, little is known about the etiology of this stability. Results from a preliminary analysis of data from 56 twin pairs who participated in the Colorado Longitudinal Twin Study of Reading Disability (Astrom et al., Twin Res Hum Genet 10:434-439, 2007) suggested that about two-thirds of the proband deficit at follow-up was due to genetic factors that also influenced deficits at their initial assessment. Although our proband sample is now nearly twice as large, it is still relatively small; thus, to increase power, we subjected data from probands, co-twins and their nontwin siblings to a novel extension of DeFries-Fulker analysis (DeFries and Fulker, Behav Genet 15:467-473, 1985; DeFries and Fulker, Acta Genet Med Gemellol, 37, 205-216, 1988). In addition to providing estimates of univariate and bivariate heritability, this analysis facilitates a test of the difference between shared environmental influences for twins versus siblings. Longitudinal composite reading performance scores at 10.6 and 15.5years of age, on average, were analyzed from 33 MZ and 64 DZ twin pairs in which at least one member of each pair had reading difficulties, and from 44 siblings of the probands. Scores were highly stable (.86±.03, across probands, co-twins and siblings) and heritability of the group deficit at initial assessment was .67±.22. Longitudinal bivariate heritability was .59±.21, suggesting that nearly 60% of the proband reading deficit at follow-up is due to genetic factors that influenced reading difficulties at the initial assessment. However, tests for special twin environmental influences were nonsignificant.

Mothers Do Matter: New Evidence on the Effect of Parents' Schooling on Children's Schooling Using Swedish Twin Data

Behrman and Rosenzweig (2002) used data on a small sample of MZ (monozygotic, identical) twin parents and their children to show that father's schooling is more important than mother's schooling for children's schooling in the U.S. Recent studies based on much larger samples of twins from registry data in Scandinavian countries reach similar conclusions. Most of these studies, however, are unable to distinguish between MZ and DZ (dizygotic, fraternal) twins. Using data from the Swedish Twin Registry, we replicate the finding that father's schooling matters more than mother's schooling in a combined sample of MZ and DZ twin parents. In contrast, results based on MZ twin parents show that mother's schooling matters at least as much as father's schooling for children's schooling. We also estimate the effect of parents' schooling separately by child gender and find this effect to be entirely driven by the impact of mother's schooling on daughter's schooling. Our results show that (1) it is vital to have zygosity information to estimate causal intergenerational effects and (2) the conclusions reached by Behrman and Rosenzweig (2002) for the U.S. do not apply in Sweden.

Enamel hypoplasia in the primary dentition of monozygotic and dizygotic twins compared with singleton controls

The study of enamel hypoplasia (EH) and opacity in twins provides insights into the contribution of genetic and environmental factors in the expression of enamel defects. This study examined prevalence and site concordance of EH and opacity in the primary dentition of 2- to 4-year-old twins and singleton controls to assess the relative contribution of genetics and the environment to the aetiology of these defects. The study sample consisted of 88 twin children and 40 singletons aged 2-4 years of age. Medical histories were obtained and the children examined for enamel defects. The prevalence of EH by teeth was 21% in monozygotic twins (MZ), 22% in dizygotic twins (DZ), and 15% in singleton controls. Twins showed a higher prevalence of EH compared with singletons (P < 0.05). Factors contributing to increase EH in twins were neonatal complications including intubation. There were no significant differences in site concordance of EH within the MZ twin pairs compared with DZ twin pairs when only presence of EH was considered, whereas a greater concordance was noted between MZ twin pairs compared with DZ twin pairs when both presence and absence of EH were considered. The results suggest that both genetic and environmental factors contribute to observed variation of EH, although it is likely that environmental factors exert a greater influence.

Social closeness and gift giving by twin parents toward nieces and nephews: An update

Twin-family designs yield an array of genetically related individuals, conducive to testing evolutionary-based hypotheses regarding social closeness. Monozygotic twins’ genetic identity makes them the “genetic parents” of their nieces/nephews. However, dizygotic twins retain customary aunt/uncle relationships with their co-twin’s children. A 2007 study found that MZ twin aunts/uncles expressed greater social closeness toward nieces/nephews than DZ twin aunts/uncles, consistent with predictions from inclusive fitness theory. Greater social closeness was expressed toward children of female co-twins than male co-twins. These issues and others were revisited in a larger sample of 419 twins. The new results parallel those from the initial study. In addition, twins with female co-twins gave more gifts to nieces/nephews than did twins with male co-twins; however, the effect of zygosity was non-significant. The value of assessing evolutionary psychological questions via behavioral-genetic methods is emphasized.

Chromatic (Red/Green) and Luminance Contrast Sensitivity in Monozygotic and Dizygotic Twin Infants

Purpose: To determine the extent to which contrast sensitivity (CS) development is governed by genetic mechanisms vs. environment, we compared CS between pairs of twin infants and pairs of unrelated infants. If genetics have a strong influence on CS, Monozygotic (Mz) twin siblings should be more similar (and more strongly correlated) than Dizygotic (Dz) twin siblings, and both Mz and Dz twins should show greater correlations than unrelated infant pairs. By contrast, if genetics have little influence, correlations should be the same for Dz and Mz twins. In this latter scenario, if both Mz and Dz twins show greater correlations than unrelated infant pairs, this suggests a role of shared environment. The current study measured Luminance (light/dark) and Chromatic (red/green) CS to assess sensitivity of the Magnocellular and Parvocellular pathways, respectively. Methods: Ten and 26 pairs of Mz and Dz twin pairs were tested (mean age = 4.5±1.5 mos). Zygosity was assessed using a questionnaire and cheek swab kits. Luminance and Chromatic CS were obtained for sinusoidal gratings using forced-choice preferential looking (∼200 trials per infant; 0.27 cycles/degree; 4.2 Hz). Results: Multiple regression was conducted on 100 runs of random Twin-1/Twin-2 orderings. Results indicated that the CS of one twin predicted 35-40% of the variance (p <0.0001) in the CS of the other twin, while controlling for postnatal age, gestational length, and gender. Simulations with randomly-matched twins suggest these findings are not due to chance. Discussion: Results suggest shared environment and/or genes influence both Luminance and Chromatic CS. With more subjects, we will address whether Luminance vs. Chromatic CS is more or less affected by shared genes/environment. Once we have enough data to compare Mz vs. Dz twins, we can ask which factor, genes or environment, is more influential using structural equation modeling (Neale et al, 2002).

A controlled study of dental erosion in 2‐ to 4‐year‐old twins

Dental erosion (DE) in children is a significant oral health issue and has become a focus for research in clinical paediatric dentistry. This study investigated DE in the primary dentition of 2- to 4-year-old twin and singleton children with regard to the genetic, medical and dietary factors associated with the condition. The 128 subjects consisted of 88 twin children (31 monozygous, 50 dizygous, 7 unknown zygosity) and singletons (n = 40) aged 2-4 years. Medical, dental, and dietary histories were obtained. The children were examined for DE using a modified index. The prevalence of DE by subject affected was 77% in monozygotic twins (MZ), 74% in dizygotic twins (DZ), and 75% in singleton controls (P > 0.1). Of the teeth scored, 12% had mild, 10% moderate, and 1% severe lesions, and DE was more severe in the older age group (P < 0.05). Concordance rates for erosion lesions in MZ and DZ co-twins were not statistically significant. The prevalence of DE and the concordance of erosion lesions were similar between MZ and DZ twins and singleton children, suggesting that the contribution of genetic factors to DE is negligible.

Automated Quantification of Inherited Phenotypes from Color Images: A Twin Study of the Variability of Optic Nerve Head Shape

Discovery and description of heritable optic nerve head (ONH) phenotypes have been haphazard. In this preliminary study, the authors test the hypothesis that inheritable phenotypes can be discovered and quantified computationally by estimating three-dimensional ONH shape parameters from stereo color photographs from the Twins Eye Study in Tasmania and determining how much of the variability in ONH shape is accounted for by genetic influence. Three-dimensional ONH shape was estimated by an automated algorithm from stereoscopic optic disc color photographs of a random sample of 172 subjects (344 eyes, 45 pairs of monozygotic [MZ] and 41 dizygotic [DZ] twins). Shape resemblances between eyes were quantified with a distance metric. The heritability of the shape resemblance was determined both through the distribution of the discongruence indices and through structural equation modeling techniques (ACE model). Significantly different discongruence indices were found for MZ (1.0286; 95% CI, 0.9872-1.0701) and DZ twins (1.4218; 95% CI, 1.2631-1.5804); larger indices for DZ twins indicated that variability was substantially determined by genetic factors. The standardized variances of the A(dditive genetic), C(ommon environmental), and (nonshared) E(nvironmental) components were 0.80, 2.00 × 10(-15) and 0.20, respectively, for all OD, and 0.79, 3.24 × 10(-14), and 0.21 for all OS. This preliminary study shows that quantitative phenotyping of the ONH shape from color images leads to phenotypes that can be measured and are largely under genetic control. The association of these inherited phenotypes with genotypes deserves confirmation and further study.

Genetic influences on brain asymmetry: A DTI study of 374 twins and siblings

Brain asymmetry, or the structural and functional specialization of each brain hemisphere, has fascinated neuroscientists for over a century. Even so, genetic and environmental factors that influence brain asymmetry are largely unknown. Diffusion tensor imaging (DTI) now allows asymmetry to be studied at a microscopic scale by examining differences in fiber characteristics across hemispheres rather than differences in structure shapes and volumes. Here we analyzed 4 Tesla DTI scans from 374 healthy adults, including 60 monozygotic twin pairs, 45 same-sex dizygotic pairs, and 164 mixed-sex DZ twins and their siblings; mean age: 24.4 years ± 1.9 SD). All DTI scans were nonlinearly aligned to a geometrically-symmetric, population-based image template. We computed voxel-wise maps of significant asymmetries (left/right differences) for common diffusion measures that reflect fiber integrity (fractional and geodesic anisotropy; FA, GA and mean diffusivity, MD). In quantitative genetic models computed from all same-sex twin pairs ( N = 210 subjects), genetic factors accounted for 33% of the variance in asymmetry for the inferior fronto-occipital fasciculus, 37% for the anterior thalamic radiation, and 20% for the forceps major and uncinate fasciculus (all L > R). Shared environmental factors accounted for around 15% of the variance in asymmetry for the cortico-spinal tract (R > L) and about 10% for the forceps minor (L > R). Sex differences in asymmetry (men > women) were significant, and were greatest in regions with prominent FA asymmetries. These maps identify heritable DTI-derived features, and may empower genome-wide searches for genetic polymorphisms that influence brain asymmetry.

Testing the Equal Environments Assumption in the Children of Twins Design

In a Children of Twins (COT) design, the environmental and genetic risk of a child is, in part, dependent upon the status of the father and the father's cotwin. The logic of the COT method breaks down if the zygosity of the twin pair is confounded with the environment provided to the child (a version of the Equal Environment Assumption, EEA). If MZ twin fathers see each other more often than DZ twin fathers, and a child's uncle is the affected twin in discordant pairs, this could increase the environmental risk of children of MZ over that of DZ discordant twins. The current study was designed to test the EEA in the COT design, specifically in children of alcohol and drug dependent fathers. Results indicated that MZ twins did have more contact than DZ twins. Regression analyses were conducted to predict child externalizing symptom counts from father's zygosity group status, level of contact with father's cotwin, and their interaction. Results found no significant interaction between father's zygosity and the higher level of cotwin contact (seen in MZ twins) in predicting several measures of offspring externalizing risk. The results of this study suggested that the COT design does not confound zygosity with differences in environmental risk exposure, findings that support the validity of the EEA within this research context.

Hippocampal volume changes in healthy subjects at risk of unipolar depression

Unipolar depression is moderately heritable. It is unclear whether structural brain changes associated with unipolar depression are present in healthy persons at risk of the disorder. Here we investigated whether a genetic predisposition to unipolar depression is associated with structural brain changes. A priori, hippocampal volume reductions were hypothesized. Using a high-risk study design, magnetic resonance imaging brain scans were obtained from 59 healthy high-risk subjects having a co-twin with unipolar depression, and 53 healthy low-risk subjects without a first-degree family history of major psychiatric disorder. High-risk twins had smaller hippocampal volumes than low-risk twins ( p < 0.04). The finding was most pronounced in DZ twins. Groups did not differ on global brain tissue volumes or regional tissue volumes assessed in exploratory voxel-wise whole cerebrum analyses. In conclusion, hippocampal volume reduction may index a predisposition to develop depression and thus may be predictive of future onset of the disorder. Further studies are needed to elucidate the role of (shared) environmental and genetic factors.

Genetic Covariation Between Brain Volumes and IQ, Reading Performance, and Processing Speed

Although there has been much interest in the relation between brain size and cognition, few studies have investigated this relation within a genetic framework and fewer still in non-adult samples. We analyzed the genetic and environmental covariance between structural MRI data from four brain regions (total brain volume, neocortex, white matter, and prefrontal cortex), and four cognitive measures (verbal IQ (VIQ), performance IQ (PIQ), reading ability, and processing speed), in a sample of 41 MZ twin pairs and 30 same-sex DZ twin pairs (mean age at cognitive test = 11.4 years; mean age at scan = 15.4 years). Multivariate Cholesky decompositions were performed with each brain volume measure entered first, followed by the four cognitive measures. Consistent with previous research, each brain and cognitive measure was found to be significantly heritable. The novel finding was the significant genetic but not environmental covariance between brain volumes and cognitive measures. Specifically, PIQ shared significant common genetic variance with all four measures of brain volume (r (g) = .58-.82). In contrast, VIQ shared significant genetic influence with neocortex volume only (r (g) = .58). Processing speed was significant with total brain volume (r (g) = .79), neocortex (r (g) = .64), and white matter (r (g) = .89), but not prefrontal cortex. The only brain measure to share genetic influence with reading was total brain volume (r (g) = .32), which also shared genetic influences with processing speed.

The genetics of general knowledge: A twin study from Croatia

The purpose of this study was to explore genetic and environmental influences on general knowledge (GK) and its relationship with intelligence. The sample consisted of 71 MZ and 78 DZ twin pairs, aged on average 17 years. General knowledge was measured with a test consisting of 60 items measuring different domains of GK including geography, history, politics, art, science, fashion, and sport. Four different measures of intelligence were used, two that measured spatial abilities (visualization and spatial orientation) and two that measured verbal abilities (word fluency and vocabulary). Univariate analyses indicated that the heritability of GK is .87, while heritability estimates for cognitive ability measures ranged from .48 to .73. Bivariate analyses between each of the four intelligence measures and GK measure were carried out using correlated factors model. Shared genetic influences were found between GK and all four intelligence measures in the .40–.80 range.

Chromatic (red/green) and luminance contrast sensitivity in Monozygotic and Dizygotic twin infants

“Purpose: To determine the extent to which contrast sensitivity (CS) development is governed by genetic mechanisms vs. environment, we compared CS between pairs of twin infants and pairs of unrelated infants. If genetics have a strong influence on CS, correlations for Monozygotic (Mz) twin pairs should be greater than those for Dizygotic (Dz) twin pairs, and both Mz and Dz twins should show greater correlations than unrelated infant pairs. By contrast, if genetics have little influence, correlations should be the same for Dz and Mz twins. In this latter scenario, if both Mz and Dz twins show greater correlations than unrelated infant pairs, this suggests a role of shared environment. The current study measured Luminance (light/dark) and Chromatic (red/green) CS to assess sensitivity of the Magnocellular (M) and Parvocellular (P) pathways, respectively. Methods: Nine and 16 pairs of Mz and Dz twin pairs for a total of 50 twin infants were tested (mean age = 5 ±1.5 mos). Zygosity was assessed using a questionnaire and cheek swab kits, when necessary. Luminance and Chromatic CS were obtained using forced-choice preferential looking (˜200 trials per infant; 0.27 cpd; 4.2 Hz; mean luminance = 20 cd/m2). Results: Correlations for both Mz and Dz twins, and for both Luminance and Chromatic CS were significant (all p values 〈 0.002). Monte Carlo simulations on randomly paired unrelated twin infants revealed non-significant correlations (mean r = 0.12 and 0.07 for Luminance and Chromatic), indicating that Mz and Dz twins show greater correlations than unrelated infant pairs, and providing evidence for effects of shared environment. For Luminance CS, the Mz and Dz correlations were nearly identical (rMz = 0.88, rDz = 0.81), suggesting negligible genetic influence. By contrast, for Chromatic CS, the Mz correlation was greater than the Dz correlation (rMz = 0.87; rDz = 0.64), suggestive of a genetic contribution. Multiple regression analyses confirmed that these significant twin effects are not accounted for by small variations in age across infants. Discussion: Results suggest shared environment influences on both Luminance and Chromatic CS, as evidenced by Mz and Dz twins showing greater correlations than unrelated infant pairs. Although more Mz and Dz twins need to be tested, preliminary data suggest less genetic influence on Luminance CS (mediated by the M pathway) than on Chromatic CS (mediated by the P pathway). Future analysis, with more subjects, will employ maximum-likelihood based structural equation modeling (Neale, et al, 2002)”