## 2 Behavioral Genetics
### Quantitative Genetics
#### 2.1.1 Twin Studies  
Twin studies, by comparing monozygotic (MZ) and dizygotic (DZ) twins, offer a robust method for studying the genetic and environmental contributions to phenotypic traits and are widely recognized as a valid methodology in quantitative genetics studies [11; 17]. MZ share 100% of their genetic material, as they are conceived by the fertilization of a single zygote, which makes them genetically identical. This genetic similarity allows MZ twins to serve as a model for investigating the genetic underpinnings of phenotypic traits, as any phenotypic differences observed between them are primarily attributable to environmental factors [18]. In contrast, DZ twins resulting from the fertilization of two separate eggs share approximately 50% of their genetic material, similar to non-twin siblings [18]. Although DZ twins differ genetically, they share the same intrauterine environment, which makes them an ideal comparison group for evaluating the relative contributions of genetic and environmental factors to phenotypic variance [18]. The comparison between MZ and DZ twins, therefore, enables a more accurate assessment of the genetic influence on a trait while accounting for environmental factors that both twin types may share. These studies are aimed at addressing the percentage of variance on traits explained by A, C, and E in order to allow researchers to understand whether the emergence of a given trait is mostly influenced by genetics, shared environmental factors, and unique environmental components [9]. To do so, twin studies estimate the correlations for a given phenotype between MZs and DZs, assuming that if the emergence of a specific trait is mostly influenced by genetics, correlations between MZs should be higher than those between DZs [19]. However, the validity of twin studies is strictly linked to the meeting of the Equal Environment Assumption (EEA). The EEA fundamentally assumes that MZs and DZs share similar environmental factors relevant to the phenotypic expression of the studied trait [9; 16]. If this condition is not met, due to potentially greater shared environmental influences among MZ twins, the validity of comparisons between MZ and DZ twins would be compromised. Consequently, this could lead to an overestimation of genetic influences, as highlighted by [PERSON] et al. [20]. Twin studies have historically faced criticism regarding their potential lack of representativeness. Nevertheless, longitudinal research has consistently demonstrated that twinsoffer a valid and generalizable framework for investigating various traits of interest within bio-psycho-social disciplines [11]. However, it is worth mentioning that the primary limitation of this research design regards the substantial difficulties associated with participant recruitment. Achieving an adequately powered sample typically necessitates the establishment of comprehensive national twin registries, a process that can span several years of rigorous effort. Furthermore, a significant proportion of twin studies have predominantly employed male-only cohorts, potentially limiting the generalizability of the findings and failing to account for the full spectrum of genetic and environmental influences across genders. Another important limitation to consider when discussing findings from twin studies concerns the impact of assessment instruments on twin data. Specifically, diagnostic questionnaires for children and adolescents can be administered to parents or teachers, or directly to the individuals themselves [21]. Evidence from the twin study literature indicates that third-party reports tend to overestimate both genetic (A) and shared environmental (C) influences, whereas self-reports are more likely to emphasize the role of non-shared environmental (E) factors [21; 22]. In particular, studies relying on parent informants consistently report inflated estimates of shared environmental influences, likely due to parents' tendency to perceive their children's experiences within the family context as highly similar [22].