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Objective: Hoarding disorder is a complex condition that significantly impacts individuals' lives, characterized by excessive acquiring, difficulty discarding, clutter, distress, and impairment. This study aimed to examine the extent to which genetics and environment influence difficulty discarding, excessive acquisition, and clutter through the implementation of a classical twin study. Method: This classical twin study, conducted between April and September 2021, enrolled 194 twins (97 pairs) from Isfahan, recruited through the Isfahan Twins Registry (ITR). A total of 194 twins, consisting of 100 monozygotic (MZ) and 94 dizygotic (DZ) twins, participated in this study. Participants aged 16-50 were invited electronically and completed an online consent form and questionnaire. Hoarding symptoms were assessed using the saving inventory-revised. Zygosity was determined using a self-report method based on Song et al.'s questionnaire. To estimate the heritability of hoarding symptoms, the classical univariate twin model was employed. Results: Based on the univariate analysis, the heritability estimates for difficulty discarding and excessive acquisition were found to be 0.43 and 0.52, respectively. However, the results did not provide support for the role of genetics in clutter. Instead, it was indicated that the common environment accounted for 0.54 of the variance in clutter, while the specific environment contributed 0.46 to this symptom. Conclusion: The difficulty discarding and excessive acquisition were found to be moderately heritable. On the other hand, considering the contribution of genetics and environment to clutter, the results raise doubts about the association of clutter with hoarding. The relatively low genetic influence suggests that this trait may overlap with other behaviors rather than hoarding. [ABSTRACT FROM AUTHOR]