The Newcastle-Ottawa Scale (NOS) [60] tool was used for the investigation of the risk of bias in case-control observational assessment. The NOS contains eight items, categorized into three domains: selection, comparability, and outcome/exposure. For each study type (i.e., family or twin), the items were adapted, and a series of response options were provided. A star system scoring was used, ranging from zero to nine stars. Details of the risk assessment of bias according to the NOS for each included study is provided in Table S2.
