Fig. 1: Theoretical diagram of the common bridging factor (cb factor).

This diagram illustrates the main strength (marked by the “√” button) and key challenge (marked by the “!” button) for the “comorbidity hypothesis” (a) and “common cause hypothesis” (b), respectively. The theoretical hypothesis for the present cb factor was drawn at (c) and was briefly introduced in the box (marked by the “i” button). These icons that used in the present figure are openly available at the web-based software (ICONFINDER, https://www.iconfinder.com/). The “√” icon by Paomedia, titled “Check, sign icon”, used under CC-BY 3.0 license, available at https://www.iconfinder.com/icons/299110/download/png/256. The “!” icon by Tahsin Tahil, titled “Error icon”, used under CC-BY 3.0 license, available at https://www.iconfinder.com/icons/381599/download/png/256. Based on the permission of the CC-BY 3.0 license, this “!” icon is modified by changing its background color to dark yellow and changing the style of the “!” mark in the figure. The “i” icon by Paomedia, titled “Info, sign icon”, used under CC-BY 3.0 license, available at https://www.iconfinder.com/icons/299086/download/png/256. Based on the permission of the CC-BY 3.0 license, this icon is modified by changing the style of this “i” mark.