I have seen studies, and from my personal experience. My dad is a single father, while my mom was a feminist who was roaming around, the most important factor seems to be the discipline and structure that fathers provide for their children, which mothers cannot comprehend.
I think this is the most obvious reason. It may also be that men work harder and are more capable of contributing due to certain biological traits (such as greater strength, speed, fewer autoimmune diseases, no menstruation, etc.). Thus bringing in more income than what single mothers can.
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