Sexism & Diversity in Comics

Building on the theme from last week, the topic for Pod Sequentialism #04 is Sexism and Diversity. Has the lack of diversity in comics been the cause of dwindling circulation of monthly comics? It certainly seems to be the case when compared to the sales of young adult novel, which are inherently more inclusive. Considering current ethnic demographics, Why are so many superheroes still so white–and so male?
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