Religion.
I know that’s not typically seen as a social problem, but religion and its effects (particularly fundamentalist religions) are behind many, if not most of the social problems today.
Consider just a few:
- religion teaches that birth control is bad. Results: too many children that live in poverty
- religion (some) teaches that white people are better than any other race.
- religion frequently denies science (stem cell research for example) and people die as a result
- religion denies personal autonomy (abortion) and women die
- religion (some) keep women subjugated, and in some societies, slaves
etc.