Many years ago, I was naive and thought that everyone had the intelligence to be successful but the effort was variable.  Then I started tutoring kids and realized that some people are just dumb, and even with hard work, are limited in their potential unless they find something that they can excel at and pays wells.  If the parents are dumb, the kids are likely to be dumb.  Being dumb means a lot of things, such as divorce, neglect, time not being spent with kids on school, lack of effort, limited success in school, etc.