The answer to this is simply Money. To be extra, it would be health. Money is a crippling thing to most people. It includes investing, economics, interest rates, retirement, saving, and more. There is so much that goes into the effects of money.
For me I have always kept a tight look on money based on my upbringing and family. Money is also such a corrupting factor to life, and happiness. They day I really saw what money really was about and reading so many books and interviews, I totally understand the world more as money fuel a lot of growth and demise of things. Learning money will change your entire family tree and legacy, trust me.