Neither fixed rate mortgages nor index funds are particularly complex. The idea behind an index fund is that it invests in the components of an index. Generall

Neither fixed rate mortgages nor index funds are particularly complex. The idea behind an index fund is that it invests in the components of an index. Generally, that is a specific basket of stocks or bonds. Stocks are perhaps the most straightforward financial instrument: pieces of ownership of a company. Bonds are a bit more complex, but they're just the right to repayment of a loan (principal plus interest). Complexity arises if the fund can't directly invest in the components of the index and most replicate it with other things instead but that's an implementation detail and an individual is perfectly capable of buying the individual components themselves if they're worried about that. A fixed rate mortgage is a loan with a fixed, compounding interest rate and a fixed term. The mortgage payment is determined based on an amortization schedule but that's an implementation detail - it could just as easily be monthly interest plus a fixed portion of the principal or something else.