Sumi (my lovely wife) and I are currently house hunting. So we've been thinking about how difficult it is to calculate the optimal amount of money we should spend on a house. And how much of it should we take as a mortgage? To work this out, we need to take into account our current income, our expected future income, our predictions for the stock market and the housing market, and the interest rate of mortgages today and in the future. On top of that, we need to consider the amount of happiness different houses will provide us, and how that level of happiness measures up against the lifetime costs of these homes. Forty-one years of education between us does not seem to be enough to figure this out.