Stocks or Bonds? Depends on how old you are, how healthy you are etc.
Why to buy bonds if stocks are performing better (just take an average for 10 years, 20 years, 30 years ...)??The answer is simple: with the age you become more and more susceptible to volatility. Peaceful retirement is more important than a capital gain.
In more precise formulation: in a pure capitalistic society one's objective is to maximize his Lifespan, which is the minimum of two random variables: BioLifespan and FinLifespan, that isLifespan = min(BioLifespan,FinLifespan) ==> max ... (1) Since both variables in (1) are well defined, it is possible to find certain "equilibrium difference" between more risky equities and (less volatile) cash/bonds; this "equilibrium difference" depends on age, gender, health, etc.The non-trivial part is the existence, value and stability of the equilibrium. In a "complete market", there should be a way, an instrument that , .