the reason people invest in bonds is because they are at least somewhat risk-avoiding. stocks have a much higher payoff rate but also you could lose quite a bit of money, due to volatility (the natural up and down swing of the market). given the fact that over any 15 year period you choose the stock market has always been up quite a bit more than bonds over the same period, you'd almost have to be pretty risk-averse. so an economist one day took a look at the combined returns of the DJIA from 1900 forward, and the combined return of bonds from the same point, and came up with this illustration for how risk averse one would have to be to buy bonds:
say you had $2000. this is all the money you have. you have a choice: you can either take a 50/50 bet that your money will either increase by 50% or decrease by 50%; or you can pay to not take the bet. so if you take the bet, you have a 50% chance of landing $1000 extra and a 50% chance of costing $1000. its a fair bet, your expected payout is $0.
now, most people are somewhat risk-averse when it comes to their money, so they'll pay some money to keep from taking the bet. $100, $200, even $500. all pretty reasonable. well, people who would invest in bonds over stocks have a risk averseness of someone paying ~$980 to keep from taking the bet. that is, they would rather have $1020 than take a bet that would leave them with $1000 or $3000.
moral of the story: put all your money in an index fund